Nipah Virus Encephalitis Outbreak in Malaysia

Abstract

Emerging infectious diseases involving zoonosis have become important global health problems. The 1998 outbreak of severe febrile encephalitis among pig farmers in Malaysia caused by a newly emergent paramyxovirus, Nipah virus, is a good example. This disease has the potential to spread to other countries through infected animals and can cause considerable economic loss. The clinical presentation includes segmental myoclonus, areflexia, hypertension, and tachycardia, and histologic evidence includes endothelial damage and vasculitis of the brain and other major organs. Magnetic resonance imaging has demonstrated the presence of discrete high-signal-intensity lesions disseminated throughout the brain. Nipah virus causes syncytial formation in Vero cells and is antigenically related to Hendra virus. The Island flying fox (Pteropus hypomelanus; the fruit bat) is a likely reservoir of this virus. The outbreak in Malaysia was controlled through the culling of >1 million pigs.

Emerging infectious diseases, especially those involving zoonosis, have become an important global health problem. This is borne out by the recent outbreaks of avian influenza in Hong Kong and of West Nile virus encephalitis in New York. The outbreak of Nipah virus encephalitis in Malaysia in 1998, which was caused by a newly emergent and deadly virus, is yet another example. This article summarizes the epidemiology of and the clinical and laboratory findings from the Nipah virus outbreak, which caused tremendous human suffering and economic loss.

Epidemiology

An outbreak of severe febrile encephalitis among pig farmers that was associated with a high mortality rate was first reported in the state of Perak, Malaysia, in September 1998. At first the deaths were thought to be due to Japanese encephalitis (JE), a disease that is endemic in Malaysia and that occurs sporadically. However, the epidemiologic characteristics of the disease were distinct from those of JE. Most of the cases occurred in men who worked with pigs, and very few case patients were young children, whereas JE virus, which is mosquito-borne, has no association with a particular occupation and is most common among young children. Mosquito control and JE virus vaccination programs did not affect the course of the outbreak. Illnesses and deaths among infected pigs eliminated the possibility that JE virus was the cause.

By February 1999, similar diseases were recognized in pigs and humans in other states in Malaysia; this was the result of the movement of infected pigs into the new outbreak areas [1, 2]. In March 1999, a cluster of 11 cases of respiratory illnesses and encephalitis was noted in Singapore among abattoir workers who handled pigs that had come from the outbreak areas in Malaysia [3]. The outbreak in Singapore ended when importation of pigs from Malaysia was prohibited, and the outbreak in Malaysia ceased when >1 million pigs were culled from the outbreak area and immediately surrounding areas [4, 5]. A total of 265 cases of encephalitis, from which 105 deaths resulted, were associated with the outbreak in Malaysia.

Clinical Features

The clinical features of Nipah virus infection are different from the features of infection caused by Hendra virus, to which Nipah is antigenically related. Hendra virus is associated with greater respiratory involvement, and only a single case of severe meningoencephalitis caused by Hendra virus has been reported. The clinical features of Nipah virus infection in 94 patients who were admitted to the University Hospital Kuala Lumpur in 1998 have been published elsewhere [6]. The mean age of these patients was 37 years, and the ratio of male patients to female patients was 4.5:1. Ninety-three percent of the patients had direct contact with pigs, usually in the 2 weeks before the onset of illness. The main features at presentation were fever (in 91 patients; 97%), headache (61 patients; 65%), dizziness (34 patients; 36%), and vomiting (25 patients; 27%). Fifty-two patients (55%) had a reduced level of consciousness and prominent brain stem dysfunction. Distinctive clinical signs included segmental myoclonus, areflexia and hypotonia, hypertension, and tachycardia, which suggests that the brain stem and the upper cervical spinal cord were involved. Thirty patients (32%) died after their condition rapidly deteriorated. An abnormal doll's eye reflex and tachycardia were associated with a poor prognosis. Fifty patients recovered fully, and 14 more recovered but had persistent neurologic deficits.

Clinical relapse was seen in 12 of the 64 surviving patients, none of whom were reexposed to infected pigs, 1 year after the initial outbreak. In these patients, either neurologic symptoms reappeared after an initial illness, or there was a long latency period between the initial finding of seropositivity after exposure to the virus and the development of neurologic symptoms. The onset of symptoms during relapses was rapid; symptoms and signs were fever, headache, focal neurologic signs, seizure, dizziness, reduced consciousness, and myoclonus.

Histopathology

Samples collected at autopsy from patients with positive results of cultures for Nipah virus showed histologic findings of endothelial damage and vasculitis, mainly in arterioles, capillaries, and venules [7]. The brain was the most severely affected organ, but other organs, including the lungs, the heart, and the kidneys, were also affected. Vasculitis of blood vessels was characterized by vessel-wall necrosis, thrombosis, and inflammatory cell infiltration of neutrophils and mononuclear cells. Syncytial cell formation was seen in the endothelium of affected blood vessels in the brain and lungs and in the Bowman's capsule of the glomerulus. Zones of microinfarction and ischemia were commonly found surrounding or adjacent to vasculitic blood vessels. In the brain, many neurons had eosinophilic cytoplasmic and nuclear viral inclusions, as is seen in association with other paramyxovirus infections.

Imaging Features

Virologic Features

In early March 1999, Vero cells inoculated with CSF specimens from 3 patients with fatal cases of encephalitis developed syncytia within 5 days [9]. The infected cells did not react with antibodies to known paramyxoviruses or other encephalitic viruses, including JE virus. However, these cells stained positively with antibodies against Hendra virus on indirect immunofluorescence assay. Cross-neutralization studies found an 8–16-fold difference in levels of antibodies to this virus, named “Nipah virus,” and levels of antibodies to Hendra viruses, which indicates that the viruses, although related, are not identical.

Electron microscopic (EM) studies of the Nipah virus demonstrated features characteristic of the family Paramyxoviridae [10]. Viruses of this family typically possess a single-stranded nonsegmental RNA genome of negative polarity that is fully encapsidated by protein. Virus particles vary in size from 120 to 500 nm. Thin-section EM studies of infected cells revealed filamentous nucleocapsids within cytoplasmic inclusions and incorporated into virions budding from the plasma membrane. Typical “herringbone” nucleocapsid structures were observed by means of negative-stain preparations.

In initial reverse-transcriptase PCR experiments, the entire nucleoprotein (N) gene and 700 nucleotides from the 3′ terminus of the phosphoprotein (P) gene were amplified [11]. Nipah virus differed from Hendra virus by 21% and 25% at the nucleotide level in the N and P genes, respectively. The predicted sequence of the N protein of Nipah virus differed from that of Hendra at 42 amino acid positions (8.0%). These findings demonstrate that Hendra virus and Nipah virus represent a unique genus within the Paramyxoviridae family. This is further supported by a study that showed that the Nipah virus genome is closely related to that of Hendra virus [12].

Virus Reservoir

Evidence of infection and, in most cases, disease was demonstrated in domestic species other than the pig, notably, cats, dogs, horses, and goats. It is believed that infected pigs were the source of infection for these species and that all but pigs are effectively “dead-end” hosts. Serum samples from rats trapped in the infected area have all yielded negative results of testing for Nipah virus.

Because of the similarity between Nipah virus and the Australian Hendra virus, it is suspected that flying foxes (fruit bats) are a natural reservoir of Nipah virus, as they are for Hendra virus. Preliminary data from surveillance of wildlife species showed the presence of neutralizing antibodies against Nipah virus mainly in the Island flying foxes (Pteropus hypomelanus) and the Malayan flying foxes (Pteropus vampyrus) [13]. This was confirmed virologically by the isolation of Nipah virus from the urine of the Island flying foxes [14]. The isolates were identified by immunofluorescence assay, using convalescent-phase serum samples from humans infected with Nipah virus, as well as Hendra virus-specific hyperimmune mouse ascitic fluid. The identification was confirmed by selective amplification of the virus genome by PCR, using 10 pairs of Nipah virus-specific primers, which yielded PCR products of the expected size and sequencing for all 10 PCR amplicons.

Control Measures

When JE virus was believed to be the cause of the outbreak in Malaysia, the government response was to control mosquito vectors by fogging in the pig farms and, at the same time, to offer immunization against JE virus to individuals at risk. These measures did not prevent the spread of the disease to other states, and they were stopped once Nipah virus had been identified as the cause of the outbreak.

Health education and health advice to people working in pig farms were given through news media and electronic media such as radio and television. Personal protection, using masks, goggles, gloves, gowns, and boots, was advocated. Because Nipah virus is a member of the Paramyxoviridae family, the virus is quite labile and is easily killed with readily available detergents. Advice was given about hand-washing after handling of infected animals and pigsties, and cages and vehicles for transporting animals were washed down with soap and water. Health care workers strictly adhered to universal precautions in the management of severely ill patients.

A 2-phase pig-culling operation was conducted that included all infected pig farms in the outbreak areas. Phase I involved culling in areas where outbreak cases occurred. More than 1 million pigs were culled. Phase II involved surveillance in all pig farms throughout the country. This process was carried out for 3 months; farms at which ⩾3 samples had positive results of testing for Nipah virus were considered to be positive farms, and all pigs at the affected farms and at farms within a 500-m radius were culled.

The discovery of the Nipah virus in flying foxes has important implications for the reorganization of the pig-rearing industry in Malaysia. The virus is found in the urine and saliva of infected flying foxes, and pigs consuming foodstuffs contaminated by these secretions can be infected. Pig farms should not be located close to fruit orchards, and fruit trees that attract flying foxes should not be grown on such farms.

Conclusion

The outbreak of Nipah virus encephalitis in Malaysia, a developing country, has taught us many valuable lessons. The initial supposition that JE virus was the cause of disease was incorrect, and much time and effort were wasted in control of vectors associated with and vaccination against JE virus. The rapid identification of the new virus required the assistance of international agencies, which was much appreciated by the Malaysian government.

We now have a good case definition for Nipah virus encephalitis, not only for humans, but also for pigs. This will make monitoring of the disease much easier in the future. The steps taken to control the spread of the disease were found to be effective, and that information will also prove to be useful. As is the case for other zoonotic infections, culling may be the most cost-effective and rapid measure for halting the spread of the disease.

A system of surveillance in pig farms has been instituted, and this will provide an early warning should the disease resurface. The discovery of the natural reservoir of the virus will greatly help in the reorganization of pig farms to prevent the reintroduction of Nipah virus into pig populations.

References