Nipah virus disease

Nipah virus disease

P W DANIELS, S SHAHIRUDIN, J AZIZ AND B L ONG
Nipah virus disease

Introduction

Nipah virus disease is caused by an RNA virus in the family Paramyxoviridae, genus Henipavirus. It has occurred only in Southeast Asia, where fruit bats of the genus Pteropus are thought to be the reservoir host of the virus. It is one of the most important emergent viral zoonotic diseases, with a case fatality rate in humans in excess of 60 per cent. It is generally an acute febrile disease. In humans neurological signs predominate, while in pigs respiratory signs manifested by paroxysmal coughing and open-mouth breathing are prominent. Adult pigs may die suddenly and sows abort, but many infections of pigs are asymptomatic. In dogs and cats the infection is pantropic with a high case fatality rate. A case of encephalitis has been diagnosed in a horse.

Although Nipah virus disease was confined to a relatively small geographic area in Southeast Asia, it was associated with significant human mortality. According to current understanding the virus has crossed once from a wildlife reservoir to cause an outbreak in a species of domestic animal, the pig, among which it was highly contagious. This chapter describes the circumstances of that outbreak, the clinical characteristics of the disease in a number of animal species and humans, and the measures that were successfully undertaken and which resulted in the control and ultimate eradication of the disease.

Cases of an unusual encephalitis were identified in pig farm workers in the Malaysian state of Perak in 1998, although, retrospectively, earlier human infections were diagnosed. In the latter months of 1998 the number of cases in Perak increased, and a number of humans died. Human cases started to occur in more southerly areas of Peninsular Malaysia and, as shown by the epidemic curve,1 rapidly escalated into a major epidemic in February, March and April 1999.

In March 1999 a paramyxovirus was isolated from human cases and identified as being related to Hendra virus,8 a paramyxovirus involved in an outbreak of fatal disease of horses and deaths in humans in Australia26 (see Hendra virus infection). The virus was shown to be novel, and was called Nipah virus7, 8 after the village of Sungai Nipah, where the patients, from whom the virus was isolated, had lived. In the ensuing investigation 1, 2, 4, 7, 10, 12, 21, 22, 29, 31 the virus isolated fromhumanswas shown to be widely disseminated among pigs on the farms on which the human cases had occurred, and also to have infected dogs, cats and horses.

Among these latter species the virus was fortunately not contagious but, nonetheless, it was associated with a high case fatality rate. Various species of bats, particularly Pteropus spp., were identified as probable wildlife reservoir hosts. The progress of the epidemic was halted by quarantine and culling of pigs on known infected pig farms as well as on those suspected to be infected.Human infection was shown to result from close contact with infected pigs.A diagnostic capability to detect infection in animals13 and a programme of serological surveillance designed to detect any other infected pig farms were established.30

No new human cases of Nipah virus disease have been diagnosed since May 1999, and no newly infected pig farms have been detected since that time.29 The last detection of seropositive pigs was in May 2000, and in June 2001 the OIE recognized that Nipah virus infection had successfully been eradicated from the Malaysian pig population.3

Nipah virus remains of concern to both veterinary and public health agencies in Southeast Asia. Bats of the genus Pteropus are distributed in many countries from Madagascar across Southeast Asia to Australia and the Pacific Islands. There remains the possibility that further outbreaks of this zoonotic disease may occur as a result of spill-over of the virus from bats to domestic animals. Research is needed to better define the geographic areas where such a risk exists, and the countries likely to be involved are developing emergency preparedness based on rapid diagnostic capability and appropriate levels of surveillance.10, 11

Aetiology

Nipah virus is a novel negative-stranded RNA virus in the family Paramyxoviridae, genus Henipavirus. The latter is a newly recognized genus that includes Nipah and Hendra viruses.7 Isolates of Nipah virus from humans and affected animals showed homology at the nucleotide level.7 Human isolates have been shown to produce Nipah virus disease experimentally in pigs and cats.24

Epidemiology

According to the hypothesis developed during the outbreak investigation, Nipah virus exists as an infection of some wild pteropid bat populations that on one occasion spilled over into domestic pigs.7, 10, 14, 21

The evidence for the association with pteropid bats is three-fold. A significant proportion of wild-caught animals from the two species present in Malaysia, Pteropus vampirus and Pteropus hyomelanus, had neutralizing antibodies to Nipah virus,21 the Nipah virus has been isolated from wild populations of P. hyomelanus in Malaysia,9 and an Australian species of pteropid bat (Pteropus poliocephalus) has been infected with Nipah virus experimentally and shown to seroconvert and to excrete the virus.25

There is no information on how the infection from bats to pigs occurred. Fruit trees grew in close...

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