Nipah virus disease is a relatively newly discovered disease of swine and humans caused by infection with a novel paramyxovirus termed Nipah virus. This disease emerged in Malaysia in 1998 and 1999. It was linked to severe encephalitis among people occupationally exposed to infected pigs in Malaysia and Singapore. The disease was eradicated from the national commercial swine population by control efforts. A number of fruit bats of the genus Pteropus seem to be reservoirs of the virus in Asia.
The etiologic agent, Nipah virus (genus Henipavirus, family Paramyxoviridae), is an enveloped, negative-sense, single-stranded RNA virus. The virus is closely related to Hendra virus, the only other member of the genus.
The outbreak in humans in Malaysia and Singapore followed contact with infected swine and resulted in encephalitis, with an ~40% mortality rate. The virus is assumed to have been introduced into the swine population from one of two Pteropus spp with detectable antibodies during the outbreak investigation. The geographic distribution of Pteropus spans the Western Pacific through Southeast Asia, south Asia and down through the coastal African islands, including Madagascar. Antibodies have been detected in several species of Pteropus, suggesting that the virus or closely related viruses occur in other areas within the range of this genus of bats.
In Malaysia, genetic analysis of viral samples from human patients and from swine strongly supports the theory of a single introduction point for the virus, with subsequent spread through the commercial swine population. Evidence of infection among several other species of domestic animals, including dogs, cats, and horses, has been reported.
In human patients, encephalitis caused by Nipah viruses, reportedly occurs regularly since 2001 in Bangladesh and more recently in adjacent regions of India. In these areas, epidemiologic studies have not supported the role of intermediary domestic species as hosts; rather, direct transmission from a flying fox reservoir of the virus.
More recently, fruit bats belonging to the family Pteropidae; however, not of the genus Pteropus, were found to harbor related viruses in Africa. In 2014, an outbreak of Nipah virus infections occurred in horses and humans in Mindanao in the southern Philippines, suggesting that an etiologic scenario similar to that for Hendra virus in horses may occur with the Nipah virus.
Nipah virus infection in pigs is assumed to occur via transfer from the reservoir bat species to pigs. Once the virus is introduced into an intensive swine husbandry setting, infection of animals within the premises is rapid, and serologic tests suggested that nearly all pigs on an affected premise are infected. Transmission between premises is thought to be by poor biosecurity procedures and movement of infected animals. Experimental infection of swine with Nipah virus in a high biosecurity facility in Geelong support the thought that transmission between swine in close contact occurs readily.
In south Asia, infection of humans seems to occur by indirect means from the reservoir fruit bats; contamination of sap collected in pots on palm trees is a recurring means of infection in regular cases occurring in Bangladesh. Other similar circumstances, such as contact with trees contaminated by bats or consumption of fruit partially eaten by bats, are other documented means of infection in Bangladesh. Human-to-human transmission, although not evident in Malaysia, has also occurred in south Asia.
Because of the danger of human infection from infected pigs and the emergency setting, clinical observations were not detailed in the field during the original epidemic of Nipah virus infection. Most pigs developed a febrile respiratory disease with a severe cough that led to the local names for the disease―“barking pig syndrome” and “one-mile cough.” Encephalitis was also noted, particularly in the sows and boars in affected facilities. The proportion of animals with each form of the disease is uncertain, although the respiratory form predominated. Overall mortality within affected facilities was also not well documented but likely was not >5% among all age groups.
Laboratory diagnosis of Nipah virus infection can be made by isolation of the virus, identification of the RNA by use of reverse transcriptase-PCR assay, detection of antigens in tissues by immunohistochemical staining with specific antibodies, or serologic tests such as indirect ELISA and virus neutralization tests. The virus is considered biosafety level 4 in the US and Australia, and stringent laboratory containment at limited laboratories is a special consideration.
Treatment of swine with Nipah virus infection was not attempted during the Malaysian emergency. Human patients required intensive care with ventilation support to manage the encephalitis; no specific treatment is available. Ribavirin was administered to some patients, but subsequent studies in laboratory animals suggest that it is ineffective.
Control of the Nipah virus epidemic/epizootic in Malaysia was dependent on the initiation of strict quarantine procedures and the slaughter of all swine from affected facilities. Adherence to appropriate biosecurity and quarantine procedures within facilities, as with other contagious diseases, is of paramount importance in preventing spread of the infection. An active surveillance and slaughter program successfully eliminated the virus from the national commercial swine population, which has remained free of infection.
Presence of the virus in reservoir species of bats in a wide geographic range emphasizes the importance of good disease surveillance and biosecurity practices to promote early detection and confine the disease to initial premises should reintroduction occur.
A subunit vaccine based on Hendra virus has proved effective in preventing Hendra virus infections in horses, and the same antigen has been shown to be effective in preventing Nipah virus infections in experimental challenge in nonhuman primates. Similarly, a cross-reactive monoclonal antibody has been demonstrated to be an effective treatment for both Hendra and Nipah virus experimental challenge in a ferret model.
Transmission of Nipah virus from infected pigs to humans was mainly in an occupational setting, and a study of risk factors associated with human infection suggests that close contact with live infected swine was the means of infection of nearly all human Nipah virus infections in Malaysia.
Continued sporadic clusters in horses and subsequent human cases with Hendra virus in Australia, with serious disease in some of these clusters, emphasize the importance of use of appropriate personal protective equipment in veterinary clinical examinations or postmortem procedures when Hendra or Nipah virus infection is suspected.
Devnath P, Masud HMAA. Nipah virus: a potential pandemic agent in the context of the current severe acute respiratory syndrome coronavirus 2 pandemic. New Microbes New Infect. 2021;41:100873.