Outbreak of Nipah encephalitis among pig farm workers in Malaysia in 1998/1999: Was there any role for Japanese encephalitis?

A study to determine the possible role of Japanese encephalitis in the Nipah encephalitis outbreak among pig-farm workers in Malaysia in 1998/1999 was done. Two hundred and twelve patients who fulfilled the criteria of Nipah encephalitis from three epicentres during the outbreak were retrospectively analysed. Twenty-nine of these patients (14%) had positive IgM serology for Japanese encephalitis, and of these, 20 patients (9%) had positive IgM serology in the cerebrospinal fluid. The 29 patients with positive IgM Japanese encephalitis serology and 183 patients with negative serology were compared. There was no significant difference in the two groups for demographic, clinical features, laboratory findings and outcome. None of the 33 fatal cases who had post mortems showed positive immunohistochemical staining for Japanese encephalitis. There was thus no evidence that the encephalitis outbreak was due to either Nipah virus or Japanese encephalitis virus; or that a patient may have suffered encephalitis concurrently due to both viruses. There was also no evidence that a subclinical Japanese encephalitis infection have aggravated a patient with Nipah encephalitis. The positive Japanese encephalitis IgM serology probably reflected the endemic Japanese encephalitis infection, which was asymptomatic. The break down in blood brain barrier associated with disseminated microinfarction seen in Nipah encephalitis possibly contributed to the high rate of positive Japanese encephalitis IgM serology in the cerebrospinal fluid. After discovery of the novel Nipah virus among some of the fatal cases 3 which suggest Nipah virus as the main culprit in the outbreak, the possibilities remained that the outbreak could be due to either of the two viruses, Nipah virus and Japanese encephalitis virus; or a patient may have suffered encephalitis concurrently from both viruses; or a subclinical Japanese encephalitis infection could have aggravated a patient with Nipah virus infection. The possible roles of Japanese encephalitis is reflected in the official reports of the outbreak from the various ministries of the government of Malaysia, which continues to label the outbreak as “Japanese/Nipah encephalitis”. 8 This paper aims to address the significance of positive Japanese encephalitis IgM serology in these patients during the outbreak.