Nipah Virus-associated Encephalitis Outbreak, Siliguri, India

Abstract

During January and February 2001, an outbreak of febrile illness associated with altered sensorium was observed in Siliguri, West Bengal, India.Laboratory investigations at the time of the outbreak did not identify an infectious agent.Because Siliguri is in close proximity to Bangladesh, where outbreaks of Nipah virus (NiV) infection were recently described, clinical material obtained during the Siliguri outbreak was retrospectively analyzed for evidence of NiV infection.NiV-specific immunoglobulin M (IgM) and IgG antibodies were detected in 9 of 18 patients.Reverse transcription-polymerase chain reaction (RT-PCR) assays detected RNA from NiV in urine samples from 5 patients.Sequence analysis confirmed that the PCR products were derived from NiV RNA and suggested that the NiV from Siliguri was more closely related to NiV isolates from Bangladesh than to NiV isolates from Malaysia.NiV infection has not been previously detected in India.D uring January and February of 2001, an outbreak of febrile illness with altered sensorium was observed in Siliguri, West Bengal, India.Siliguri is an important commercial center with a population of ≈500,000.It is near borders with China, Bangladesh, Nepal, and Sikkim.The outbreak occurred among hospitalized patients, family contacts of the patients, and medical staff of 4 hospitals.Japanese encephalitis, which is endemic in this area, was initially suspected, but the age group affected and the epidemiologic features suggested a different disease.Laboratory investigations conducted at the time of the outbreak failed to identify an infectious agent (1).Nipah virus (NiV), a recently emergent, zoonotic paramyxovirus (2), was implicated as the cause of a highly fatal (case-fatality ratio 38%-75%), febrile human encephalitis in Malaysia and Singapore in 1999 (1) and in Bangladesh during the winters of 2001, 2003, and 2004 (3-6).The natural reservoir of NiV is presumed to be fruit bats of the genus Pteropus.Evidence of NiV infection was detected in these bats in Malaysia, Bangladesh, and Cambodia (7-10).In the Malaysian outbreak, NiV was introduced into the pig population, and most of the human cases resulted from exposure to ill pigs (2).However, an intermediate animal host was not identified during the Bangladesh outbreaks, which suggests that the virus was transmitted either directly or indirectly from infected bats to humans.Human-to-human transmission of NiV was also documented during the outbreak in Faridpur, Bangladesh (4,5).Because the clinical manifestations of the cases in Siliguri were similar to those of NiV cases in Bangladesh (3-6), and because Siliguri is near affected areas in Bangladesh, a retrospective analysis of clinical samples was undertaken to determine if NiV was associated with the Siliguri outbreak. Methods Case Definition and Clinical Sample CollectionA team of physicians and epidemiologists from the National Institute of Virology, Pune, India, along with local public health authorities, visited Siliguri.Investigations were conducted with the assistance of health authorities from West Bengal State and staff from the North Bengal Medical College Hospital.Medical records of patients who were hospitalized during the study period were examined, and their family members or caretakers were interviewed.Areas of the town in which cases occurred and the houses of patients who died were visited.Contacts and family members of patients who died were also interviewed.A broad working case definition was used for case detection.The case definition evolved over the course of the outbreak on the basis of information from case-patients admitted to different hospitals, review of the line list of patients, and interviews with contacts in the community.A suspected patient was one >15 years of age with acute Nipah Virus-associated EncephalitisOutbreak, Siliguri, India