An outbreak of viral encephalitis occurred among pig industry workers in Malaysia in September 1998 to April 1999. The encephalitis was attributed to a new paramyxovirus, Nipah virus. This is a description of the clinical features of 103 patients treated in the Seremban Hospital with characterization of the prognostic factors.

Clinical case records and laboratory investigations were reviewed. The case definition was: patients from the outbreak area, direct contact or in close proximity with pigs, clinical or CSF features of encephalitis.

The mean age was 38 years, 89% were male, 58% were ethnic Chinese, 78% were pig farm owners or hired workers. The mean incubation period was 10 days. The patients typically presented with nonspecific systemic symptoms of fever, headache, myalgia and sore throat. Seizures and focal neurological signs were seen in 16% and 5% respectively. In the more severe cases, this was followed by drowsiness and deteriorating consciousness requiring ventilation in 61%. Autonomic disturbances and myoclonic jerks were common features. The mortality was high at 41%. Systolic hypertension, tachycardia and high fever were associated with poor outcome. On the other hand, 40% recovered fully. As for the other 19%, the residual neurological signs were mostly mild.

Nipah virus caused an encephalitis illness with short incubation period and high mortality. The prognosis for the survivors was good.