Three newly recognized encephalitogenic zoonotic viruses spread from fruit bats of the genus Pteropus (order Chiroptera, suborder Megachiroptera) have been recognised over the past decade. These are: Hendra virus, formerly named equine morbillivirus, which was responsible for an outbreak of disease in horses and humans in Brisbane, Australia, in 1994; Australian bat lyssavirus, the cause of a severe acute encephalitis, in 1996; and Nipah virus, the cause of a major outbreak of encephalitis and pulmonary disease in domestic pigs and people in peninsula Malaysia in 1999. Hendra and Nipah viruses have been shown to be the first two members of a new genus, Henipavirus, in the family Paramyxoviridae, subfamily Paramyxovirinae, whereas Australian bat lyssavirus is closely related antigenically to classical rabies virus in the genus Lyssavirus, family Rhabdoviridae, although it can be distinguished on genetic grounds. Hendra and Nipah viruses have neurological and pneumonic tropisms. The first humans and equids with Hendra virus infections died from acute respiratory disease, whereas the second human patient died from an encephalitis. With Nipah virus, the predominant clinical syndrome in humans was encephalitic rather than respiratory, whereas in pigs, the infection was characterised by acute fever with respiratory involvement with or without neurological signs. Two human infections with Australian bat lyssavirus have been reported, the clinical signs of which were consistent with classical rabies infection and included a diffuse, non-suppurative encephalitis. Many important questions remain to be answered regarding modes of transmission, pathogenesis, and geographic range of these viruses.
The last decade of the 20th Century saw the introduction of an unprecedented number of encephalitic viruses emerge or spread in the Southeast Asian and Western Pacific regions (Mackenzie et al, 2001; Solomon, 2003a). Most of these viruses are zoonotic, either being arthropod-borne viruses or bat-borne viruses. Thus Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, has spread through the Indonesian archipelago to Papua New Guinea (PNG) and to the islands of the Torres Strait of northern Australia, to Pakistan, and to new areas in the Indian subcontinent; a strain of tick-borne encephalitis virus (TBEV) was described for the first time in Hokkaido, Japan; and a novel mosquito-borne alphavirus, Me Tri virus, was described from Vietnam. Three novel bat-borne viruses emerged in Australia and Malaysia; two, Hendra and Nipah viruses, represent the first examples of a new genus in the family Paramyxoviridae, the genus Henipaviruses, and the third, Australian bat lyssavirus (ABLV) is new lyssavirus closely related to classical rabies virus. These viruses will form the body of this brief review.
In the interval 1994-1999, in Australia, Malaysia and Singapore, epizootic and epidemiological episodes of meningoencephalitis and severe acute respiratory syndromes were reported. Highly lethal in horses, swine and humans, the episodes were proved to be caused by the "new" viruses Hendra (HeV) and Nipah (NiV). At the same time three "new" viral agents have been isolated: Lyssavirus, Menanglevirus and Tupaia paramyxovirus. The intense contemporary circulation of people, animals and food products together with changes in human ecosystem favor new relations between humans and the "natural reservoirs" of biologic agents with a pathogenic potential for domestic and peridomestic animals and humans.
Since 1994, a number of novel viruses have been described from bats in Australia and Malaysia, particularly from fruit bats belonging to the genus Pteropus (flying foxes), and it is probable that related viruses will be found in other countries across the geographical range of other members of the genus. These viruses include Hendra and Nipah viruses, members of a new genus, Henipaviruses, within the family Paramyxoviridae; Menangle and Tioman viruses, new members of the Rubulavirus genus within the Paramyxoviridae; and Australian bat lyssavirus (ABLV), a member of the Lyssavirus genus in the family Rhabdoviridae. All but Tioman virus are known to be associated with human and/or livestock diseases. The isolation, disease associations and biological properties of the viruses are described, and are used as the basis for developing management strategies for disease prevention or control. These strategies are directed largely at disease minimization through good farm management practices, reducing the potential for exposure to flying foxes, and better disease recognition and diagnosis, and for ABLV specifically, the use of rabies vaccine for pre- and post-exposure prophylaxis. Finally, an intriguing and long-term strategy is that of wildlife immunization through plant-derived vaccination.