Haemophilus influenzae

MICROBIOLOGY

Haemophilus influenzae is a small, non-motile, non-spore forming, Gram-negative pleomorphic rod that can be either encapsulated (serotypes a-f) or unencapsulated (non-typeable H. influenzae). Haemophilus influenzae normally exists as a commensal in the human upper respiratory tract, but can cause disease, either by invasion of the blood stream or by contiguous spread. Before the introduction of Hib conjugate vaccines, serotype b (Hib) was the most common etiology of invasive disease, and was a common cause of pediatric pneumonia, meningitis, and bacteremia (75, 90, 280). In areas that have introduced Hib conjugate vaccine into routine infant immunization programs, non-typeable H. influenzae and serotypes a and f have become the most common etiologies (112, 269).

EPIDEMIOLOGY

Carriage

Humans are the only natural host for Hi and transmission occurs through respiratory droplets and direct contact with respiratory secretions. Oropharyngeal carriage of Hib is an important component of disease pathogenesis. In the pre-vaccine era, Hib carriage ranged from near 0% to over 15% with the highest carriage prevalence occurring in Australia (global Hib carriage prevalence has been reviewed recently (91). Interestingly, carriage prevalence did not correlate strongly with disease incidence. For example, prevalence among US populations and Asian populations both were about 5% while disease incidence in the US was much higher.

Hib Incidence and Clinical Presentation

Before the introduction Hib conjugate vaccination, the annual incidence of Hib meningitis among children age <5 years in North and South America, Northern Europe, Australia/Pacific Islands, and the Middle East was approximately 15 to 60 per 100,000 (90, 138, 280). In Africa, documented pediatric meningitis incidences were higher, ranging from 30 to 60 per 100,000 per year, despite the likelihood that many cases went undocumented. By contrast, documented annual incidences were 3 to 25 per 100,000 in Southern Europe and 1 to 10 in Asia. Other than true differences in incidence, differences in incidence may relate to a variety of issues including patient access to care, use of lumbar puncture (versus presumptive diagnosis and treatment), and laboratory diagnostics. However, these factors are unlikely to explain the large differences in incidence that have been documented particularly since several studies in Asia and Southern Europe were well implemented and thus unlikely to be affected by these issues. An additional theory is that areas with high population use of antimicrobial therapy (such as Southeast Asia and Southern Europe) may have less disease because early and frequent antibiotic use blunts disease progression, decreases carriage, or makes existing disease more difficult to diagnose (90).

Besides meningitis, Hib may cause other invasive disease syndromes including bacteremia or sepsis with or without pneumoniae and epiglottis, as well as septic arthritis (138, 241). For poorly understood reasons, in Northern Europe, North America, and Australia – but not other areas – epiglottis was a common presentation of invasive Hib disease, in some instances as common or almost as common as meningitis (94, 253). Consequently, in these areas total invasive disease incidences were 1.5 to 2.7-fold higher than meningitis incidences while in Africa, Asia, and Southern Europe this ratio was 1.2 to 1.5.

Indigenous populations in the North American Arctic and Australia have reported exceptionally high incidence rates, ranging from 140 to 530 per 100,000 per year among children age <5 years (104, 106, 272). At these levels, 0.7% to 2.6% of all children experienced Hib meningitis before their fifth birthday. Although genetic and cultural differences have been postulated, no satisfactory theory exists to explain the magnitude of disease seen in these populations.

Perhaps the most common presentation of Hib disease is non-bacteremic pneumonia. This syndrome is particularly difficult to document and thus for most areas of the world, no incidence estimate is available. A randomized clinical trial of Hib vaccine in The Gambia in Western Africa evaluated vaccine impact on a variety of clinical syndromes. In this study, children under age 2 years that received vaccine had 20% fewer pneumonias associated with alveolar consolidation than unvaccinated children and 6.5% fewer severe pneumonias regardless of radiological findings (185). No estimate of vaccine-preventable incidence was made.

A subsequent vaccine probe trial in Indonesia evaluated the vaccine preventable incidence of several pneumonia syndromes by subtracting the incidence of disease among children who received vaccine from the incidence among control children. In this study, there was no impact against pneumonia with alveolar consolidation and a 4.8% reduction in any clinically severe pneumonia, remarkably similar to results from The Gambia. The vaccine-preventable incidence of any clinically severe pneumonia was 250 per 100,000 child-years of follow-up and for any clinical pneumonia was over 1500 per 100,000 child-years of follow-up (93), the highest Hib disease incidence ever documented. These data provide intriguing evidence that Hib pneumonia may commonly present with milder symptoms, at least in the circumstances of this clinical trial where intensive case finding and treatment may have led to blunting of natural disease progression. Interestingly, the vaccine preventable incidence of clinical meningitis was 158 per 100,000 child-years of follow-up among children age <2 years, presumably all due to Hib since Hib vaccine is not protective against other etiologies. This occurred despite a low incidence of laboratory-confirmed Hib meningitis, suggesting that previous Asian studies may have also underestimated Hib meningitis incidence.

Finally, a study from urban Dhaka, Bangladesh (10) used a quasi-randomized vaccine distribution followed by a case-control assessment process to evaluate Hib vaccine impact on pneumonia. Similar to results from The Gambia, a reduction of over 20% in pneumonia with a lobar infiltrate was seen in the vaccinated compared to the unvaccinated population.

Non-typeable Haemophilus influenzae

Serotype b is not the only Haemophilus influenzae serotype associated with disease, particularly during the post-Hib conjugate vaccine period. While frequently Hib remains the most common serotype due to incomplete vaccination or vaccine failure, serotype a is also common (183, 217), and as with Hib frequently causes invasive disease and occurs primarily among young children. For example, annual invasive Hia disease incidence among Navajo children age <5 years was 20.2 per 100,000 (174); while this was considerably lower than the pre-vaccine Hib invasive disease incidence in this population, it is as high as pre-vaccine Hib incidence in many settings (280). In the North American Arctic, annual invasive Hia incidences among children age <2 years were 5.7 and 79.1 per 100,000 in Alaska and Northern Canada respectively (152). Almost all of these cases occurred among indigenous children who experienced annual incidences of 20.9 and 101.9 per 100,000 in the two areas. Other sites have reported serotype f as the most common non-b typeable Haemophilus influenzae (242, 269).

Non-encapsulated or non-typeable Haemophilus influenzae also may cause a substantial proportion of invasive Haemophilus influenzae disease (242, 269). These organisms infrequently cause meningitis, presenting more commonly with pneumonia, otitis media, sinusitis, and bacteremia (242).

In theory, an increasing recognition of non-b Haemophilus influenzae disease could be due to a true increase in occurrence related to serotype replacement. However, to date no convincing evidence exists that serotype replacement occurs following Hib conjugate vaccine (149). More likely, Hib conjugate vaccine introduction led to increased emphasis on laboratory surveillance, improvement in laboratory methods, and identification of non-b serotypes.

Case Fatality Ratios

As with sequelae, case fatality ratios will vary substantially depending on the setting. For example, mortality for Hib meningitis in the United States, Europe, the Middle East, and Australia during the pre-vaccine period varied from 0% to 8% (90, 280). In Asia, with a great range in economic development and health care services, mortality varied from 0% (in Hong Kong) to 14% in Taiwan. In South America, mortality varied from 9% to 37% (the latter in rural Brazil). In Africa, mortality was 5% in South Africa, while in four other countries it varied from 16% to 43%. It is worth noting that case fatality ratios in all cases are those for patients who present for care and have a diagnostic test performed. In settings where many patients do not have health care access (such as rural Africa), the case fatality ratios will be even higher than those reported here.

Risk Factors

Documented risk factors usually are those related to increased risk of exposure to the Hib organism and include daycare attendance, household crowding, and exposure to many siblings (5, 203, 252, 262, 286). The second category is increased susceptibility to invasive disease once infected and includes short duration of breastfeeding (203, 238, 252), parental smoking, and frequent non-invasive Hib infections (252). Given the extremely high incidence of disease among some populations, such as Australian Aboriginal and Alaska Native children (90, 280), it is possible that genetic susceptibility also plays a role although this has not been documented to date.

Adult Disease

The incidence of H. influenzae invasive disease in adults was 0.2-1.7/100,000 (63, 75, 78, 196, 235, 236, 243). In the United Kingdom in 1990-95, the commonest clinical presentations in adults were pneumonia with bacteremia (65%) or bacteremia alone (18%) and the highest rates of disease occurred among those age 65-74 years (230). Seventy percent of adults with invasive Hib disease had an underlying predisposing illness and pre-existing disease also was associated with a higher mortality (82% in those with substantial underlying disease disease, compared with 21% in adults without) (230).

CLINICAL MANIFESATIONS

H. influenzae causes invasive and localized diseases. Meningitis and epiglottitis are the most serious consequences of infection and are principally caused by H. influenzaetype b. Meningitis makes up about two thirds of invasive disease. Lower respiratory tract infections, otitis media and sinusitis are usually caused by contiguous spread of non-typeableH. influenzae. There is an overlap between the two disease patterns so that non-typeable H. influenzae occasionally invade, especially in immunocompromised persons, the elderly and neonates; conversely H. influenzae type b account for a minor percentage of localized infection. H. influenzae also causes cellulitis, osteomyelitis, septic arthritis, cholecystitis, endocarditis, pericarditis, acute exacerbations of chronic bronchitis or cystic fibrosis, genitourinary infections and neonatal-maternal sepsis.

In addition to syndromes discussed above, Hib may also causes cellulitis, osteomyelitis, otitis media, sinusitis, endocarditis, pericarditis, and neonatal-maternal sepsis (20, 122, 179, 186, 277). 

Sequelae

Sequelae following Hib meningitis are well known. One of the most common is hearing loss, which has been documented in 3% to 6% of children in developed countries (107, 125, 198). In other countries, such as those in Asia and Africa, the prevalence of documented hearing loss has approached 30% (95, 129, 152, 157, 178, 271,283). Other reported sequelae include seizure disorder (prevalence, 0 to 28%), mental retardation (prevalence 0 to 15%), vision impairment (prevalence, 2% to 5%), and cerebral palsy or major motor deficit (prevalence, 3% to 16%). In almost all cases, sequelae are more common among children from resource-poor settings. This may reflect delay in accessing treatment, sub-optimal case management, the influence of associated conditions (such as HIV infection or malnutrition) or other factors. Sequelae following Hib pneumonia are less well documented but likely include bronchiectasis (240).

LABORATORY DIAGNOSIS

Conventional Methods

To determine disease etiology, blood, CSF or tissue samples are collected from the patient prior to giving antibiotics. Presumptive early diagnosis of meningitis can be made by identifying pleomorphic Gram-negative rods by Gram stain of CSF. Antigen detection kits such as latex agglutination also are useful for confirmation of microscopy findings, particularly in situations where antibiotics were given before collection of CSF, and in remote field situations where microbiology may be difficult to perform. With skilled technicians and well-kept kits, latex agglutination has excellent performance for detection of Haemophilus influenzae, even under difficult conditions (195). Identification of bacteria is obtained by direct culture on Brain Heart Infusion (BHI) agar or in BHI broth. Haemophilus influenzae characterization is accomplished by analysis of colony morphology, standard biochemical tests (143), and X and V factors requirements. The organism has an absolute requirement for two blood components, factors X and V, which have to be supplied, usually as haemin and NAD, respectively. These cofactors are supplied as discs, either X alone, V alone, or X and V together; and the organism will only grow around the X/V disc and between the X and V discs. Speciation is accomplished through assessment of growth on BHI plates without added factors X and V. For capsular serotyping, Haemophilus influenzae isolates are analyzed by slide agglutination using polyvalent Haemophilus influenzae capsular-serotyping sera from commercial sources. Isolates positive for the presence of a polysaccharide capsule are confirmed by agglutination using antisera specific for types a through f.

Antibiotic Susceptibility

Haemophilus influenzae is fastidious, requiring NAD and iron in the form of hemoglobin, hematin, or hemin (258). A variety of susceptibility testing media providing these growth factors have been developed by groups such as the Clinical and Laboratory Standards Institute (CLSI), British Society for Antimicrobial Chemotherapy (BSAC), Comité de l’Antibiogramme de la Socité Française de Microbiologie (SFM), Deutsches Institut Für Normung, and Swedish Reference Group of Antibiotics (77). These media include Haemophilus test medium (HTM); Mueller Hinton broth with 2% lysed horse blood and 15µg of NAD per ml; IsoSensitest broth with 2% lysed horse blood and 15µg of NAD per ml; and IsoSensitest broth-based HTM (132). Antimicrobial susceptibility of Haemophilus influenzae is determined by disc diffusion. The following antibiotics can be tested: ampicillin (2µg and 10 µg), amoxicillin-clavulanic acid (30 µg), cefaclor (30 µg), ceftriaxone (30 µg), chloramphenicol (30 µg), ciprofloxacin (5 µg), clarithromycin (15 µg), moxifloxacin (5µg), and trimethoprim-sulfamethoxazole (25 µg). Detection of ß-lactamase-negative ampicillin-strains can be accomplished by using two concentrations of ampicillin (141).

Haemophilus influenzae has a baseline, wild-type population with a defined, usually narrow, range of intrinsic activity of antimicrobial drug class (260). Determination of susceptibility breakpoints should correlate with clinical outcome to be useful for patient management. Comparison of CLSI (41) BSAC (160) and pharmacokinetic/pharmacodynamic (PK/PD) (49, 131) breakpoints applicable to oral and parenteral agents relevant to H. influenzae are given in Table 1 (258). Susceptibility of worldwide isolates of H. influenzae to 23 antimicrobials is shown in Table 2 (258).

Molecular Laboratory Diagnosis

Molecular methods including multiplex conventional PCR, real-time PCR and others have been shown to be a feasible and useful adjunct for etiologic identification of bacterial meningitis, particularly in areas where community antibiotic use is common. A single multiplex PCR was developed (46, 47) based on Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae, which together are responsible for upwards of 80% of cases of bacterial meningitis in developed and developing countries (74). Nucleic acid amplification methods for the nonculture detection of H. influenzae in clinical specimens are designed to detect the bexA gene (46) as a marker forH. influenzae based on the bexA gene sequence of the Hib Eagan strain (267). The bexA gene encodes the capsulation-associated BexA protein present in all H. influenzaestrains that express one of the six capsular polysaccharides (types a to f). Consequently, PCR assays that target only the bexA gene will not detect invasive disease cases caused by nontypeable H. influenzae (227, 285).

PATHOGENESIS

 Most serious infections, predominantly by encapsulated strains, transpire within a few days of colonization. The invasive route is via the bacterium breaching the epidermis and endothelium, giving rise to bacteremia. Secondary spread to other tissues follows, producing meningitis and rarely other focal infections. Contiguous spread is the predominant route by which non-typeable H. influenzae cause localized disease such as pneumonia, otitis media and sinusitis, and by which H. influenzae type b causes epiglottitis.

SUSCEPTIBILITY IN VITRO AND IN VIVO

The percentage of isolates susceptible to different classes of antimicrobials varies substantially between countries, but some generalizations can be made for each class.

Single Drug Therapy

Ampicillin

Prior to the 1970’s all isolates of H. influenzae were uniformly susceptible to ampicillin or amoxicillin (189-191) but in 1972, H. influenzae containing the TEM-1 ß-lactamase were first recognized in Europe. By 1973, TEM-1 was reported as a cause of treatment failure in H. influenzae meningitis. A second ß-lactamase, ROB-1, emerged later (224) and in 1990 accounted overall for 8% of ampicillin resistance, 13% in North America compared with 0.3% in France (153). ß-Lactamases have now been documented in H. influenzae in virtually all parts of the world and are present in over 95% of ampicillin resistant isolates (139). Respiratory outbreaks of ampicillin resistant non-typeable H. influenzae have been reported (4) (115). Resistance has increased to a level that has curtailed ampicillin’s usefulness (see Table 1) (42, 211). In studies spanning 1986-1993, the prevalence of ampicillin resistance in H. influenzae type b has been twice that in non-typeable H. influenzae, 14-64% in type b versus 8-34% in non-typeable H. influenzae isolates (see Table 1) (62, 105, 139, 161, 210-212). Recently, ß-lactamase expressing H. influenzae accounted for 17-31% of all isolates in North America, 21% in Latin America, and in one global snapshot of the world the prevalence ranged from 6% in China to 32% in Spain (1, 23, 59, 163, 225). The prevalence of resistance in non-typeable H. influenzae from urogenital and neonatal sources is low, an indication that ampicillin is still useful for maternal and neonatal infections (213). Theß-lactamase inhibitors, clavulanate, sulbactam and tazobactam are active against TEM-1 harboring H. influenzae and reduce the minimal inhibitory concentration (MIC) by 8-64 fold. Between 0.1% and 2.6% of all H. influenzae isolates have developed resistance through alterations in penicillin binding proteins, PBP s 3, 4 and 5 (40, 89, 153, 241); most are non-typeable H. influenzae strains isolated from sputum of patients with chronic lung disease (61, 210) but they have also been implicated in meningitis and endocarditis treatment failure (165). They are insensitive to ß-lactamase inhibitors and have a decreased susceptibility to some oral cephalosporins. Further mutations in penicillin binding proteins may lead to third-generation cephalosporin resistance, but has not yet been reported. Extended spectrum β-lactamases that are found in Enterobacteriaceae and confer third-generation cephalosporin and carbapenem resistance have not spread to H. influenzae (85). The in vitro activities of antimicrobials againstH. influenzae have been monitored in the United States, Canada, Europe, Latin America and the Asian-Pacific region as part of the SENTRY antimicrobial surveillance program (118), see Table 1.

Chloramphenicol

Chloramphenicol resistance, mediated by chloramphenicol acetyltransferase in >90% of resistant H. influenzae, emerged in 1976 (61). In North America, the prevalence is <1% (see Table 1) (69, 257) and in much of Europe it is <5%, however in pockets of Europe and Asia resistance has become a major problem, reaching 24.9% in Spain, 10.9% in Belgium, 8 to 36% in Thailand (139, 161). Outbreaks of chloramphenicol resistant H. influenzae type b have occurred (29). Chloramphenicol resistant isolates are often multi-drug resistant. Chloramphenicol is still considered a valuable drug for susceptible H. influenzae (182). Bone marrow suppression is almost always dose-related and reversible, and permanent hematological toxicity is extremely rare.

Cephalosporins

During the 1980’s, as a result of excellent in vitro activity and cerebrospinal fluid penetration, third-generation cephalosporins became the preferred treatment for invasive H. influenzae disease (34). First-generation cephalosporins, such as cefaclor and cephalexin, have limited activity against H. influenzae (see Table 2) (61, 210). Cefaclor resistance has a prevalence of 1.4-5.5% in North America and 2% in Europe. Second- and third-generation oral cephalosporins, cefuroxime axetil, cefamandole, cefixime, ceftibuten and cefpodoxime proxetil have MIC’s <0.5 µg/ml. Resistance is generally less than 2% for cefuroxime and cefamandole, and to newer cephalosporins has not been documented (219). Cefotaxime, ceftazidime, ceftriaxone and moxalactam all have excellent cerebrospinal fluid penetration and are extraordinarily active against H. influenzae (MIC’s mostly <0.1 µg/ml) (83, 133).

Macrolides and Azalides

In vitro susceptibility testing of many antibiotics is confounded by the fastidious growth of H. influenzae. The composition of media, concentration of CO2 during incubation (14, 84) and the bacterial inoculation density all affect results of disc diffusion tests; breakpoints vary between laboratories and discrepancies in reported prevalence of resistance are common (61, 201, 257). Breakpoints for erythromycin range from 0.5 to 4 µg/ml, whereas 8 µg/ml for H. influenzaecultured on Haemophilus Test Media has been proposed by the National Committee for Clinical Laboratory Standards (70). Consequently, 90% erythromycin resistance is reported in Barcelona (139) but only 1.1% in Austria (61). Even where H. influenzae is deemed susceptible to erythromycin, the MIC s of isolates are usually high, at between 2 and 8 µg/ml (average 6.4 µg/ml; see Table 2). H. influenzae appears to have an inherent resistance to erythromycin and should therefore theoretically have a limited role in treatment. Azithromycin and clarithromycin are considerably more active (108, 123, 201, 202), whereas roxithromycin has similar activity to erythromycin (96). Clarithromycin is metabolized to 14-OH-clarithromycin, which is 2-4 fold more active than clarithromycin itself (21), thus in vitro susceptibility testing with only the parent drug inaccurately reflects in vivo sensitivity (15). Clarithromycin and 14-OH-clarithromycin have additive effects against 82% of H. influenzae isolates and are synergistic against 8%. Guidelines for disc diffusion interpretation have been suggested by the National Committee for Clinical Laboratory Standards (137), but do not incorporate the addition of 14-OH-clarithromycin to clarithromycin containing discs, partly because the most appropriate concentration of metabolite and the ratio to parent drug are unknown. An alternative proposal is to increase the clarithromycin breakpoint to take account of the greater in vivo activity (106). In one study, microbroth dilution tests showed azithromycin to have the lowest MIC (0.5-4 mg/L) followed by clarithromycin and roxithromycin (4 mg/L) (50).

Tetracycline and Aminoglycosides

Both groups of antimicrobials are active against H. influenzae and resistance is of low prevalence, but the availability of more efficient and safer drugs has limited the use of tetracycline and aminoglycosides. Tetracycline resistance is a plasmid encoded, energy-dependant process similar to the class B effluxsystem of resistance in Enterobacteriaceae. It is frequently associated with ampicillin and chloramphenicol resistance. Areas of high tetracycline resistance in the world are Spain 25.4%, Belgium 17.8% and Thailand 36% (61). Aminoglycoside resistance in H. influenzae is mediated by drug modification and inactivation or by alteration of the ribosomal target. Aminoglycosides used in broad-spectrum therapy of acute exacerbations of cystic fibrosis are initially effective but H. influenzae becomes resistant swiftly.

Newer Broad Spectrum Antimicrobials

Flouroquinolones, aztreonam, moxalactam and carbapenems are used to treat neutropenic and other highly susceptible patients with infections of unknown aetiology. These agents are all extremely active against H. influenzae, but the organism is an infrequent cause of sepsis in such patients (27). Quinolones are likely to be used with increasing frequency where multi-drug resistance exists. Resistance to quinolones has been reported, but remain very rare and does not at present restrict empiric use of this class of drug (11, 22) (45, 101, 127, 225). In Canadian isolates, fluoroquinolones including gemifloxacin, trovafloxacin, moxifloxacin, grepafloxacin, gatifloxacin, levofloxacin and ciprofloxacin were all highly active against H. influenzae, with MIC (103) < 0.03 mg/L.23 In Japanese isolates, meropenem and cefotaxime were the most potent antimicrobials active against ß-lactamase producing strains (175).

Combination Therapy

Trimethoprim-Sulfamethoxazole

H. influenzae develops resistance through over-production or altered structure of dihydrofolate reductase (60). Trimethoprim-sulfamethoxazole resistance is of low prevalence in North America and the United Kingdom, but has become significant in Spain and Asia (see Table 1) (139), and almost 27% of invasive isolates were resistant in Latin America (59). A polysaccharide capsule has been shown to confer tolerance to trimethoprim-sulfamethoxazole for some H. influenzae type b. That is they have a high MBC of > 4/76 µg/ml but a low MIC of <0.03/0.6 µg/ml. One consequence is that a proportion of H. influenzae type b with virulence potential may persist in the nasopharynx of carriers after trimethoprim-sulfamethoxazole therapy whereas relatively benign non-typeable H. influenzae strains may be eradicated (283).

Rifampicin (+ cephalosporin)

Resistance emerges rapidly in H. influenzae after a four day course of prophylactic rifampicin, nevertheless the community prevalence of resistance is low, <1%, perhaps as a reflection of the seldom use of this drug for treatment. No case of rifampicin invasive disease has yet been documented (244). When rifampicin is used in treatment it is often co-administered with a cephalosporin, and these 2 drugs exhibit synergy against H. influenzae (100).

Erythromycin-Sulfisoxazole

There appears to be little advantage of this combination over that of erythromycin, a drug which alone is of unproven efficacy.

Multiple Drug Resistance

In the late 1970’s, chloramphenicol combined with ampicillin was the treatment of choice for life-threatening H. influenzae infections. Ampicillin and chloramphenicol combined resistant H. influenzae were first recovered in Thailand in 1980. In most areas of Europe and North America, multi-drug resistance has a prevalence of only 0.6%, but the prevalence of multi-drug resistance in Barcelona is 45% in type b isolates and 57% in type b causing meningitis; in Taiwan it is 51% inH. influenzae type b (34, 78). Multi-drug resistance is encoded by at least 3 distinct but similar plasmids and can potentially spread with ease (64, 154).

Summary of In Vitro Sensitivities

Over the course of two decades, the prevalence of ampicillin resistant H. influenzae has increased markedly and is twice as frequent in type b as in non-typeable H. influenzae. Fortunately, resistance to other antimicrobials has not escalated at a comparable rate. Amoxicillin-clavulanate, cefuroxime axetil and cefpodoxime proxetil remain active and rates of resistance to trimethoprim-sulfamethoxazole, chloramphenicol, cefaclor, tetracycline, azithromycin and clarithromycin are generally low, prevalence 1-5%, but with geographical exceptions. Third-generation cephalosporins are uniformly active and a variety of broad-spectrum drugs, which are usually reserved for compromised patients, are highly effective (34). Interpretative criteria for cefaclor, loracarbef, cefprozil and cefixime resistance have recently been revised for testing on Haemophilus Test Medium (68).

Susceptibility In Vivo

Determinants of in vivo susceptibility are drug pharmacokinetics (see section III), the activity of metabolites and post-antibiotic effects. Bactericidal assays using volunteers or patients serum or cerebrospinal fluid have previously been used to confirm that the pathogen deemed susceptible in vitro is indeed killed in vivo, but for H. influenzae infections is little used as a clinical tool. In the development of antimicrobials for H. influenzae infections, animals have been vital in determining drug kinetics and efficacy, for example the rabbit meningitis model (169, 219). Clarithromycin has a greater efficacy than azithromycin in murine H. influenzae pulmonary infections and otitis media in gerbils (202) In the rabbit meningitis model, imipenem treatment of ampicillin/chloramphenicol resistant H. influenzae reduced the bacterial load by only 49% in 24 hours compared with 92% for cefotaxime and ceftriaxone (226). Although interpretation should be cautious, animal models may guide therapeutic choice for newer drugs, but such information contributes little to data derived from drug trials in man. Post-antibiotic effects have been most elegantly demonstrated for azithromycin and clarithromycin by analysis of H. influenzae growth curves after incubation in broth containing antibiotics at concentrations 10 times greater than the MIC, removing the antibiotics by washing, and then allowing growth to restart in broth containing antibiotics at concentrations equivalent to 0.1-0.3 of the MIC (192). Reduced growth rates were observed for up to 19.6 hours after a 2-hour incubation at 10 x MIC followed by 0.3 x MIC. Clarithromycin had a shorter post-antibiotic effect than azithromycin. 14-OH-clarithromycin and desacetyl cefotaxime are active metabolites and consequently the parent compounds have in vivo efficacies greater than expected from bacterial killing studies conductedin vitro.

 

ANTIMICROBIAL THERAPY

General

The antibiotics used for known or presumptive H. influenzae infections can be categorized into several groups:

               i) Those with minimal activity against H. influenzae (erythromycin).

               ii) Active against H. influenzae, but either the pharmacokinetics or side effects are unfavorable and limit their usefulness (aminoglycosides and tetracycline).

               iii) Active against susceptible H. influenzae, but cannot be used where antibiotic resistance prevalence is high (ampicillin, chloramphenicol, tetracycline and trimethoprim-sulfamethoxazole ).

               iv) Highly active agents with good tissue penetration appropriate for therapy of non-invasive infections (second- and third-generation oral cephalosporins, azithromycin andclarithromycin).

               v) Extremely active with reliable cerebrospinal fluid penetration, used in the treatment of invasive disease (third-generation parenteral cephalosporins).

               vi) Oral cephalosporins still being assessed for their full potential in H. influenzae infections (cefpodoxime, proxetil, ceftibuten, cefetamet).

               vii) Newer agents to which H. influenzae are highly susceptible (fluoroquinolones, aztreonam and carbapenems). The reliability of cerebrospinal fluid penetration has often not been fully established.

Special Situations

CNS Infections

Meningitis

H. influenzae meningitis is predominantly caused by H. influenzae type b in children under five years old. Non-typeable H. influenzae is an infrequent meningitis pathogen in neonates, immune compromised persons and where there is a basilar skull fracture or cerebrospinal fluid fistula. Cerebrospinal fluid penetration is a major determinant of drug choice and can be estimated from antibiotic concentrations in volunteers (without meningeal inflammation) and in patients (with inflammation) following parenteral antibiotic administration (see Table 3). Meningeal inflammation markedly affects penetration so that at the start of treatment hydrophilic antibiotics such as β-lactams achieve bactericidal levels in cerebrospinal fluid, but as recovery ensues penetration declines and concentrations fall to around 0.5-2% of peaks in serum (219). Lipophilic drugs such as chloramphenicol and rifampicin enter the cerebrospinal fluid adequately even in the absence of inflammation. Serum protein binding inversely correlates with meningeal penetration, and although cefotaxime is highly protein bound it achieves sufficient cerebrospinal fluid levels for therapeutic success (18, 219). As the cerebrospinal fluid is physiologically hidden from the immune system and bacterial killing is impaired, a bactericidal antimicrobial is preferred to a bacteriostatic one. The speed of cerebrospinal fluid sterilization is proportionate to the neurological sequelae rate (54) and microbial killing is maximal when antibiotic concentrations are 10-20 fold higher than the MBC (see Tables 2 and 3).

Clavulanate and other ß-lactamase inhibitors reach therapeutic levels in cerebrospinal fluid and are active against most ampicillin resistant H. influenzae type b. Chloramphenicol has outstanding cerebrospinal fluid penetration; however, geographical variations in resistance, largely unjustified apprehension about side effects ofchloramphenicol and the perceived comparative safety of cephalosporins have led to the declining use of chloramphenicol. Third-generation cephalosporins are extremely active against H. influenzae causing meningitis with achievable cerebrospinal fluid concentrations being 100- to several 1000- fold greater than the MIC (28, 37). They are the preferred antibiotics for meningitis treatment (17, 18, 32, 133, 167). Cefotaxime, ceftriaxone or ceftazidime treatment of meningitis, compared with chloramphenicol, may result in fewer neurological sequelae (197), and are at least as effective as the combination of chloramphenicol and ampicillin (6, 111, 134, 193, 220). Ampicillin and chloramphenicol may be considered preferable in countries were the per capita expenditure on health care is low (255). Especially as in many such countries, even where the individual prevalence of ampicillin resistance or chloramphenicol resistance is high, the prevalence of combined ampicillin/chloramphenicol resistance is often low (197).

Initial studies of cefuroxime in meningitis indicated an equivalent efficacy to cefotaxime or ceftriaxone, but delayed cerebrospinal fluid sterilization is more frequent with cefuroxime (9% versus 0% at 24 hours, or 12% vs. 2% at 18-36 hours), with reports of cerebrospinal fluid cultures remaining positive until the 5th day of treatment, and breakthrough or secondary bacteremia, mastoiditis and epiglottitis occurring between 10 and 17 days after the start of therapy (151, 171, 233). Hearing impairment is more common after cefuroxime therapy (18% vs. 11%, or 17% vs. 4%) and third-generation cephalosporins have been found to be consistently better (16, 274, 281). Oral cephalosporins are considered inadequate therapy for meningitis because cases of breakthrough meningitis have been reported in children receiving cephalexin, cefaclor and cephamandole for otitis media (61, 215).  However, the role of oral cefixime in meningitis needs further evaluation (188).

Carbapenems and aztreonam penetrate into the cerebrospinal fluid, but levels may be unreliable (170). Imipenem-cilastatin, but not meropenem, result in more convulsions during meningitis therapy when compared with third-generation cephalosporins (145, 146, 278). Fluoroquinolones have a potential future role in H. influenzaemeningitis, they penetrate cerebrospinal fluid well and limited reports indicate that they are efficacious in meningitis. Perfloxacin and ciprofloxacin have been most studied (176). Aztreonam or fluoroquinolones are being considered as alternative therapies for H. influenzae meningitis (219).

Current dosage recommendations, based on measured serum and cerebrospinal fluid concentrations, rate of bacterial killing in cerebrospinal fluid and drug side effects, are given in Table 4.

Cerebrospinal Fluid Shunt Infections

H. influenzae rarely causes meningitis in patients with cerebrospinal fluid shunts. In one analysis of 27 cases, 22 were caused by H. influenzae type b and 5 by non-typeable H. influenzae (276). Ronan found that most pathogens infecting cerebrospinal fluid shunts did so within 30 weeks of the last neurosurgical operation, whereas H. influenzae was notable in that infections occurred later, at 33 weeks to 4 years after surgery, indicating that shunt infection by H. influenzae is usually blood borne, as with meningitis in children without shunts (222). Meningitis in patients with cerebrospinal fluid shunts who have not had recent surgery should be treated with antimicrobials that cover H. influenzae. The treatment of any shunt infection often necessitates removal of the shunt with a period of externalization before replacement. However, apparent successful treatment of some H. influenzae infections with antibiotics alone have been reported, including treatment with a combination of rifampicin and ceftriaxone (116, 117, 222) and with cefotaxime followed by ampicillin  (276). A large multi-centre study would be necessary to determine whether H. influenzae shunt infections are particularly amenable to therapy without surgical intervention.

Respiratory Tract Infections

Lower Respiratory Tract Infections

Pneumonia has been estimated to cause over three million deaths in children younger than five years of age in developing countries (261). In industrialized countries, the incidence of pneumonia is around 40/1000 pre-school children and 9/1000 in 9-15 year olds. Worldwide, H. influenzae is the second commonest bacterial pathogen in community acquired pneumonia (237). Non-typeable H. influenzae strains cause most H. influenzae pneumonia, especially in the elderly and in those with chronic respiratory diseases such as acute exacerbations of chronic bronchitis, cystic fibrosis and bronchiectasis. H. influenzae type b accounts for less than 2% of bacterial pneumonia in hospitalized patients, and mainly occurs in young children. H. influenzae is one of five bacteria that most frequently cause acute bronchitis, although the commonest aetiology is viral. Seventy percent of parapneumonic effusions occur in children under 2 years old and, in children between 7 and 24 months old, H. influenzaeis the commonest pathogen. Ninety percent are H. influenzae type b and 10% are non-typeable H. influenzae (82). Effusions caused by H. influenzae type b are particularly likely to progress to empyema.

Empiric therapeutic regimens for community treated pneumonia are directed at H. influenzae and Streptococcus pneumoniae (+ atypical bacteria) and include either a second-generation cephalosporin (+ erythromycin), amoxicillin plus a ß-lactamase inhibitor or one of the newer macrolides, azithromycin or clarithromycin. Second-generation cephalosporins are often replaced by third-generation for treatment in hospitalized patients.

Concentrations of antibiotics considered representative of those in the lower respiratory tract during infection are measured in mucosal biopsies obtained during bronchoscopy (102), in bronchial secretions obtained during intubation (36), and in lung tissues and tonsillar tissue procured during thoracic surgery and tonsillectomy (81,166), see Table 3.

With the rise in prevalence of ß-lactam resistance (67, 69) and reports of ampicillin resistant outbreaks (115), the usefulness of ampicillin or amoxicillin has been curtailed. Amoxicillin/clavulanate or amoxicillin/sulbactam (52), show good distribution into the lung and remain useful in the treatment of lower respiratory tract infections, with a >80% satisfactory outcome for treatment of a range of pathogens (110, 130). Ampicillin resistant non-typeable H. influenzae that have altered penicillin binding proteins are almost all obtained from lower respiratory tract infections, but at present the prevalence of H. influenzae with this mechanism of resistance is insufficiently high to alter therapeutic choice. Reports of significant resistance to trimethoprim-sulfamethoxazole and tetracycline in S. pneumoniae limit the use of these drugs for empiric treatment of lower respiratory tract infection, even where H. influenzae is the suspected pathogen. The prevalence of resistance to these drugs amongst respiratory non-typeable H. influenzae isolates remains low (65).

Oral first-generation cephalosporins, cephalexin, cephradine and cefadroxil, were all previously considered useful for lower respiratory tract infection therapy, but have now been replaced by cephalosporins with increased activity against H. influenzae and improved pharmacokinetics.

The second-generation cephalosporins, cefuroxime, cefuroxime axetil and cephamandole are insensitive to the TEM-1 ß-lactamase. Cefaclor has limited use because of protracted skin reactions, especially in children, although it has indistinguishable clinical efficacy from many other antibiotics (80, 256), but when compared with azithromycin the bacteriological cure rate was only 61% versus 93.5% (56). Cefaclor and cefuroxime have been shown to produce clinical cure/improvement in 93-95% of adults with pneumonia or with acute exacerbations of chronic bronchitis and are as efficient as amoxicillin (44), though 1 of 10 patients with proven H. influenzae lower respiratory tract infection treated with cefuroxime and 2 of 5 receiving cefaclor are not bacteriologically cured.

Oral third-generation cephalosporins, cefixime, ceftibuten, cefpodoxime proxetil are licensed for treatment of lower respiratory tract infections but the use of such broad spectrum antimicrobials should be reserved where narrower spectrum agents are available. Cefixime has a long half-life, 3 hours compared with 0.5 hours for cefaclor (164), and can be given once daily for lower respiratory tract infections. Cefixime does not penetrate well into sputum, but reaches useful levels in bronchial mucosa (see Table 5). In comparative trials, cefixime had a similar efficacy to amoxicillin/clavulanate, cefaclor, cephalexin, cefuroxime axetil and clarithromycin, and in non-comparative studies had a high clinical cure/improvement rate for lower respiratory tract infections, up to 96% in a large multicenter study of acute bronchitis or acute exacerbations of chronic bronchitis in adults, especially where the pathogen was H. influenzae (164, 191). Cefixime may have a specific role in the future when the antibiotic is switched from intravenous to oral administration, where broad spectrum antibiotic cover has been initiated empirically with a third-generation cephalosporin. Ceftibuten has less activity against S. pneumoniae, limiting its role as an empiric agent, whereas cefpodoxime proxetil is the most active of this group against all respiratory tract pathogens. Cure/improvement rates of cephalosporins for lower respiratory tract infections, and acute exacerbations of chronic bronchitis are similar to rates achieved with conventional antibiotics, except for first-generation cephalosporins. Cefpodoxime has a similar efficacy to conventional antibiotics and to parenteral cephalosporins, with 90-100% satisfactory clinical response rates in lower respiratory tract infections, 80% cure in acute exacerbations of chronic bronchitis and 91-100% in community acquired pneumonia where H. influenzae is the second commonest pathogen (88).

Third-generation parenteral cephalosporins, cefotaxime, ceftriaxone, ceftazidime, ceftizoxime and others are used for severe and life-threatening pneumonia. H. influenzae is extremely sensitive to these agents, as are most community acquired respiratory pathogens, and achievable lung concentrations are high, well above the MIC90 ofH. influenzae and remains so for the duration of the dosing interval. Ceftriaxone has the advantage of once daily administration. Cefpirome is reserved for severe sepsis and the treatment of complicated lower respiratory tract infection.

Azithromycin and clarithromycin have proven efficacy in upper and lower respiratory tract infections and can be considered as alternatives to amoxicillin/clavulanate, erythromycin plus a sulphonamide, or a cephalosporin (103, 201, 202). They attain particularly high tissue and intracellular concentrations. Azithromycin tissue concentrations can be up to 100 fold higher than serum levels. The tissue half-life is four days, and levels remain above the MIC90 for up to 10 days after a 3 day course (81, 158). Clinical cure/improvement rates for lower respiratory tract infection range from 90-100% for azithromycin and clarithromycin (56, 80, 124, 173, 187,191, 201, 202). Bacteriological cure rates in the same studies were 72-100% (108). The efficacy of both was at least comparable to amoxicillin/clavulanate, cefaclor, cefixime and cefuroxime (55, 80, 120, 173, 191, 201, 202, 234). The development of azithromycin resistance has been shown to contribute to disease recurrence one month after treatment of chronic bronchitis (202).

Fluoroquinolones are characterized by their broad antibacterial spectrum, with excellent activity against H. influenzae, and rapid bactericidal action. Particularlyciprofloxacin, but also sparfloxacin (120), and norfloxacin (136), have been extensively prescribed in lower respiratory tract infection. For H. influenzae, cure rates are greater than 90% (79). In cystic fibrosis, fluoroquinolones are used to treat acute exacerbations (122) or used in maintenance treatment (119), but are primarily directed at Pseudomonas superinfection. H. influenzae, particularly non-typeable H. influenzae, is a common pathogen in young cystic fibrosis patients (177). Non-typeable H. influenzaeare rarely cultured consecutively from the same cystic fibrosis patient for more than a few months and in the majority are rapidly cleared from sputum with conventional therapy, such as amoxicillin/clavulanate (181). In the unusual circumstance of persistent H. influenzae infections, fluoroquinolones would be appropriate antimicrobials to use. Although ciprofloxacin resistance in chronic lung disease has been reported (13). General guidelines for the treatment of infective exacerbations in cystic fibrosis are to make a microbiological diagnosis where possible before initiating treatment, use high-dose bactericidal antibiotics for 14 days, avoid prophylactic regimens (except for maintenance therapy in chronic Pseudomonas infections) (119), be aware of altered pharmacokinetics in cystic fibrosis patients, especially increased total body clearance of ß-lactams - hence shorter half-life and lower sputum concentrations, and aim to eradicate H. influenzae in the lower respiratory tract whenever present (121, 122).

Parapneumonic Effusion/Empyema

The presence of an effusion demands a diagnostic thoracentesis and an empyema requires drainage; in one large retrospective study of 227 patients, 21 of 40 empyemas caused by H. influenzae were deemed too small for closed drainage and were treated by needle aspiration. Antibiotics were given for a median duration of 13 days (82). Antibiotic choice should be the same as for invasive H. influenzae type b disease, usually a parenteral cephalosporin. Therapeutic doses ofcefuroxime, cefotaxime and ceftriaxone all give rise to pleural fluid concentrations of up to 7 µg/ml (144), well above the MIC for H. influenzae.

Acute Exacerbations of Chronic Bronchitis

Greater than 50% of bacterial isolates from the sputum of patients with chronic bronchitis are non-typeable H. influenzae. Antibiotics are given to treat acute exacerbations of chronic bronchitis when they occur, or given as long term prophylaxis, or as suppressive therapy in advanced disease. The benefit of antibiotics has been extremely difficult to demonstrate conclusively, partly because many acute exacerbations of chronic bronchitis are viral in origin, because patients enrolled into trials are at very different stages of disease and because improvement often occurs without antibiotics (186). Consequently, only marginal differences between antibiotics are demonstrated in comparative studies. There is little solid evidence on which to make a choice between amoxicillin, amoxicillin/clavulanate,trimethoprim-sulfamethoxazole, tetracycline, erythromycin, chloramphenicol and cefaclor or other oral cephalosporin. Sub-optimal pharmacokinetics in the lungs of patients with chronic bronchitis and higher antibiotic resistance rates may also contribute to the poor responses. Newer antibiotics with excellent in vitro activity against H. influenzaeshould be more effective, but this is seldom demonstrated (7). Because of the difficulty in interpreting comparative trials, amoxicillin alone is still recommended by some for acute exacerbations of chronic bronchitis. A more logical choice would be amoxicillin/clavulanate or a cephalosporin. Recommended dosages of antibiotics for H. influenzaepneumonia are given in Table 5.

Otitis Media and Sinusitis

H. influenzae is aetiological in 10-30% of acute otitis media compared with S. pneumoniae in 20-40%.11 In chronic otitis media, H. influenzaepredominates, i.e., 15% vs. 7% of S. pneumoniae, and the rate of ß-lactamase producing H. influenzae is up to three times higher than in acute otitis media. More than 90% ofH. influenzae are unencapsulated, and 15-30% of them produce ß-lactamases; only a minor percentage are H. influenzae type b (186). H. influenzae is a primary pathogen in the now rare but serious intracranial complications of chronic otitis media, mastoiditis and subdural empyema. Sinusitis has a comparable pathogenesis and microbiology and is treated similarly (186). Prevention of otitis media caused by H. influenzae will best be achieved by vaccination, but this will only be possible once a non-typeable H. influenzae vaccine becomes available (209, 246).

Ampicillin resistant H. influenzae are widely prevalent. Although it would seem logical to abandon amoxicillin in preference to other antimicrobials, for many physicians amoxicillin remains the first choice for treatment of acute otitis media (144). One recommendation is to use alternatives to amoxicillin only if two treatment failures occur in any one winter season. It is difficult to establish that amoxicillin has a reduced efficacy because so many bouts of acute otitis media are caused by viruses and even where H. influenzae or other bacteria are recovered by tympanocentesis, spontaneous resolution of the acute otitis media is common. Comparison of drugs is impeded by lack of a universal definition of otitis media, difficulty in making bacteriological diagnoses and often no clear outcome measures within studies. The penetration of amoxicillin and alternative antibiotics into middle ear effusions is given in Table 3 (159). Clinical comparative studies in the treatment of otitis media have identified little differences between amoxicillin, amoxicillin/clavulanate (72, 162, 194), trimethoprim-sulfamethoxazole (123, 194, 196), erythromycin-sulfisoxazole (148, 221), cefaclor (162, 194) cefixime (30),cefprozil, cefuroxime, cefpodoxime, clarithromycin and azithromycin (43, 199, 201). In general, the numbers of patients in trials of otitis media therapy are too small to assess equivalence of two agents, this often requiring several times the number of patients as is necessary to demonstrate a difference between drugs. Thus, most studies conclude that a difference between two antibiotics cannot be shown, but do not predict with confidence that two antibiotics are equally efficacious (144). The situation is exemplified by the continued use of erythromycin. H. influenzae has inherent resistance to erythromycin in vitro and erythromycin penetrates less well than most other antibiotics into middle ear effusions and yet trials do not show it to be less effective (148, 221). Comparative trials of maxillary sinusitis treatment similarly show little difference between antibiotics (35,140, 250). The clinicians choice of antibiotic in otitis media therefore rests on knowledge of each drugs pharmacokinetics and H. influenzae (and other pathogens) sensitivity to the various antibiotics (Tables 2 and 3), as well as palatability and administration frequency. Some general observations are that cefaclor appears to have a lower efficacy than cefuroxime axetil, cefixime or amoxicillin/clavulanate. Cefixime has exceptional stability against H. influenzae β-lactamases. Drugs with questionable efficacy: erythromycin, cefaclor and possibly amoxicillin-alone, should perhaps be avoided in preference to those with predicted better in vivo efficacy: amoxicillin/clavulanate, cefuroxime axetil, cefixime, cefprozil, ceftibuten and cefpodoxime proxetil.

Table 5 gives some recommended dosages of antibiotics for otitis media.

Other Infections

Hib is one of the principal causes of bacteraemic infection in young children; in addition to meningitis H. influenzae type b may cause orbital cellulitis and epiglottitis (99), vertebral osteomyelitis (19), septic arthritis, psoas abscess (57), rarely septicemia and very rarely endocarditis. If H. influenzae type b is the pathogen in apparently localized disease, there is a high likelihood of systemic spread and another concurring focus. For instance in H. influenzae type b infections, 75% of patients with pneumonia have bacteremia and 15% have meningitis; 30% with empyema have meningitis; and 50% with infective endocarditis have pneumonia or meningitis (82). non-typeable H. influenzae cause mainly localized disease, otitis media, sinusitis, acute tracheobronchitis and conjunctivitis (186). Occasionally, non-typeable H. influenzae are implicated in invasive disease one of the most serious manifestations of which is Brazilian purpuric fever, a rare fulminant septicemic illness with a case-fatality rate of more than 60% (109,249).

Non-typeable H. influenzae, especially biotype IV, cause invasive neonatal-maternal infections (33, 277) with endometritis (213) leading to bacteremia in the mother and fulminant septicemia and meningitis in the premature neonate (156, 186). Localized disease has been reported in the genitourinary tract, presenting as urinary tract infection (71, 179, 213, 268), urethritis (248), orchidoepididymitis, Bartholin abscess (265), cervicovaginitis, salpingitis (97) and spontaneous abortion (213). Meningitis can be caused by non-typeable H. influenzae, especially where there is a breach of the meninges, either after trauma (247), or due to congenital sinuses (214). Whether it is non-typeable H. influenzae or type b that cause infection, the treatment for invasive disease is as for meningitis, usually with a third-generation cephalosporin. Cefotaxime penetrates well into bone (73), and joints, with joint fluid concentrations at two hours after administration greater than in serum (for instance levels of 29 µg/ml), and penetrates into skin (2-6.3 µg/ml), and the genitourinary tract. Bone and joint infections require 3-6 weeks of therapy (245). Ceftriaxone penetrates into bone efficiently and has been given once daily, and once daily in the community after resolution of the acute stages of disease (28, 53).

ENDPOINTS FOR MONITORING THERAPY

A 7 day course of parenteral antibiotics is satisfactory for most cases of uncomplicated H. influenzae meningitis (167, 171), but conventionally 10 days has been recommended. There are no long-term comparisons of sequelae rates for 10 day, 7 day or shorter regimens (171). It is prudent to continue antibiotics for 7-10 days depending on severity, or for 3-5 days after effervescence of fever and signs of meningitis. A repeat lumbar puncture is considered unnecessary, except where pyrexia and clinical state have not improved after 48 hours or more of therapy.

Duration of therapy for lower respiratory tract infection is dictated by disease severity. As a guide, 7 or more days, or 3-5 days after pyrexia has receded, is appropriate for hospital acquired pneumonia. Intravenous therapy can be switched to oral after there has been no fever for 24-48 hours. For oral therapy of community acquired pneumonia, 5-7 days is usually satisfactory, though a 3 day course of a newer macrolide/azalide may also be adequate.

A 10 day course of antibiotics for acute otitis media has become the standard duration of therapy, based on the necessity to treat Group A streptococcal pharyngitis for this duration. However, a single intramuscular dose of ceftriaxone, which produces therapeutic levels for 3 days, or 3 days of azithromycin or 5 days of cefaclor have been shown to be as effective as 10 days of amoxicillin or amoxicillin/clavulanate (199). The 10 day rationale has not been adopted in Europe where 5-7 days is considered sufficient for uncomplicated acute otitis media. Adenoidectomy, antihistamines and decongestants are of unproven benefit in chronic otitis media, and the appropriate time for myringotomy with ventilation tube insertion is an area of contention. The effectiveness of treatment is assessed by the presence of otoscopic findings, pyrexia and symptoms at 48 hours and by repeat otoscopy at the end of therapy.

Resolution of symptoms and pyrexia at around 48 hours is a useful guide to the effectiveness of therapy for most H. influenzae infections.

VACCINES

Vaccine impact One of the most successful chapters in the history of vaccinology has been the development of the H. influenzae type b vaccine and its introduction into North America, Finland and the United Kingdom (112, 235) from 1989 to 1992, and then subsequently into many other European countries and Australia (26, 114) followed by South America (168, 218), Africa (2, 48, 58, 155, 168, 184, 216), and most recently Asia. Currently the World Health Organization recommends that all countries introduce Hib conjugate vaccine into routine infant immunization programs (279). Prior to vaccination, the weighted worldwide incidence of invasive Hib disease (excluding pneumonia) in patients <5 years old was estimated to be 71/100,000, indicating 357,000-445,000 cases/year worldwide including 108,500 deaths. If pneumonia is included, these figures amount to 2.2 million cases and 520,000 deaths from Hib (and this may be a conservative estimate) (196). In over a decade of use in the United States, Hib has almost been eradicated (223). In the world as a whole, the global situation has dramatically improved, but much of the world’s population, primarily children living in Asia, remain unvaccinated. Vaccines against non-typeable H. influenzae remain in the development phase (209).

Indications

H. influenzae type b vaccine should be considered for the prevention of invasive disease in any infant and the World Health Organization has now called for routine infant immunization of all children in the world (279). Conjugate vaccines consist of the polyribosyl phosphate (PRP) capsule derived from H. influenzae type b covalently attached to a protein carrier. Four commercial conjugate vaccines are available: Act-Hib (Sanofi Aventis) is formulated from native PRP linked to tetanus toxoid (PRP-T); HibTITER (Wyeth) is a PRP oligosaccharide connected to CRM (241), a mutant diphtheria toxoid; PedvaxHIB (Merck) is native PRP attached to the outer membrane protein complex of serogroup b Neisseria meningitidis (PRP-OMP); and ProHIBit is composed of sized PRP linked to the diphtheria toxoid (PRP-D) (147). All have been extensively evaluated in trials in man (62, 76, 228, 229, 263) and the reduction in invasive Hib disease as well as carriage has been impressive (11, 12). PRP-T in the UK has a protective efficacy in infants of over 98%, and is protective after just two doses (25). PRP-D appears to be the least immunogenic and though efficacious in Finland it was not protective in Alaska Native children, perhaps as a result of a younger age of presentation in Alaska Native infants. PRP-OMP is the most immunogenic after just one dose of vaccine and may be the best choice for immunization in the developing world. Nevertheless, at least within several years of introduction and at a national level, all currently marketed formulations have been successful in dramatically reducing Hib incidence even in rural and impoverished settings with high pre-vaccine incidences such as those seen in Africa (2, 48, 58, 155, 168, 184, 216).

While vaccine impact against invasive Hib disease is well documented, impact against the primary Hib syndrome, that is, non-invasive pneumonia, is less well understood. As indicated above, in developing country contexts, Hib vaccine has been shown to reduce severe or hospitalized pneumonia by approximately 5%, corresponding to incidences of approximately 250 per 100,000 children age less than 2 years per year (93, 196). What is not known is what percentage of all non-invasive Hib pneumonias this represents, and thus what the overall vaccine impact is against this outcome.

Doses and Schedules

Single doses of PRP-T, -OMP and -D contain 10ug, 15ug and 20ug or PRP, respectively. HbOC consists of 10ug of oligosaccharide. Each dose is administered in a volume of 0.5ml, given intramuscularly. Primary courses of 3 doses are given in the first few months of life, such as in 2, 3, 4 month; 2, 4, 6 month; or 3, 4, 5, month schedules. In North America and other developed countries, a booster dose is recommended at 12-15 months of age. This booster dose was implemented because of concerns regarding waning immunity. A natural experiment was carried out in the United Kingdom where no booster dose was provided. Initially, this schedule seemed to work (25). Subsequently, though, older children and adolescents experienced a substantial increase in invasive disease, probably because of a combination of waning immunity and lack of natural boosting because of the Hib conjugate vaccine’s dramatic reduction in Hib carriage (113, 172). This effect may start occurring in developing countries as well, as has been noted recently in The Gambia (126). One solution to this may be a single dose of vaccine during the second year of life, but this would have to be carefully evaluated, particularly in countries that have traditionally had a substantial disease burden among young infants. Another option would be to provide two doses during the first 14 weeks of life followed by a booster dose at 9 to 12 months along with measles (or MMR). Either of these changes would require rethinking the entire Expanded Program on Immunizations schedule.

Under most circumstances, the various forms of the conjugate vaccine can be used interchangeably (31). However, when carriage prevalence and disease burden are high, and disease is more concentrated among young infants, PRP-OMP may lead to more substantial and sustained disease reduction, due to its ability to elicit protective antibody levels after a single dose (62). This effect has been seen on a population basis among Alaska Native children, who experienced a marked increase in disease burden when PRP-OMP was replaced by HbOC (239).

Adverse Effects

No association with major adverse events have been reported in the first decade of use. Hib disease may still occur in partially vaccinated infants and in children with immune deficiencies. Vaccine failures should be considered for immunological investigations.

Vaccine Impact

The introduction of new vaccines had altered the epidemiology of meningitis over the last decades, mostly due to the introduction of the Hib vaccine conjugate(196,235). Immunization has almost eradicated invasive type b disease in those countries that have implemented national vaccination programs (196). In The Netherlands, where vaccination was introduced in 1993, the incidence of invasive Hib disease among children younger than 5 years of age dropped from 28.7 per 100,000 in 1992 to 0.8 in 2002 (266). Similar reductions of Hib disease have been observed in most other countries where vaccination has been implemented in the national vaccination programs and has led to a near eradication of invasive Hib disease among children whether in developed (86, 128, 259, 264) or developing country settings (2, 48, 58, 155, 168, 184, 216, 218).

Economic studies, including cost-effectiveness analyses, provide another measure of vaccine impact. In developed countries, and even at prices much higher than those that exist currently, Hib vaccine was cost-saving and thus was an obvious intervention (87, 208, 284). In developing countries, however, vaccine was cost-effective but not cost-saving, even at the greatly reduced prices available through UNICEF currently (3, 92). In developed countries, cost savings were accrued largely because of the long-term medical and institutional care costs resulting from Hib meningitis survivors with long-term sequelae such as hearing loss, cerebral palsy, and mental retardation. Services for these outcomes in developing countries, however, are minimal or non-existent and thus no costs are saved. Even direct medical costs are relatively low and thus outweighed by vaccine costs. As countries develop and provide greater services, Hib vaccine will become more cost effective even without a reduction in vaccine price.

 

PREVENTION

Rifampicin Prophylaxis

The risk of H. influenzae type b invasive disease in an unvaccinated household contact is increased 500-800 fold compared with the general population and is greatest in infants under 2 years old (51). Rifampicin prophylaxis is recommended for all household contacts of a case of H. influenzae type b meningitis if there is an infant of <2 years old or an immune compromised person in the house (200). Where necessary all children should have their vaccinations brought up to date (135). Rifampicin eradicates H. influenzae from the nasopharynx in 95% of carriers and is given on the presumption that clearing carriage prevents infection. Only one prospective study however, has clearly shown a benefit (8, 51). In countries that have introduced H. influenzae vaccination programs, the benefits may be marginal. Ceftriaxone clears nasopharyngeal carriage and when used for treatment, eliminates the need for rifampicin prophylaxis in the index case. Where a single child has meningitis in a day care centre the role of prophylaxis is controversial, one proposal is to give rifampicin to all unvaccinated children under two years old if contact has been for 25 hours or more per week or if a second case arises within 60 days (200).

 

CONTROVERSIES

Dexamethasone

For H. influenzae meningitis, it is generally agreed that dexamethasone therapy significantly reduces the incidence of sensorineural deafness (231). Although a 1989 meta-analysis of all studies concluded that there was no benefit, scrutiny of selected studies from Dallas and Switzerland did show an advantage (232). In a large prospective study, the incidence of hearing loss was 3.3% in dexamethasone recipients and 15.5% in controls, whereas the recent meta-analysis determined the incidence of neurological or audiological sequelae in steroid and non-steroid groups as 7% vs. 16%, respectively. Routine use of empirical dexamethasone for pediatric meningitis remains controversial because a clear benefit in S. pneumoniae and N. meningitidis meningitis has not been established. In many countries, H. influenzae type b has been virtually eradicated by implementation of national vaccination so that administration of steroids to meningitis patients before microbial diagnosis is likely to be less beneficial than in non-vaccinating countries where H. influenzae still predominates (270). Steroids must be commenced prior to the first dose of antibiotic (24). Cerebrospinal fluid parameters return to normal faster with dexamethasone treatment, yet dexamethasone does not affect cerebrospinal fluid sterilization or cephalosporin concentrations (232). Dexamethasone therapy has been associated with rebound fever and carries a risk of significant bleeding in the absence of coagulopathy of 0.5% (204, 232, 270). Dexamethasone 0.15 mk/kg 6 hourly for 4 days is the standard regimen; some consider a 2 day course as effective and two days may be associated with less secondary fever (204, 251).

Vaccine and Immunization Issues

As discussed above, the optimal vaccine schedule has not been identified although it is clear that a schedule limited to the first half of infancy is not adequate to eliminate disease. Vaccine cost also remains a significant impediment to worldwide Hib vaccine introduction. While most countries in the world have now introduced vaccine into routine infant immunization programs, many resource-poor countries have done so with support from the Global Alliance for Vaccines and Immunizations and it is not clear that they will continue vaccinating when this support ends.

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Table 1: Breakpoints Used to Determine Susceptible, Intermediate, and Resistant Categories for H. influenzae Based on PK/PD, BSAC, and CLSI Interpretative Breakpoints.

Antimicrobial	Breakpoint (µg/ml) based ona
	PK/PD		BSAC			CLSI
	S	R	S	I	R	S	I	R
Parenteral agents
Ampicillin	£2	³4	£1	--	³2	£1	2	³4b
Ampicillin-sulbactan	£2	³4	--	--	--	£2	--	³2
Piperacillin-tazobactan	£8	³16	--	--	--	£1	--	³2
Cefuroxime sodium	£4	³4	³4	--	³2	£4	8	³16
Cefotaxime	£2	³4	£1	--	³2	£2	--	--
Ceftriaxone	£2	³4	£1	--	³2	£2	--	--
Cefepime	£4	³8	--	--	--	£2	--	--
Ceftazidime	£8	³16	£2	--	³4	£2	--	--
Meropenem	£4	³8	£4	--	³8	£0.5	--	--
Imipenem	£4	³8	£4	--	³8	£4	--	--
Ertapenem	£1	³2	£2	--	³4	£0.5	--	--
Parenteral and oral agents
Erythromycin	£0.25	³0.5	£0,5	1-8	³16	--	--	--
Clarithromycin	£0.25	³0.5	£0.5	1-16	³32	£8	16	³32
Azithromycin	£0.12	³0.25	£0.25	0.5-4	³8	£4	--	--
Doxycyclin	£0.25	³0.5	--	--	--	--	--	--
Trimethoprim-sulfamethoxazolee	£0.5/9.5	³1/19	£1.6/30.4	--	³32-60.8	£0.5/9.5	1/19-2/38	³4/76
Ciprofloxacin	£1	³2	£0.5	--	³1	£1	--	--
Ofloxacin	£2	³4	£0.5	--	³1	£2	--	--
Gemifloxacin	£0.25	³0.5	£0.25	--	³0.5	--	--	--
Levofloxacin	£2	³4	£1	--	³2	£2	--	--
Gatifloxacin	£1	³2	£1	--	³2	£1	--	--
Moxifloxacin	£1	³2	£0,5	--	³1	£1	--	--
Rifampicin	ND	ND	--	--	--	£1	2	³4
Chloramphenicol	£4	³8	£2	--	³4	£2	4	³8
Oral Agents
Amoxicillin (1,5g/d; 45 mg/kg/d	£2	³4	£1	--	³2	--	--	--
Amoxicillin (3-4g/d; 90 mg/kg/d	£4c	³8c	--	--	--	--	--	--
Amoxicillin-clavulanate (1,5g-250mg/d; 45-6,4mg/kg/d	£2c	³4c	£1	--	³2	£4	--	³8d
Amoxicillin-clavulanate (4g-6,4mg/d; 45mg/kg/d	£4c	³8c	--	--	--	--	--	--
Cafaclor	£0.5	³1	£1	--	³2	£8	16	³32
Cefuroxime axetil	£1	³2	£1	--	³2	£4	8	³16
Cefixime	£1	³2	--	--	--	£1	--	--
Cefprozil	£1	³2	--	--	--	£8	16	³32
Cefdinir	£0.5	³1	--	--	--	£1	--	--
Cefpodoxime	£0.5	³1	--	--	--	£2	--	--
Telithromycin	£0.5f	³1f	£0.5	1-2	³4	£4	8	³16
Tetracycline	£2g	³4g	£1	--	³2	£2	4	³8

^aS, susceptible; I, intermediate;; R, resistant; ND, Not defined; --, no breakpoint available

^bCLSI breakpoint used to define beta-lactamase negative ampicillin susceptible (BLNAS)

^cBreakpoints are expressed as amoxicillin component; testing breakpoints are expressed as amoxicillin component; testing was performed using 2:1 ratio of amoxicillin/clavulanic acid

^dBreakpoints used to defined beta-lactamase positive amoxicillin-clavulanate-resistant (BLPACR)

^eBreakpoints are expressed as trimethoprim component;  testing was performed using a 1:19 ratio of trimethoprim/sulfamethoxqazole.

^fLimited information is currently available to determine PK/PD breakpoints

^gThe microbiological breakpoint was used in absence of PK/PD studies (26, 29, 68, 102).

Table 2: Susceptibility of Worldwide Isolates of H. influenzae (n=8,523) to 23 Antimicrobials and MIC₅₀s and MIC₉₀s^a

Antimicrobial	MIC50s (µg/ml)	MIC90sa (µg/ml)	%S by PK/PD	CLSI
				%S	%R
Ampicillin	0.25	>16	NA	81.9	17.0
Amoxicillin	0.5	>16	81.6	83.2	16.8
Amoxicillin-clavulanate lower dose	0.5	1	98.1	99.6	0.4
Amoxicillin-clavulanate higher dose	0.5	1	99.6	NA	NA
Cafaclor	4	16	1.4	89.7	3.6
Cefuroxime axetil	1	2	83.6	98.1	0.7
Cefixime	0.03	0.06	99.8	99.8	NA
Ceftriaxone	£0.004	0.008	100	100	NA
Cefprozil	2	8	22.3	92.5	2.6
Cefdinir	0.25	0.5	92.0	97.6	NA
Erythromycin	4	8	<0.5	NA	NA
Clarithromycin	8	16	<0.”	79.6	0.9
Azithromycin	1	2	<1.2	99.5	NA
Chloramphenicol	0.5	1	98.1	97.9	1.9
Doxycycline	0.5	1	28.9	NA	NA
Trimethoprim-sulfamethoxazole	0.12	>4	78.3	78.3	17.0
Ciprofloxacin	0.015	0.03	99.9	99.9	NA
Ofloxacin	0.03	0.06	99.9	99.9	NA
Gemifloxacin	0.004	0.015	99.9	NA	NA
Levofloxacin	0.015	0.015	99.9	99.9	NA
Gatifloxacin	0.008	0.015	99.9	99.9	NA
Moxifloxacin	0.015	0.03	99.8	99.8	NA

^AData are from the Alexander Project 1998 to 2000 (68), S, susceptible; R, resistant; NA, not available.

Table 3: Some Reported Drug Concentrations in Body Compartments Following Therapeutic Doses of Antibiotics (ug/ml)

Drug	Cerebrospinal  fluid	Respiratory tract	Middle earfluid
Penicillin V			6
Ampicillin	03	025-06	02-22
Amoxicillin	2	09-72	18-23
Clavulanate		1-16
Sulbactam	42	03
Erythromycin			06
Sulfisoxazole			37
Trimethoprim			19
Sulfamethoxazole			17
Chloramphenicol	57
Ciprofloxacin	035-056
Rifampicin	027
Cephradine	<15	13
Cephalexin		03
Cefuroxime	03-217	24
Cefotaxime	1-15	06
Ceftriaxone	1-135*	19-27
Ceftazidime	1-30	56-7
Cefpodoxime		11
Cefpriome		72
Cefamandole	094
Cefixime	02-117	<02-24
Azithromycin		9
Clarithromycin		9-17
Aztreonam	3
Imipenem	21**
Meropenem	28

 *(up to 58)       

**(up to 86 in early phase of acute bacterial meningitis)

Adapted from references: 36, 38, 102, 146, 159, 164, 166, 180, 188, 207

Table 4: Recommended Drug Dosages for Meningitis                                               

	Adults - total daily dose  (dosing interval; hours)	Children - mg/kg/day (dosing interval; hours)
Ampicillin	12 (4)	200-400 (3-6)
Chloramphenicol	4 -6 (6)	75-100  (6-8)
Cefotaxime	8-12 (6-8)	200  (6)
Ceftazidime	6 (8)	125-200 (6-8)
Ceftriaxone	4  (12-24)	100  (12-24)
Meropenem		120 (8)
Rifampicin	06 (24)	10-20  (12-24)
Dexamethasone		06 (6)

Adapted from references: 105, 219

Table 5: Dosages of Antibiotics Used in H influenzae Pneumonia and Otitis Media

	Total Daily Dose (mg/kg/day)
Drug	Adults (dosing interval; hours)	Children (dosing interval; hours)
Ampicillin	4g (6)	50-200 (6)
Amoxicillin	15g (8)	30-60 (8)
Trimethoprim-sulfamethoxazole	288g (12)	8 (12)
Cefaclor	15g (8)	40 (8)
Cefuroxime	500 (12)	30 (12)
Third-generation cephalosporins	as for meningitis	as for meningitis
Cefixime	200-400 (12-24)	8 (12-24)
Ceftibuten	400 (24)	9 (24)
Cefpodoxime	200-400 (12)	8 (12)
Azithromycin	500 (24)	200-400mg/day (24)
Clarithromycin	500 (12)	15 (12)
Ciprofloxacin	05-15(12)	0-20 (12)

Compiled from references: 111, 164, 187, 205

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