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SummaryBackgroundIn March 1999, an outbreak of encephalitis and pneumonia occurred in workers at an abattoir in Singapore. We describe the clinical presentation and the results of investigations in these patients.MethodsClinical and laboratory data were collected by systemic review of the case records. Serum and cerebrospinal fluid (CSF) samples were tested for IgM antibodies to Nipah virus with an IgM capture ELISA. Reverse-transcriptase PCR was done on the CSF and tissue samples from one patient who died.FindingsEleven patients were confirmed to have acute Nipah-virus infection based on raised IgM in serum. Nipah virus was identified by reverse-transcriptase PCR in the CSF and tissue of the patient who died. The patients were all men, with a median age of 44 years. The commonest presenting symptoms were fever, headache, and drowsiness. Eight patients presented with signs of encephalitis (decreased level of consciousness or focal neurological signs). Three patients presented with atypical pneumonia, but one later developed hallucinations and had evidence of encephalitis on CSF examination. Abnormal laboratory findings included a low lymphocyte count (nine patients), low platelet count, low serum sodium, and high aspartate aminostransferase concentration (each observed in five patients). The CSF protein was high in eight patients and white-blood-cell count was high in seven. Chest radiography showed mild interstitial shadowing in eight patients. Magnetic resonance imaging (MRI) showed focal areas of increased signal intensity in the cortical white marker in all eight patients who were scanned. The nine patients with encephalitis received empirical treatment with intravenous aciclovir and eight survived.Interpretation Infection with Nipah virus caused an encephalitis illness with characteristic focal areas of increased intensity seen on MRI. Lung involvement was also common, and the disease may present as an atypical pneumonia. [ABSTRACT FROM AUTHOR]