Hendra virus

Authors: Mohamad Aljofan, Ph.D.

Previous author: Joe McCormack,M.D.


The Henipavirus genus represents a group of paramyxoviruses that are some of the deadliest of known human and animal pathogens. There are two known viruses within the Henipavirus genus, Hendra virus (HeV) and Nipah virus (NiV). However, a novel virus, cedar virus (CedPV), was isolated from Australian fruit bats during HeV surveillance activities and that genome characterization of cedar virus, identified a close like to Hendra virus and Nipah suggesting their inclusion as a third member of the genusHenipaviruses (819). Henipaviruses are responsible for severe disease outbreaks in humans, pigs and horses, with an estimated human mortality rate between 50 and 100% making them one of the most deadly viruses known to infect humans (18).

Hendra Virus (HeV)

Hendra virus, previously referred to as equine morbillivirus, was first identified in an outbreak in September 1994 in Hendra, a suburb of Queensland, Australia, whichresulted in the deaths of 14 horses and 2 humans who had close contact with the infected horses (22).  The naturally acquired symptomatic infection, characterized by a rapidly progressive illness involving the respiratory system and/or the central nervous system (CNS), has so far been recognized in horses, humans as well as domestic animals (40). However, there is potential for other species to be infected, with significant consequences for animal and human health. After its first emergence in Queensland, hendra virus continued to reemerge and spread to other areas in Queensland and neighboring state of New South Wales (118).

Nipah Virus (NiV)

Nipah virus was first isolated in 1999 from cerebrospinal fluid (CSF) of an encephalitic patient from Sungai Nipah village (6). Molecular analyses indicated that the isolated virus was a paramyxovirus closely related, but not identical, to Hendra virus (29). The newly emerged virus was named Nipah virus after the village of the index case (7,36). Initially, the virus caused mild respiratory and neurological disease in swine and was associated with an acute febrile encephalitic and fetal disease in humans with estimated case fatality rates of approximately 35–75% in humans and approximately 5% in swine (29).


Hendra Virus

The first Hendra virus outbreak occurred in a racing horse stable in 1994, where two people who worked at the stable were infected, along with 21 horses (2330). The second outbreak occurred in Mackay, approximately 1,000 kilometers north of Brisbane. A farmer who had assisted at the postmortem of two of his horses that had died at about the same time as the Hendra outbreak had also became infected (2324). In both outbreaks the mode of transmission was not clearly identified but the closeness of contact between human cases and horses in the Hendra stable and the postmortems in Mackay suggested that transmission by hands and/or inhalation were responsible (24). During a 17 year period, 1994-2011, there were a total of 14 known “spill over” events infecting 45 horses and seven humans, four of which were fatal (11). All seven recognized human cases of HeV infection have occurred in Queensland, Australia (32). The regular outbreaks of HeV-related disease that have occurred in Australia since 1994 have all been characterized by acute respiratory and neurological manifestations, with low morbidity but high case fatality rates infection  (11).  However, there has been an unprecedented rise in the number of Hendra virus outbreaks (Table 1), with 18 reported cases in 2011, more than 8 confirmed events in 2012 and about 7 events in 2013 (8).

Nipah Virus

The first Nipah virus outbreak in Malaysia resulted in 265 human cases including 105 fatal cases (7), and 11 cases and one death were reported among pig abattoir workers in Singapore. The Malaysian outbreak was controlled by the culling of over one million pigs and strict quarantine measures on pig movements (25).

Nipah virus was shown to be associated with high case fatality rates of approximately 35–75% in humans and roughly 5% in pigs (29). The prior knowledge of HeV outbreaks and the similarities between Hendra virus and Nipah virus facilitated the rapid identification of fruit bats as the reservoir hosts of Nipah virus (12). Furthermore, antibodies to henipaviruses have been detected in five different bat species from Australia, Malaysia, Bangladesh, India, Cambodia, Thailand, Indonesia and Madagascar (1526,42).

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Clinical Manifestations

The clinical syndrome associated with Hendra virus infection in horses was predominantly associated with a rapid progress febrile illness culminating in fulminating respiratory disease (30), some of the early cases exhibited some mild neurological signs including muscle twitching and restlessness (28). However, in the later outbreaks such as the 2008 Redlands outbreak in Queensland, all of the five infected horses displayed various neurological signs; including ataxia, depression, disorientation, head tilt, facial nerve paralysis, circling, head pressing, stranguria and recumbent period (11).  In contrast, young Nipah virus infected pigs presented with a febrile respiratory illness with epistaxis, dyspnea and coughing, while the older ones presented with neurological signs including agitation, muscle fasciculation, ataxia, paresis, and seizures (21).

Despite some of the variations, the general clinical manifestations of henipavirus infections in humans and other animals appear to be similar. After a short incubation period, an acute illness manifesting as acute encephalitis or pulmonary disease with a high mortality may occur. In some patients relapsing encephalitis or late onset of encephalitis could complicate initial recovery from the acute illness months or years later (40).

Laboratory Diagnosis

There are currently no diagnostic tests to detect early animal responses to henipavirus infection in use (37). Further elucidation of the excretion of Hendra virus during the course of Hendra virus infections in horses has shown that Hendra virus can be detected in nasal excretions even in the preclinical phase (18), while early studies of Nipah virus infections in pteropid bats showed that detection of virus even in known infected animals could be problematic (20). However, there are a number of diagnostic approaches to Hendra virus and Nipah virus testing with different degrees of strength and shortcomings.

Molecular diagnosis include conventional polymerase chain reaction (PCR), which can be used for detection of viral genetic materials in different types of specimens, including fixed, or fresh tissue, various swabs, cerebrospinal fluid or urine samples (9). Real-Time PCR or quantitative PCR (qPCR) has been used for the development of sensitive and specific molecular tests for henipaviruses. The most commonly used form is the TaqMan test which utilizes a specific 30-minor groove binder-DNA probe targeting a short sequence between the two priming sites (14).

Virus isolation remains an important primary diagnostic approach for henipavirus infections (9). For confirmation of new cases or outbreaks or identification of new host(s) for henipavirus, virus isolation method is highly desirable. However, due to their classification as dangerous zoonotic agents it is absolutely essential that isolation of virus from suspected henipavirus infections should only be conducted where laboratory biosafety can be guaranteed (37)

Immunohistochemistryis one of the most useful tests for Hendra virus and Nipah virus detection, especially where formalin-fixed tissues are presented as the main form of specimen for diagnosis. The initial immunohistochemical investigation of Hendra virus-infected tissue used a convalescent human serum, but a range of polyclonal and monoclonal antisera is now available (13343839).

Serological tests are highly important for investigation of infection status in the natural reservoir, bats, and in the identification of new animal hosts which may or may not present acute symptoms following henipavirus infection. This is best illustrated by the recent diagnosis of a sero-positive dog in a property where lethal infection of horses byHendra virus was confirmed as part of the 2011 Hendra virus outbreak investigations jointly conducted by Biosecurity Queensland and the Australian Animal Health Laboratory (AAHL) (27). The other serological method is enzyme linked immunosorbent assay (ELISA), which is the most affordable serological test for diagnosis of henipavirus infection. The most used indirect ELISA for detection of IgG antibodies to HeV is based on inactivated HeV-infected Vero cell lysate (9).

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Histopathological findings from  Nipah virus infected animals and guinea pigs, revealed evidence of endothelial and/or epithelial syncytial cells as well as mural lymphohistiocytic vasculitis with fibrinoid vascular change (35). The autopsy results of the first  Hendra virus human victim revealed lesions of congestion in his lungs, haemorrhage and oedema associated with histological chronic alveolitis with syncytia  (22).  In humans, henipavirus infections can also result in a clinically calm period following an apparent recovery from an acute infection, which can later recrudesce as encephalitis. Clinically, severe henipavirus infection in humans will present as a severe respiratory disease, encephalitis or a combination of both, include neurological symptoms (e.g. headaches, drowsiness, disorientation, myoclonus, motor, and sensory loss), respiratory disorders (observed in Hendra virus-infected horses, Nipah virus infected pigs, and 25–40 % of Nipah virus-infected humans), unstable blood pressure, and, in one case, vision loss (616,17). Totally 40–92 % of  Nipah virus-infected humans succumb to acute encephalitis with an average time of 10 days from fever onset to death, while 3–7 % of infected patients exhibit a late onset or relapsed encephalitis months to years after the initial infection (631).


There are  variable differences in virus replication and giant cell (syncytia) formation amongst different cell lines and that  Nipah virus  was noticed to replicate to a higher titre compared to  Hendra virus (2).  While, the mechanism behind the observed difference is still unclear, it may be valid to speculate that the rapid in vitro replication of  Nipah virusis related to its high fatality rate; and that significant differences between the two viruses make one virus generate more viral particles than the other.


There are no clinically available antivirals for the treatment of henipaviruses. The only antiviral compound with demonstrated clinical efficacy against henipaviruses is ribavirin, a broad spectrum antiviral often used to treat Hepatitis C infection (32). A limited non-randomized clinical trial of ribavirin during the initial Nipah virus outbreak in Malaysia indicated a reduction in mortality of acute Nipah virus encephalitis (5). An in vitro study of ribavirin showed a more than 50 fold reduction in Hendra virus infection (41). Although, these two studies have shown that ribavirin is effective both in vitro and in a clinical trial, there are a number of studies indicating that ribavirin is also associated with a range of side effects such as haemolytic anaemia (10). This might result in worsening of cardiac disease that has led to fatal and nonfatal myocardial infarctions (31).

A potentially effective post-exposure therapy is the human monoclonal antibody (mAb) known as m102.4. This antibody has been used in humans, and in Queensland, Australia they have a limited license to use in high risk exposures. Bossart et al. (2011), illustrated that the use of this neutralizing human monoclonal antibody in African green monkeys challenged with a lethal dose of Hendra virus provided 100% survival rate. While, the animal subjects displayed some neurological signs of the disease, they have all survived the infection by day 16 (3, 4).

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There are no adjunctive therapies available. However, a number of research laboratories are currently, working at  evaluating potential adjunctive therapies to enhance the monoclonal antibody m102.4, which has already been shown (in non-human primates)  to suppress severe Hendra virus infection when administered during the incubation period.


No therapeutic endpoints have been established.


As  part  of  finding  a  potential  treatment  against  the  lethal  infections,  several  vaccine  candidates  have  been  tested  for  potential protection. Nevertheless, the  only effective therapy  against  Hendra  or  Nipah  virus  infection is a recently discovered  Hendra  virus  vaccine, commercially known  as  Equivac  HeV®,  which is  a  soluble  form of  the  G  glycoprotein (4). The vaccine however, is licensed for equine uses only.


The exact route of infection is unknown, however, horses may contract Hendra virus infection whenever, they have close contact with infected bats (e.g. bites or urine) or body fluids of an infected horse. Virus may be present in the infected horse's body fluids, particularly nasal secretions.

All humans confirmed with HeV infections had contact with Hendra virus infected horses. The absence of human cases among healthcare workers and among family members suggests that Hendra virus is not easily transmissible from person-to-person. However, all of the confirmed human cases became infected following high level exposures to respiratory secretions and/or blood of Hendra virus infected horses, for instance, assisting with post mortem examination, performing certain veterinary procedures or having extensive exposure to respiratory secretions. The virus was shown to be killed by drying, heat or cleaning with detergent.

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Table 1:  This Table Summarizes HeV Outbreaks Since its Initial Emergence Until the Last Reported Case in 2013

Period   Outbreaks Fatalities
Animals Humans
1994- 2000 3 23 3
2000-2005 2 2 1
2005-2010 11 19 3
2010-2013 33 40 0

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