Nipah Virus Infection: Introduction
(Porcine respiratory and neurologic syndrome, Barking pig syndrome) |  |
| Nipah virus disease is a newly discovered disease of swine and humans associated with infection with a new paramyxovirus given the name Nipah virus. This disease emerged in Malaysia in 1998 and 1999. It was linked to severe encephalitis among humans occupationally exposed to infected pigs in Malaysia and Singapore. The disease was eradicated from the national commercial swine population by control efforts. Fruit bats of the genus
Pteropus
appear to be reservoirs of the virus. |
| Etiology and Epidemiology: |
| 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 (
Hendra Virus Infection: Introduction), the only other member of the genus. The human outbreak in Malaysia and Singapore followed contact with infected swine and resulted in encephalitis and ~40% case mortality. The virus
is assumed to have been introduced into the swine population from 1 of the 2 species of
Pteropus
found with Nipah virus antibodies during investigation of the outbreak. The geographic range of
Pteropus
includes all of southeast and south Asia, and several species have been found with antibodies, 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 virus from human and swine clinical materials strongly supported a single introduction of the virus with spread through the commercial swine population. There was evidence of infection among several other species of domestic
animals including dogs, cats, and horses. |
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| Transmission and Pathogenesis: |
| Infection in pigs is assumed to have been a transfer from the reservoir bat species to pigs. Once the virus was introduced into an intensive swine husbandry setting, infection of animals within premises was rapid, and serologic tests suggested that nearly all pigs on an infected premise were infected. Transmission between premises was 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 supported that transmission between swine in close contact occurred readily. |
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| Clinical Findings: |
| Because of the danger of human infection and the emergency setting, clinical observations were not detailed in the field during the epidemic. 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 it probably was not >5% among all age groups. |
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| Diagnosis: |
| Laboratory diagnosis can be made by isolation of the virus, identification of the RNA by use of reverse transcriptase-PCR, 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 USA and Australia, and stringent laboratory containment at limited laboratories is a special consideration. |
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| Treatment: |
| Treatment of affected swine was not attempted during the Malaysian emergency. Humans required intensive care with ventilation support to manage the encephalitis; no specific treatment is available. Ribavirin was administered to some patients and results of one study suggested that it may reduce mortality, but subsequent studies in laboratory animals suggest that it is ineffective. |
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| Control and Prevention: |
| Control of the 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 and it
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. |
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| Zoonotic Risk: |
| Transmission of the virus from infected pigs to humans was largely in an occupational setting, and a study of risk factors associated with human infection suggests that close contact with live infected swine is the means of infection of nearly all human Nipah virus infections. |
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