Hendra Virus Disease and Nipah Virus Encephalitis
Henipavirus is a genus of the family Paramyxoviridae, order Mononegavirales containing two members, Hendravirus and Nipahvirus. The henipaviruses are naturally harboured by Pteropid fruit bats (flying foxes) and are characterised by a large genome, a wide host range and their recent emergence as zoonotic pathogens capable of causing illness and death in domestic animals and humans.[1]
Emergence
Hendra virus (originally Equine morbillivirus) was discovered in September 1994 when it caused the deaths of fourteen horses, and a trainer at a training complex in Hendra, a suburb of Brisbane in Queensland, Australia.[6]
The index case, a mare, was housed with 23 other horses after falling ill and died two days later. Subsequently, 19 of the remaining horses succumbed with 13 dying. Both the trainer and a stable hand were involved in nursing the index case and both fell ill within one week of the horse’s death with an influenza-like illness. The stable hand recovered while the trainer died of respiratory and renal failure. The source of virus was most likely frothy nasal discharge from the index case.
A second outbreak occurred in August 1994 (chronologically preceding the first outbreak) in Mackay 1000 km north of Brisbane resulting in the deaths of two horses and their owner.[7] The owner assisted in autopsies of the horses and within three weeks was admitted to hospital suffering from meningitis. He recovered, but 14 months later developed neurologic signs and died. This outbreak was diagnosed retrospectively by the presence of Hendra virus in the brain of the patient.
A survey of wildlife in the outbreak areas was conducted and identified pteropid fruit bats as the most likely source of Hendra virus with a seroprevalence of 47%. All of the other 46 species sampled were negative. Virus isolations from the reproductive tract and urine of wild bats indicated that transmission to horses may have occurred via exposure to bat urine or birthing fluids.[8].
Nipah virus
Nipah virus was identified in 1999 when it caused an outbreak of neurological and respiratory disease on pig farms in peninsular Malaysia, resulting in 105 human deaths and the culling of one million pigs.[7] In Singapore, 11 cases including one death occurred in abattoir workers exposed to pigs imported from the affected Malaysian farms. The Nipah virus has been classified by the Centers for Disease Control and Prevention as a Category C agent (http://www.bt.cdc.gov/agent/agentlist-category.asp).
The outbreak was originally mistaken for Japanese encephalitis (JE), however, physicians in the area noted that persons who had been vaccinated against JE were not protected, and the number of cases among adults was unusual [16] Despite the fact that these observations were recorded in the first month of the outbreak, the Ministry of Health failed to react accordingly and instead launched a nationwide campaign to educate people on the dangers of JE and its vector, Culex mosquitoes.
Symptoms of infection from the Malaysian outbreak were primarily encephalitic in humans and respiratory in pigs. Later outbreaks have caused respiratory illness in humans, increasing the likelihood of human-to-human transmission and indicating the existence of more dangerous strains of the virus.
Based on seroprevalence data and virus isolations, the primary reservoir for Nipah virus was identified as Pteropid fruit bats including Pteropus vampyrus (Malayan flying fox) and Pteropus hypomelanus (Island flying fox), both of which occur in Malaysia.
The transmission of Nipah virus from flying foxes to pigs is thought to be due to an increasing overlap between bat habitats and piggeries in peninsular Malaysia. At the index farm, fruit orchards were in close proximity to the piggery, allowing the spillage of urine, faeces and partially eaten fruit onto the pigs.[17] Retrospective studies demonstrate that viral spillover into pigs may have been occurring in Malaysia since 1996 without detection.[7] During 1998, viral spread was aided by the transfer of infected pigs to other farms where new outbreaks occurred.