Giardia lamblia (Giardiasis)

Authors: Rodney D. Adam, M.D.

Previous Authors (2nd Edition 2002):Jeanine F.J.B. Nellen, M.D.,Joost O.M. Zaat, M.D., Ph.D.,Peter Speelman, M.D., Ph.D.


Giardia is a binucleate flagellated protozoan parasite first seen in 1681 by Antoni van Leeuwenhoek, inventor of the microscope, in his own diarrheal stool. Lambl, in 1859, described the genus Giardia more extensively and the human variant has been named after him, although not without controversy, in that Giardia duodenalis and Giardia intestinalis are also used frequently.

Classification of the protozoa (protists) has been primarily on the basis of morphology, in part because there is great uncertainty and controversy regarding the molecular relationships of many of these organisms. One recent classification system places Giardia within the infrakingdom Excavata, Phylum Metamonada, Subphylum Trichozoa, Superclass Eopharyngia, Class Trepomonadea (38) while another system declines to give a specific rank, but lists the hierarchy as follows: Excavata, Fornicata, Eopharyngia, Diplomonadida (4), With either system, Giardia species belong to the diplomonads (two bodies), which are unique in their possession of two nuclei that are nearly identical in morphology and time of replication. (This is in contrast to the ciliates, such as Paramecium, which have a micronucleus that contains the entire genome, and a macronucleus that has amplified portions of the genome and is the place where transcription occurs). The diplomonads include free-living and parasitic organisms, but Giardia is the only one recognized as a human pathogen. Giardia was initially divided into species on the basis of the host of origin with the resultant description of dozens of species. Subsequently, Filice described three distinct species on the basis of light microscopic studies; G. agilis, which is long and slender and found in amphibians,G. muris, which is nearly round and is found in rodents, and G. duodenalis (G. lamblia), which is found in birds and mammals (70). Subsequently, electron microscopy was used to identify further species within G. lamblia; Giardia psittaci (61) in psittacine birds, Giardia ardeae in herons (62) and Giardia microti in voles (208). (see Table 1 for a listing of Giardia species and genotypes). Each of these species distinctions is also supported by molecular comparisons. Those parasites that are still categorized with G. lamblia are all mammalian parasites, but even these organisms are divided into at least eight different genotypes or assemblages (1,180). Many of which have substantially different ranges of host specificities (Table 1). Only two of these genotypes, A and B (Nash Groups 1/2 and 3 (150,151) are found in humans. These two genotypes are so different from each other that they should probably be considered as separate species (3,71,150,151). On the other hand, most isolates from dogs and cats have been from genotypes C and D, and therefore, do not pose a significant risk for human infection. However, controversy remains because occasionally, organisms from Genotype A (and rarely B), are also found in dogs or cats (180). However, in most cases, it is not clear whether there has been transmission between humans and cats or dogs, and if so, whether it was from zoonotic or anthropozoonotic (from human to animal). In contrast, there is reasonably good epidemiologic evidence to suggest that beavers can be a source for human infections (53,136,155).
Life cycle


G. lamblia is the most commonly identified intestinal protozoan and is a common cause of diarrheal disease throughout the world. Infection is transmitted by ingestion of the cyst, which is found in fecally contaminated locations and has prolonged survival in a cool moist environment. Beyond these generalizations, the epidemiology varies markedly depending on the setting. Many developing regions have water of poor quality that is regularly contaminated by human feces. In these settings, infection is nearly universal early in childhood and recurrence occurs very quickly after treatment (83). In the US, and likely in many other temperate developed areas, the epidemiology is very different from what is found in the tropics. Infections frequently occur as water-borne outbreaks and the percent of people with symptomatic disease appears much higher (213). Many of the cases are the result of recreational water use, which is thought to explain the two-fold increase in cases from June through October. The incidence is several-fold higher in children ages one to nine (especially one to four), which could be due to day care exposure, more frequent recreational water exposure, and the lack of protection acquired from prior exposure. Less commonly, outbreaks have been associated with food ingestion, either as a result of an infected food handler or by handling food after changing diapers for an infected child (162,171,211). Men who have sex with men also appear to be at increased risk of giardiasis (132,185). Household pets have been proposed as potential sources of human giardiasis, but there are no convincing reports of transmission of giardiasis from dogs or cats to humans. Since these animals usually carry genotypes that differ from those found in humans, it is likely that cross-transmission does not occur frequently.

Case rates per 100,000 population vary from 1.4 in Louisiana to 30 in Vermont for an overall average of about 7. These rates reflect the generally increased frequency of infections in the northern states and contrast with prevalence rates of 2-5% estimated from fecal samples. Giardiasis is substantially underdiagnosed, so the rate of 7 per 100,000 is a substantial underestimate. In contrast, the rates determined from fecal specimens are generally from symptomatic patients, resulting in a marked overestimate by this approach. The rate of hospitalization for giardiasis has been estimated at 2 per 100,000, which is similar to that of shigellosis (121).


After ingestion of G. lamblia cysts, the typical incubation period before development of symptoms is one to two weeks, but it can be as short as one to two days and ranges to over a month (160). The presence or absence of symptoms after infection varies greatly with the type of population. Rates of symptomatic disease have ranged from 100% in Finish travelers returning from Saint Petersburg, Russia (105), to completely asymptomatic excretion of cysts in children living in endemic regions (96). The reasons for these differences are not well understood, but it is likely that at least part of the difference is a reflection of partial immunity that protects from symptomatic disease. This possibility is supported by an outbreak in a ski resort in Colorado in which the clinical attack rate was much higher for tourists than for local residents despite drinking the same water (the source of infection) (102).The local residents had probably had prior exposure and developed protection from symptomatic disease.

Studies of multiple outbreaks of symptomatic giardiasis (26,113,138,162,187,209) indicate that overall, 95% of symptomatic patients have diarrhea, Other signs orsymptoms seen in about 50-70% of patients include abdominal cramps, foul-smelling stools, malaise or weakness, nausea, greasy stools weight loss, decreased appetite, and bloating or flatulence. Fever is seen early in the course in about 20% of patients. In contrast, the symptomatic patients in some of the clinical trials for treatment of giardiasis had much lower rates of diarrhea. For example, of the 164 symptomatic patients that were included from a total of 267, abdominal pain (79%) and poor appetite (41%) were more frequent than chronic diarrhea (32%). Thus, even among symptomatic patients the clinical presentation varies significantly according to patient selection.In contrast to the high attack rates found in nonendemic regions, symptoms are the exception rather than the rule in highly endemic regions and case controlled studies have frequently found no correlation between Giardia infection and symptoms. Some have even suggested a protective role for Giardia in that setting (42).The relative importance of the organism, host genetics, host microbiota and pre-existing immunity remain to be determined.

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The diagnosis of giardiasis is generally made by the identification of cysts or trophozoites in fecal specimens (standard ova and parasite examination). Trophozoites are found less frequently, but may be more associated with symptomatic infection. Shedding of cysts is somewhat intermittent, so a total of three fecal samples over a period of several days is generally recommended. Fecal coproantigen detection using enzyme immunoassays (EIA) or immunochromography detection and fluorescent antibody (FA) assays of fecal specimens are being used increasingly (60,75,76,104,115,128,148,176,178,203,214). These methods all provide increased sensitivity so that one fecal sample is probably comparable to three samples used for ova and parasite identification. The FA test provides combined diagnosis of Giardia and Cryptosporidium, and is slightly more sensitive and specific, while the coproantigen detection methods are much less labor-intensive. Nucleic acid detection methods have also been studied extensively and include conventional and real time PCR as well asLoop mediated isothermalamplification (LAMP) (94,115). Nucleic acid methods probably have greater sensitivity but questions remain regarding their specificity. Perhaps a greater sensitivity is allowing the detection of low levels of infection that are not associated with clinical manifestations. In addition, nucleic acid detection tests are more expensive and laborious than immunologic methods and have not yet gained widespread usage in clinical settings.

There are some patients with chronic diarrhea and malabsorption consistent with giardiasis who have repeatedly negative fecal examinations. In some of these patients, Giardia trophozoites can be identified by endoscopic aspiration from the small intestine (97,198). This approach has the advantage that a biopsy can be done to identify other etiologies of malabsorption such as celiac disease or Whipple's disease. Alternatively, the string test can be used (84,177).With this approach, the patient swallows a string with a capsule on the end for four hours to overnight. The end of the string is then examined microscopically for the presence of Giardia trophozoites. Whether these patients could be identified by PCR has not been systematically evaluated.

There are no antibody or antigen-based serologic assays available for the diagnosis of giardiasis.


Infection is initiated when the cyst is ingested from a fecally contaminated environmental source. The cyst is the environmentally stable and nearly inert form of the organism and can remain viable for up to a month in a moist environment at 4oC. The survival is less at higher temperatures and in dryer climates, perhaps explaining why the US prevalence is higher in the northern than in the southern states (213). Patent infection can be initiated by the ingestion of as few as 10 cysts (175). Excystation into trophozoites is triggered during passage through the gastric acid of the stomach and into the bile-rich duodenum (24). However, it should be noted that the acidic environment is not required as demonstrated by in vitro excystation under conditions of neutral pH (68) as well as the observation that gastric acidity does not protect from infection.

The trophozoite is the vegetative form of the parasite, which then colonizes the surface of the small intestine and causes the associated symptoms. The ventral surface of the trophozoite consists of a concave surface that attaches to the intestinal, most likely via a suction generated by the ventral disk (59,90,91). Although receptor-based attachment has been proposed, the organisms attach nearly as well to smooth inanimate objects such as glass nearly as well as to intestinal mucosa, emphasizing the primary importance of the physical attachment. Trophozoites have 4 pairs of flagella, providing the motility required within the small intestine. The mechanism(s) by which Giardia causes diarrhea and malabsorption are not yet well understood. There is evidence for villous flattening with epithelial barrier dysfunction and impaired epithelial transport (202). However, the mechanism by which this occurs is unclear since no toxin has been identified and the degree of inflammatory response is relatively limited (109). In contrast, trophozoites may actually attenuate the immune response as well (43). A study using an in vitro model of Giardia trophozoites (both Genotypes A and B) showed that they adhered to Caco-2 cells under the control of the trophozoite lipid raft (98). Adhesion resulted in disorganization of the F-actin cytoskeleton, followed by losses of numerous functional brush border proteins. Thus, direct and immunopathogenic mechanisms may be important. In addition, a recent study using an adult mouse model and Genotype B isolates showed symptomatic disease in mice that was accentuated by malnutrition, suggesting possible parallels with human infections (17).

The antibody-mediated secretory immune response has long been considered the major means of host response and elimination of the infection based in increased prevalence and severity in patients with hypogammaglobulinemia and possibly with isolated IgA deficiency. However, evidence from an animal model of giardiasis suggests that the cell mediated immune response may also be an important contributor (188). Trophozoites are coated by a cysteine-rich protein that undergoes antigenic variation in vitro (2,147) in an animal model (5), and in human volunteers, which may contribute to the prolonged infections that are typical of giardiasis.

Trophozoites replicate by binary fission in the small intestine, and until recently, Giardia has been assumed to be asexual. However, population genetic evidence for recombination among different organisms within a single genotype suggests a sexual or parasexual cycle of replication (40,41). There is evidence for exchange of nuclear contents within single encysting organisms (37,170), so perhaps encystation also provides an occasional means of exchanging genetic material between organisms.

More distally in the small intestine, some of the trophozoites are differentiated into cysts, a process which is triggered either by exposure to certain increased bile concentration and a slightly alkaline pH (118) or possibly due to cholesterol starvation (123). Encystation occurs during binary fission of the trophozoite after partial completion of cytokinesis, so that a cyst has four nuclei and will release two trophozoites after subsequent excystation in the next host.


In vitro studies of antimicrobial activity have been very useful in helping to guide clinical studies of drugs with potential anti-giardial activity. However, it should be recognized that these studies are much more difficult than those used for most bacteria, and even in comparison to the yeasts. These difficulties result from the requirement for anaerobic growth and the slow generation time (8-12 hours). In addition, these studies must be done with laboratory-adapted strains since axenization (adaptation to in vitro cultivation without other organisms) is laborious and not always successful.

A variety of different methods have been used to assay drug susceptibility. One method determined the drug concentration that would decrease trophozoite numbers by 50% in 3 days (127). Other approaches have included a radiometric approach to look for a 50% reduction in uptake of 3H thymidine (25), 50% reduction in parasite adherence (67), and a colorimetric assay for products released by killed trophozoites (110). A mouse model for susceptibility has even been used to circumvent the difficulty of axenizing Giardia (120). Methods using growth inhibition versus loss of adherence give varying results for different classes of drugs, supporting the proposition that direct comparison of results across classes of antimicrobial agents is inappropriate (45).

Another in vitro approach determined the degree of growth inhibition after two days of drug exposure in comparison to untreated trophozoites. Aminoglycoside susceptibilities were correlated with the Giardia rDNA sequence, which predicted the activity of paromomycin and hygromycin against Giardia, but the lack of activity for other aminoglycosides such as gentamicin (54), The same approach was used to show the activity of the benzimidazoles (56) and some activity for the more lipophilic tetracyclines, such as doxycycline (55). More recently, loss of viability as determined by loss of parasite motility has been used to identify nitroimidazoles with activity against organisms that have reduced in vitro susceptibility to metronidazole (206). The tricyclic, 3-chloroimipramine, also has anti-Giardia activity (210).

A variety of natural and synthetic flavonoids have demonstrated in vitro activity against Giardia as reviewed in (133). Some of these agents were identified on the basis of their roles as natural remedies for gastrointestinal illnesses, and in some cases, the activity is greater than that of metronidazole (114). Numerous recent studies have analyzed the in vitro effects of a variety of agents against Giardia, including analogs of drugs known to be active, plant materials and other agents (8, 21, 28, 29, 30, 31, 32, 48, 49, 50, 57, 58, 74, 92, 93, 99, 100, 103, 118, 124, 125, 126, 137, 142, 143, 153, 154, 164, 169, 172, 194, 195, 196, 197, 200, 201, 204, 207). Methods with higher throughput have also been used to identify bioactive compounds with in vitro activity against Giardia (135, 169, 184). Several of the studies of drug resistance have demonstrated the ability to develop laboratory strains that have markedly reduced susceptibility to metronidazole or albendazole.

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Treatments of Choice

Since 1972, the most studied drugs in numbers of studies and patients have been metronidazole, tinidazole, and albendazole (see Table 2 for a list of drugs used for treatment of giardiasis). These drugs have all had excellent side effect profiles. The efficacies of metronidazole and tinidazole have been uniformly excellent, while some studies of albendazole have shown inferiority to the nitroimidazoles. Therefore, metronidazole and tinidazole should be considered the drugs of choice. Of the two, tinidazole is effective when given as a single dose, greatly improving the ease of administration and compliance in comparison to metronidazole. However, the cost may be greater than generic forms of metronidazole, limiting its availability in some areas. Secnidazole and ornidazole appear to be as effective as tinidazole, but have been studied less and are not available in the US.

Albendazole probably has the most favorable side effect profile and may be an optimal choice when its anti-helminthic activity is desired. Quinacrine is difficult to obtain in the US and treatment is more likely to be limited by toxicity, but the drug if highly effective and may be especially useful in combination with a nitroimidazole for refractory cases of giardiasis.

Clinical Trials

Numerous trials have been conducted to determine the comparative efficacies of different treatment regimens (Table 3). The studies included in the table include most of the comparative trials that have been reported in English-language articles since 1970 that are indexed on Medline. Many of the studies have been included in recent systematic reviews or meta-analyses (85, 166, 190) (Table 3).

The more recent studies were identified surveying all articles with the key word of giardiasis that reported comparative trials. Most of the studies have one or more significant flaws, such as small numbers, inadequate follow-up, or non-standard treatment regimens, Very few of the studies were placebo-controlled, but for those that were, the spontaneous resolution of Giardia cyst excretion, was relatively uncommon. Most of the studies have been performed in developing countries that are endemic for giardiasis, but a few have been performed in the US and other western countries. The majority of studies included only symptomatic patients, but a substantial minority included asymptomatic patients as well. When the percentage of symptomatic patients was available, it was included in the table. However, despite the limitations, the relatively large number of studies makes it possible to glean valuable information from these reports. Some of the key points are as follows:

(1) All nitroimidazoles that have been tested have demonstrated remarkable efficacy. All except metronidazole (i.e. tinidazole, secnidazole and ornidazole) have very good efficacy after single dose treatment. For metronidazole, a number of studies have demonstrated rather poor efficacy after single dose, but longer courses have been very successful. The side effect profiles have been similar for all agents with gastrointestinal side effects being very common, but serious side effects are rare.

(2) The benzimidazoles, primarily albendazole, also have reasonably good efficacy, but short course (one day) have poor efficacy. In addition, mebendazole has shown quite poor efficacy in some of the studies. In general, these drugs have been very well tolerated. Not only have they been without serious side effects in these studies; they lack the transient gastrointestinal side effects that are common with the nitroimidazoles.

(3) None of the agents has an efficacy of 100%. That means that for any drug treatment course, there is the possibility of clinical failure, which is not necessarily the result of drug resistance.

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Drugs with Demonstrated Efficacy

Nitroimidazoles:The nitroimidazoles have a rather unique mechanism of action that results in broad spectrum activity against anaerobic protozoa and bacteria. In the setting of an anaerobic or microaerophilic environment, pyruvate:ferredoxin oxidoreductase (PFOR) reduces ferredoxin or flavodoxin, which then donates an electron to metronidazole (or another nitroimidazole), activating the drug within the cell. The reduced drug becomes a toxic radical which covalently binds to DNA(183).The DNA destruction is proposed as the mechanism of killing, although cellular proteins may also be affected. The validity of this proposed mechanism in Giardia is supported by studies in which transfection was used to inhibit PFOR expression in Giardia, resulting in increased survival in oxygen and decreased susceptibility to metronidazole (46). These drugs are mutagenic in vitro, which led to their delayed acceptance as therapeutic agents, but carcinogenesis in humans has not been documented (20).These drugs all have excellent bioavailability with nearly 100% oral absorption. Metronidazole and tinidazole are both available in the US, although metronidazole has never been approved for treatment of giardiasis. Serious side effects are rare with these drugs, and all have similar side effect profiles which include a metallic taste in the mouth, nausea and vomiting, and anorexia. In addition, there is a potential disulfiram-like interaction with alcohol, so ethanol ingestion is prohibited during treatment.

Metronidazole: is the most studied drug for treatment of giardiasis and is available worldwide at low costs. Useful reviews of pharmacology and mechanism have been published (72, 183).Treatment courses ranging from single dose to 10 days have been used. The results with single dose therapy have been poor with parasitologic cures of 36-76% (Table 3). In contrast, cure rates have been much better with 5 (75-100%), 7 (89-100%), and 10 days (83-100%). These 5-10 day courses have generally used doses of 250 mg to 750 mg tid for adults, or the equivalent dose for children. It is not clear whether the higher dose results in significantly improved outcomes.

Tinidazole:has been used worldwide for decades and recently became available in the United States (73). Tinidazole has also been evaluated in numerous studies, but in contrast to metronidazole, these have almost exclusively been as single dose treatment courses. Response rates have ranged from 80-100% with most studies showing responses of >90%.

Secnidazole and ornidazoleare other nitroimidazoles with pharmacokinetic properties similar to those of tinidazole. These agents have also been studied primarily in single dose treatment courses (82).Secnidazole has been studied with a dose of 30 mg/kg, yielding response rates of 79-100%, and ornidazole has been studied using doses of 20-40 mg/kg, yielding response rates of 92-100%.

Benzimidazoles:The benzimidazoles are broad spectrum anti-helminthic agents that bind to beta-tubulin, causing irreversible damage to the parasite (129). They are poorly absorbed from the gastrointestinal tract of humans, but absorption is improved by food ingestion, especially with a high fat content (47). These agents are embryotoxic in animals, causing a craniofacial abnormality, although at higher blood levels than have documented in humans.

Albendazole has an oral bioavailability in humans of 1-5% and after absorption, is metabolized to the active drug, albendazole sulphoxide (47). It is the most extensively studied benzimidazole and has yielded the best clinical response rates (Table 3). When used as single dose therapy, response rates were 50-75%. These rates improved to 62-97% (all with a 5 day course, and all but one study gave response rates of at least 90%. In addition, albendazole has been very well tolerated with low rates of side effects.

Mebendazole has been studied in one to 10 day courses with response rates varying from 12 to 91%. In general, the response rates have been unsatisfactory.

Nitazoxanide: Nitazoxanide is a relatively new antimicrobial agent that has activity for Giardia in addition to E. histolytica, Cryptosporidium, Trichomonas, and bacteria that include Helicobacter pylori (12, 15). After absorption from the gastrointestinal tract, the drug is very rapidly converted to its active metabolite, tizoxanide, so that the parent drug cannot be detected in the serum. Nitazoxanide is proposed to work as a noncompetitive inhibitor of the PFOR reaction (95) perhaps by inhibiting the binding of pyruvate to the thiamine pyrophosphate cofactor (12).

Nitazoxanide has been studied in children using 3 day courses, yielding response rates of about 70-80%, and has been very well tolerated. It is approved in the US for treatment of giardiasis in children and adults.

Furazolidone: Furazolidone is a nitrofuran that became popular for treating giardiasis in children because of its better tolerability than quinacrine and because it is available in liquid suspension (77). The drug is well absorbed orally and has a half life of about 10 minutes. It has a broad spectrum of antimicrobial activity, including gram positive and gram negative aerobic bacteria. In Giardia, the drug is reduced and activated in the trophozoite, possibly via NADH oxidase. The toxicity of the metabolites is mediated via DNA binding and destruction. Serious toxicity has been rare, but the drug is mutagenic in bacteria and carcinogenic in animals. In addition, hemolysis can occur in the setting of G6PD deficiency and use with MAO inhibitors is contraindicated.

Furazolidone has been evaluated in a number of clinical studies, and has generally resulted in cure rates of 80-94%. It has been administered 4 times daily for 7 to 10 days.

Quinacrine: Quinacrine was developed as an antimalarial agent, and for many years was considered the treatment of choice for giardiasis in the US (77). The drug is well absorbed from the gastrointestinal tract and has a half life of 5-14 days. The drug intercalates with the DNA of the trophozoite, but does not localize to the nuclei, so the mechanism of inhibition of trophozoites is unclear. It also reduces cyst viability (165). Perhaps the most troubling toxicity has been the occasional report of toxic psychosis. A bitter taste and vomiting are common, especially in children, sometimes limiting its efficacy. It is no longer generally available in the US, but is available from a compounding pharmacy (see Table 2).

Several clinical studies have evaluated quinacrine, yielding cure rates of 77-100%. In most studies, the drug was given in three doses per day for 5 to 10 days. Two studies have observed that the efficacy of quinacrine combined with another agent (metronidazole) was very high for cases of refractory giardiasis (140, 145) (see Treatment Failure below).

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Other Agent

Certain aminoglycosides have activity against Giardia; most notably paromomycin and neomycin. Paromomycin has been evaluated in a few earlier studies, demonstrating an efficacy of about 55-90% (77). It is commonly recommended for treatment of giardiasis, at least during the first trimester of pregnancy because of its perceived safety in that setting. Bacitracin with or without zinc and with or without neomycin, and neomycin were studied in 80 patients with about 20 in each group (13).The response rate for each group ranged from 90 to 100%.

Specific Clinical Situations

Asymptomatic Infection: Asymptomatic carriage of Giardia is very common in certain endemic areas, with recurrence of infection very soon after effective treatment (83). In this setting, there do not appear to be long term adverse consequences of untreated infection (96). Therefore, asymptomatic infections in highly endemic areas should usually not be treated. However, in areas of low endemicity, symptomatic infection can result from exposure to asymptomatic carriers, so these carriers should be treated. Likewise, infections in people with increased risks of transmission such as food handlers and day care workers should be treated.

Treatment Failure: As noted above, treatment failures occur and may be due to a number of reasons. Cysts are much more resistant to drug exposure than are trophozoites, and may also be exposed to less drug on the basis of their location within the intestine. Even for the trophozoites, the relative importance of high luminal vs. high serum concentrations is not always clear, since the nonabsorbed aminoglycosides, the poorly absorbed benzimidazoles, and the very well absorbed nitromidazoles all have substantial efficacy for treatment of giardiasis.

It is certainly possible that true drug resistance occurs in clinical isolates, but documentation is generally impossible for individual cases because of the difficulty of in vitro cultivation. The few published reports correlating in vitro and in vivo resistance have yielded conflicting results (127, 131, 189, 204). In addition, as a tetraploid organism, Giardia would be expected to develop resistance much more slowly than haploid organisms. Therefore, it is more accurate to refer to treatment failure or clinical resistance. A few reports have addressed the treatment of patients with refractory giardiasis. In one report, six patients had giardiasis lasting from 6 months to over one year and had already failed one or more treatment courses that included metronidazole. Five of the 6 patients were successfully treated with the combination of metronidazole and quinacrine (145). In a large water-borne outbreak of giardiasis in Norway there were 38 out of 1200 cases that were considered refractory to metronidazole (140). These 38 patients were treated with a progressive approach using one week of albendazole plus metronidazole, followed by paromomycin for those failing the first regimen and three weeks of quinacrine plus metronidazole for those failing the second regimen. Thirty responded to metronidazole plus albendazole. Six of the 8 failures were treated with paromomycin and 3 responded. The remaining 3 were treated with quinacrine plus metronidazole and all responded. These studies suggest that combination therapy with metronidazole and albendazole or quinacrine will be successful in most cases of treatment failure.

In addition to documented treatment failures, many people have persistent symptoms after presumably successful treatment of giardiasis, which could be due to inability to find Giardia in fecal samples or altered intestinal anatomy or function after diarrhea. A study of the above-noted outbreak of giardiasis in Norway revealed that some people had persistent symptoms, but negative fecal samples. These people were randomized and treated either with albendazole/metronidazole (Giardia-specific treatment) or with tetracycline/folate (for persistent gastrointestinal symptoms after diarrheal illness) (88). The lack of an outcome difference between the two groups suggested that the persistence of symptoms was not due to cryptic giardiasis. Instead, these patients had an illness consistent with irritable bowel syndrome, an entity that is common after a variety of gastrointestinal infections. Patients with laboratory proven giardiasis from the same outbreak had a prevalence of 39.4% for irritable bowel syndrome and 30.8% for chronic fatigue, approximately three-fold higher than for uninfected case controls (89).

Pregnancy: Pregnancy poses a significant complication for the treatment of giardiasis, since few of the available agents are known or presumed to be safe during early pregnancy. The benzimidazoles (mebendazole and albendazole) are embryotoxic in animals during organogenesis (although at levels higher than obtained with these drugs in humans). The nitromidazoles (metronidazole, tinidazole, secnidazole, ornidazole) are mutagenic in bacteria, so acceptance of these agents during pregnancy has been very slow. However, metronidazole has subsequently been evaluated in multiple studies during second and third trimesters of pregnancy for treatment of bacterial vaginosis or trichomoniasis, including some showing improved outcome when patients with symptomatic vaginosis at risk for premature delivery were treated (129, 139). These agents are overall, the most efficacious for treating giardiasis and appear reasonable to use during the second and third trimesters for symptomatic giardiasis, especially since the nutritional consequences of giardiasis may outweigh any as yet theoretical risk of these drugs. However, paromomycin, a nonabsorbed aminoglycoside is often recommended during pregnancy, especially when the disease is mild or asymptomatic.

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A lactase free diet can be tried during treatment and shortly after, because deficiency of disaccharidases can occur due to damage to the brush border. Lactase deficiency during giardiasis is very common (101, 186), and enzyme deficiency may persist for weeks to months. Therefore, elimination of lactose from the diet is commonly recommended for patients with giardiasis. Antidiarrheal drugs may be used but are usually not necessary. Likewise, oral rehydration solution or intravenous fluid administration is advised for volume depletion, but that rarely occurs with giardiasis. Probiotics have been studied for the treatment and prevention of a number of intestinal pathogens because of the known effect of probiotics on gastrointestinal flora. Saccharomyces boulardii has been evaluated as an adjunctive agent to metronidazole, and those treated with metronidazole along had an 83% parasitologic response rate while those treated with metronidazole in addition to S. boulardii had a 100% response (22). These probiotic agents have not been approved by the FDA so there is no standardization in the US, and those who choose to use them must obtain them from health food stores.


Resolution of symptoms and/or parasitological cure are the endpoints of therapy. When resolution of symptoms is achieved in individual patients, repeat stool specimens are not required. Treatment failure that is documented by persistence of parasites may be the result of drug resistance, but true drug resistance has not been clinically correlated with treatment failures, perhaps because of the difficultly in adapting the organisms to in vitro cultivation.

Whether asymptomatic carriers should be treated remains controversial, but probably depends on the setting. Treatment of asymptomatic carriers in non-endemic areas may prevent symptomatic cases, but reinfection occurs so frequently in some endemic regions that asymptomatic carriers should not be treated. Carriers of who work in day care centers or in food preparation should have documentation of parasitologic cure by stool specimens. When there is reason to suspect cross-infection in a family, all carriers in the family should be treated till parasitological cure is documented.


Vaccines for humans are not available, but a commercial Giardia vaccine is available in the USA for dogs and cats in preventing clinical symptoms and reduction of cyst shedding. It protected dogs and cats from infection when orally challenged with Giardia lamblia derived from a symptomatic human source (157). A safety study in 817 dogs showed only mild injection-site reactions in 3 %, but no systemic reaction. Whether the vaccine is prevents naturally occurring infection has not been determined. The vaccine has also been proposed as an immunotherapeutic agent for persistently infected animals (158) but subsequent studies have not demonstrated efficacy for this purpose (11, 193).


Since Giardia is transmitted via a fecal-oral route, prevention efforts should be directed at interrupting these routes of transmission. One of the most frequent means of acquisition of infection is by ingestion of contaminated water, typically from fresh water streams or shallow wells. Since cysts remain viable for longer at lower temperatures, the risk is sometimes higher in colder climates. Water from these sources should be purified, either by boiling for one minute or by filtration with a filter designed for eliminating parasites from water (pore size < 1um). Iodine or chlorine treatment also has activity against the cysts, but is much less effective for Giardia than for most bacterial or viral pathogens, requiring more prolonged exposure. Food is a much less frequent source, but outbreaks have been associated with infected food handlers. These infections are difficult to prevent, but uncooked foods should be avoided in endemic areas. In developed countries, direct fecal-oral transmission occurs most frequently in settings where small children are cared for; these infections can be reduced by good handwashing techniques and appropriate use of gloves. Male homosexual activity has been documented as a means of transmission of giardiasis, and female: female sexual transmission has been documented for Entamoeba histolytica, which has a similar mechanism of transmission (182). Presumably, these occur by direct fecal-oral contact, and may not be prevented by the usual barrier methods. Household pets are usually infected by non-human genotypes of G. lamblia, but handwashing after pet contact is still reasonable because they are occasionally infected with the "human" genotypes.

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Table 1. Giardia species and G. lamblia genotypes

Species G. lamblia
G. agilis   Amphibians
G. muris   Rodents
G. ardeae   Herons
G. microti   Rodents
G. psittaci   Psittacine birds
G. lamblia
Humans, other mammals
Humans, other mammals
Dogs, cats
Dogs, cats
Marine mammals

Table 2. Drugs used for treatment of giardiasis

Class Drug Oral bioavailability (%) Half life Dose (adult and max ped dose) and duration Availability in US FDA approved for giardiasis Common side effects Concerns
Nitroimidazole Metronidazole
6-10 h
5 mg/kg tid (250 mg)
5 to 10 days
metallic taste, nausea, anorexia DNA mutagenesis and possibility of carcinogenesis
12 h
50 mg/kg (2 gm)
single dose
metallic taste, nausea, anorexia DNA mutagenesis and possibility of carcinogenesis
11-14 h
40 mg/kg (2 gm)
single dose
metallic taste, nausea, anorexia DNA mutagenesis and possibility of carcinogenesis
14-20 h
30 mg/kg (1.5 gm)
single dose
metallic taste, nausea, anorexia DNA mutagenesis and possibility of carcinogenesis
Benzimidazole Albendazole
8-12 h
15 mg/kg qd (400 mg)
5 days
Aminoglycoside Paromomycin
10 mg/kg tid (500 mg)
 5 to 10 days
Nitrofuran Furazolidone
10 m 2 mg/kg qid (100 mg)
7 to 10 days
  Adverse effect on reproduction, mutagenesis
Others Nitazoxanide parent drug not detected 1.3-1.8 h Adult 500 mg bid, 100 mg bid (ages 1-3), 200 mg bid (ages 4-11)
3 days
Quinacrine good 5-14 d 2 mg/kg tid (100 mg)
5 to 7 days
nausea and vomiting Toxic psychosis, common nausea and vomiting

Abbreviations: Y (yes), N (no), NA (not applicable), h (hours), m (minutes), d (days)

1 No longer generally available in the US, but available from Expert Compounding Pharmacy (Panorama Compounding Pharmacy) in Lake Balboa, CA; Phone (818) 988-7979 (availability confirmed on November 25, 2014).

Table 3. Controlled Trials for Treatment of giardiasis: 1970 to 2014

Ref Granados et al., 2012 Pasupuleti et al., 201 Solaymani-Mohammadi and Singer, 2013 Year Study type Age of patients % sympto-matic Location # Subjects
Bassily et al., 1970 X 1970 ROL Both Egypt 80
Garg, 1972 1972 ROL Children 100 India 81
Gazder and Banerjee, 1978 X 1977 ROL Children 100 India 100
Bakshi et al., 1978 1978 ROL Children 100 India 186
Krishnamurthy and Saradhambal, 1978 1978 ROL Children 100 India 60
Kavousi, 1979 X 1979 AT Children 100 Iran 160
Jokipii and Jokipii, 1979 1979 AT Adults 100 Finland 85
Craft et al., 1981 1981 ROL Children NS US 58
Kyronseppa and Pettersson, 1981 1981 ROL Adults 100 Finland 50
Jokipii and Jokipii, 1982 1982 AT Adults 100 Finland 105
Murphy and Nelson, 1983 1983 ROL Children 100 US 22
Speelman, 1985 1985 ROL Children 52 Bangladesh 33
Bassily et al., 1987 1987 NS Both NS Egypt 80
Gupta et al., 1989 1989 ROL Adults 100 India 20
Gascon et al., 1989 X X 1989 ROL Adults NS Spain 23
Quiros-Buelna, 1989 X 1989 ROL Children 100 Mexico 82
Gascon et al., 1990 X 1990 ROL Both NS Spain 19
Oren et al., 1991 1991 ROL Children 87 Israel 75
Nigam et al., 1991 1991 ROL Both 61 India 75
al-Waili and Hasan, 1992 1992 ROL Children 100 Iraq 44
Hall and Nahar, 1993 X X X 1993 ROL Children NS Bangladesh 334
Dutta et al., 1994 X X X 1994 ROL Children 100 India 150
Kalayci et al., 1995 X 1995 ROL Children 100 Turkey 45
Andrews et al., 1995 1995 ROL Both NS Romania 83
Misra et al., 1995 X X X 1995 ROL Children 100 India 57
Rastegar-Lari and Salek-Moghaddam, 1996 1996 ROL Children 67 Iran 52
Bulut et al., 1996 X X 1996 ROL Children 100 Turkey 48
Cimerman et al., 1997 X 1997 ROL Children 61 Brazil 267
Pengsaa et al., 1999 X 1999 ROL Children 66 Thailand 113
Ortiz et al., 2001 X X 2001 ROL Children 100 Cuba 110
Rossignol et al., 2001 2001 DBPC Both 100 Egypt 36
Sadjjadi et al., 2001 X 2001 ROL Children NS Iran 100
Ozbilgin et al., 2002 2002 NS Children 100 Turkey 175
Pengsaa et al., 2002 2002 ROL Children 48 Thailand 84
Escobedo et al., 2003a X 2003 ROL Children 100 Cuba 146
Escobedo et al., 2003b X 2003 ROL Children NS Cuba 165
Karabay et al., 2004 X X X 2004 ROL Adults 100 Turkey 57
Yereli et al., 2004 X X X 2004 ROL Children 34 Turkey 107
Besirbellioglu et al., 2006 2006 DBPC Adults 53 Turkey 65
Canete et al., 2006b X 2006 ROL Children ? Cuba 122
Canete et al., 2006a 2006 ROL Children 100 Cuba 122
Escobedo et al., 2008 X 2008 ROL Children 100 Cuba 137
Canete et al., 2012 X X 2012 DBRCT Adult 100 Cuba 150
al-Waili and Hasan, 1992 X X 1992 Children 100 Iraq 44
Alizadeh et al., 2006 X X X 2006 ROL Both 100 Iran 120
Canete et al., 2010 X 2010 Children NS Cuba 122
Teles et al., 2011 X 2011 Both  NS Brazil 100
Almirall et al., 2011 X 2011 Adult 100 Cuba 126
Bhandari and Upadhyay, 1972 X 1972 ROL Children NS India 54


Ref Drug Dose1 Duration % Cure2
Bassily et al., 1970 Quinacrine 100 mg tid 5 days 100
Metronidazole 250 mg tid 10 days 95
Furazolidone 100 mg qid 7 days 80
Garg, 1972 Metronidazole 20 mg/kg tid 7 days 96
Furazolidone 2 mg/kg tid 7 days 94
Gazder and Banerjee, 1978 Metronidazole 50 mg/kg single dose 36
Tinidazole 50 mg/kg single dose 80
Bakshi et al., 1978 Tinidazole 50 mg/kg single dose 88
Metronidazole 50 mg/kg single dose 47
Krishnamurthy and Saradhambal, 1978 Tinidazole 50 mg/kg single dose 97
Metronidazole 50 mg/kg single dose 50
Kavousi, 1979 Quinacrine 8 mg/kg/d tid 5 days 94
Metronidazole 250 mg tid 5 days 100
Jokipii and Jokipii, 1979 Metronidazole 2.4 gm single dose 50
Metronidazole 2.4 gm 2 days 77
Tinidazole 2 gm Single dose 93
Craft et al., 1981 Quinacrine 6 mg/kg/d 10 days 77
Furazolidone 8 mg/kg/d tid 10 days 89
Kyronseppa and Pettersson, 1981 Metronidazole 2 gm single dose 76
Tinidazole 2 gm single dose 88
Jokipii and Jokipii, 1982 Ornidazole 1.5 gm single dose 100
Tinidazole 1.5 gm single dose 98
Murphy and Nelson, 1983 Furazolidone 8 mg/kg/d tid 5 days 20
Furazolidone 8 mg/kg/d tid 10 days 92
Speelman, 1985 Metronidazole 2.4 gm single dose 65
Tinidazole 2 gm single dose 94
Metronidazole 2 gm/d 3 days 93
Tinidazole 2 gm single dose 100
Bassily et al., 1987 Metronidazole 500 mg/d 10 days 95
Tinidazole 2 gm Single dose 90
Ornidazole 1 gm Single dose 97
Gupta et al., 1989 Tinidazole 2 gm single dose 100
2. Tinidazole 150 mg tid 7 days 100
Gascon et al., 1989 Metronidazole 250 mg tid 7 days 89
Mebendazole 200 mg tid 1 day 14
Quiros-Buelna, 1989 Furazolidone 250 mg/d tid 10 days 92
Metronidazole 750 mg tid 10 days 96
Gascon et al., 1990 Mebendazole 200 mg tid 5 days 12
Metronidazole 250 mg tid 7 days 91
Oren et al., 1991 Ornidazole 40 mg/kg single dose 92
Metronidazole 20 mg/kg/d tid 7 days 100
Nigam et al., 1991 Metronidazole 50 mg/kg single dose 54
Tinidazole 50 mg/kg single dose 98
al-Waili and Hasan, 1992 Mebendazole 200 mg tid 5 days 91
Metronidazole 200 mg tid 5 days 86
Hall and Nahar, 1993 Albendazole 600 mg single dose 62
Albendazole 400 mg/d 3 days 81
Metronidazole 375 mg/d 5 days 97
Albendazole 800 mg single dose 75
Albendazole 400 mg/d 5 days 95
Metronidazole 375 mg/d 5 days 97
Dutta et al., 1994 Albendazole 400 mg/d 5 days 97
Metronidazole 7.5 mg/kg tid 5 days 97
Kalayci et al., 1995 Mebendazole 200 mg tid 10 days 73
Metronidazole 5 mg/kg tid 10 days 93
Furazolidone 2 mg/kg qid 10 days 80
Andrews et al., 1995 Bacitracin zinc 240,000 U/d bid 10 days 100
Bacitracin 240,000 U/d bid 10 days 95
Neomycin 240,000 U/d bid 10 days 91
Bacitracin Zn +Neo 120,000 U/d bid ea. 10 days 90
Misra et al., 1995 Albendazole 400 mg/d 5 days 96
Metronidazole 7.5 mg/kg tid 5 days 100
Rastegar-Lari and Salek-Moghaddam, 1996 Secnidazole 30 mg/kg single dose 100
Metronidazole 20 mg/kg/d tid 10 days 94
Bulut et al., 1996 Mebendazole 100 mg tid 1 day 42
Mebendazole 100 mg tid 10 days 58
Metronidazole 15 mg/kg/d 7 days 93
Ornidazole 40 mg/kg Single dose 100
Cimerman et al., 1997 Secnidazole 30 mg/kg single dose 92
Tinidazole 50 mg/kg single dose 90
Pengsaa et al., 1999 Albendazole 400 mg/d 3 days 50
Tinidazole 50 mg/kg Single dose 96
Ortiz et al., 2001 Nitazoxanide 200 mg bid, ages 4-11, 100 mg bid, ages 2-3 3 days 71
Metronidazole 250 mg bid ages 6-11, 125 mg,ages 2-5 5 days 75
Rossignol et al., 2001 Nitazoxanide 500 mg bid 3 days 71
Sadjjadi et al., 2001 Mebendazole 200 mg tid 5 days 86
Metronidazole 15 mg/kg/d tid 7 days 90
Ozbilgin et al., 2002 Ornidazole 20, 25, or 30 mg/kg single dose 96
Ornidazole 25 mg/kg/d bid 5 days 100
Metronidazole 20 mg/kg/d tid 7 days 89
Pengsaa et al., 2002 Albendazole/
400 mg/20 mg/kg single dose 74
Albendazole 800 mg single dose 50
Tinidazole 50 mg/kg single dose 91
Escobedo et al., 2003a Mebendazole 200 mg tid 3 days 78
Secnidazole 30 mg/kg Single dose 79
Escobedo et al., 2003b Chloroquine 10 mg/kg bid 5 days 86
Albendazole 400 mg/d 5 days 62
Tinidazole 50 mg/kg Single dose 91
Karabay et al., 2004 Albendazole 400 mg/d 5 days 96
Metronidazole 500 mg tid 5 days 100
Yereli et al., 2004 Albendazole 10 mg/kg/d 5 days 90
Metronidazole 20 mg/kg/d tid 5 days 89
Besirbellioglu et al., 2006 Metronidazole + Saccharomyces Mtz 750 mg tid +Sac 250 mg bid 10 days 100
Metronidazole 750 mg tid 10 days 83
Canete et al., 2006b Mebendazole 200 mg tid 1 day 64
Tinidazole 50 mg/kg Single dose 82
Canete et al., 2006a Mebendazole 200 mg tid 5 days 79
Quinacrine 2 mg/kg tid 5 days 84
Escobedo et al., 2008 Tinidazole 50 mg/kg single dose 90
Nitazoxanide 7.5 mg/kg bid 3 days 78
Canete et al., 2012 Albendazole 400 mg/d 5 days 83
Metronidazole 250 mg tid 5 days 85
al-Waili and Hasan, 1992 Mebendazole 200 mg tid 5 days 91
Metronidazole 200 mg tid 5 days 86
Alizadeh et al., 2006 Albendazole 400 mg/d 5 days 90
Metronidazole 250 mg tid 5 days 77
Canete et al., 2010 Metronidazole 5 mg/kg tid 5 days 74
Chloroquine 10 mg/kg bid 5 days 85
Teles et al., 2011 Secnidazole 30 mg/kg 2 gm max single dose 84
Mentha crispa 2 gm single dose 48
Almirall et al., 2011 Secnidazole 2 gm single dose 90
Mebendazole 200 mg tid 3 days 86
Bhandari and Upadhyay, 1972 Metronidazole 20 mg/kg/d tid 5 days repeated in 2 weeks 86
Metronidazole 35 mg/kg single dose repeated in 2 weeks 80

Abbreviations: ROL (Randomized open label), AT (alternating treatment), DBPC (double blind placebo controlled), NS (not stated)
1 For studies including adults and children, the adult dose is given.
2 For most studies, the cure rate was determined by parasitologic cure shortly after completion of treatment.



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