Hendra virus is a novel zoonotic member of the family Paramyxovirus. With the closely related Nipah virus, it comprises the genus Henipavirus. Both viruses have emerged from wildlife, and both cause fatal disease in livestock and humans. Bats are the natural hosts of henipaviruses; evidence of infection is widespread in Pteropus bat species (flying foxes) across their range, and is becoming increasingly evident in other genera. Hendra virus, first described in 1994 in Australia, continues to cause sporadic equine and human cases in that country; Nipah virus, first described in Malaysia in 1999 following an outbreak of disease in pigs and attendant humans, now causes almost-annual clusters of encephalitic disease in humans in Bangladesh. The 'atypical' Hendra virus cases in eastern Australia in 2008 sharpened focus on virus strain diversity, pathogenicity and transmissibility, and on drivers for spillover from bats. There is increasing evidence that (like Nipah virus in Bangladesh) a cluster of Hendra strains exists, that aspects of the ecology and/or biology of flying foxes play a role in spillover, and that patterns of spillover are changing. This paper expands a previous discussions of Hendra virus emergence and management (6, 8). Emphasis is given to the importance of understanding agent and host ecology.
Hendra virus was first described in September 1994, in an outbreak of disease in horses in Australia. Twenty horses and two humans were infected, with the resultant deaths of 13 horses and one human (10). A further ten spillover events have been identified to date, resulting in a total of 37 equine cases and six human cases (Table 1).
Table 1. Confirmed Hendra virus cases, Australia
Prior to the Redlands cluster in mid-2008, the predominant clinical manifestation in horses was an acute febrile respiratory syndrome (4, 5, 10), although minor neurological symptoms were sometimes observed. The Redlands cases exhibited symptoms of severe central nervous system involvement and an absence of respiratory involvement (7).
Low infectivity and a high case fatality rate are features of Hendra virus infection in both horses and humans. Infection appears not to transmit readily from bats to horses, nor from horse to horse, nor from horses to humans, however, once infected, horses have a 75% probability, and humans a 50% probability, of a fatal outcome. All six human cases of Hendra virus infection to date have been attributed to exposure to infected horses. There is no evidence of bat-to-human or human-to-human transmission (11, 13). Infection in flying foxes causes no evident disease, and appears widespread, both taxonomically and geographically; archived serum samples from the 1980s reveal an antibody prevalence similar to that evident in contemporary surveys, indicating that Hendra virus infection is endemic in Australian flying foxes.