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Update: Outbreak of Nipah Virus -- Malaysia and Singapore, 1999

During March 1999, health officials in Malaysia and Singapore, in collaboration with Australian researchers and CDC, investigated reports of febrile encephalitic and respiratory illnesses among workers who had exposure to pigs (1). A previously unrecognized paramyxovirus (formerly known as Hendra-like virus), now called Nipah virus, was implicated by laboratory testing in many of these cases. Febrile encephalitis continues to be reported in Malaysia but has decreased coincident with mass culling of pigs in outbreak areas. No new cases of febrile illness associated with Nipah virus infection have been identified in Singapore since March 19, 1999, when abattoirs were closed. This report summarizes interim findings from ongoing epidemiologic and laboratory investigations in Malaysia and Singapore.


As of April 27, 1999, 257 cases of febrile encephalitis were reported to the Malaysian Ministry of Health (MOH), including 100 deaths. Laboratory results from 65 patients who died suggested recent Nipah virus infection. Since April 4, new encephalitis cases have been reported in the states of Negeri Sembilan and Selangor. However, the number of new cases reported decreased from a peak of 46 during March 13-19 to four during April 10-16 (Figure_1).

The apparent source of infection among most human cases continues to be exposure to pigs. Of 65 serologically confirmed cases of Nipah virus-associated encephalitis in Negeri Sembilan, 56 (86%) case-patients reported touching or handling pigs before onset of illness. Of the 56 case-patients, 36 (64%) reported contact with pigs that appeared to be ill.

Human-to-human transmission of Nipah virus has not been documented. In a survey of nurses and physicians who cared for encephalitis patients during the outbreak and pathologists who conducted postmortem examinations of case-patients, none developed an encephalitic illness or had acute serologic evidence confirming recent Nipah virus infection. To further define risk factors for human transmission, other groups being surveyed include case-patients and their families, pig workers, abattoir workers from 10 Malaysian states, soldiers involved in pig culling, and veterinary workers with potential exposure to Nipah virus-infected animals.

Outbreak control in Malaysia has focused on culling pigs in the states of Perak, Negri Sembilan, and Selangor; approximately 890,000 pigs have been killed. Other measures include a ban on transporting pigs within the country, education about contact with pigs, use of personal protective equipment among persons exposed to pigs, and a national surveillance and control system to detect and cull additional infected herds.

Field and laboratory studies have been initiated to investigate the potential for Nipah virus infection among animal species other than commercially raised pigs. Lung, kidney, spleen, and heart tissues from one necropsied dog demonstrated positive immunohistochemical staining using hyperimmune Hendra antibodies. Virus was isolated from kidney and liver tissues from this dog. Nucleotide sequencing of product from reverse transcriptase polymerase chain reaction amplification of RNA extracted from these tissues confirmed Nipah virus infection.


No new cases of febrile illness associated with Nipah virus have been documented in Singapore after pig importation from Malaysia ceased and abattoirs were closed on March 19. During March 13-19, 11 abattoir workers developed febrile encephalitic or respiratory illnesses associated with acute Nipah virus infection. Epidemiologic investigations are under way to determine risk factors for Nipah-associated illness among abattoir workers in Singapore, and laboratory studies among abattoir, laboratory, and health-care workers are continuing to determine whether Nipah virus exposure may have led to mild or asymptomatic illness.

Reported by: Vector-Borne Disease Control Section, Disease Control Div, Institute for Medical Research, Ministry of Health; Dept of Medical Microbiology; Univ Hospital; Univ of Malaya; General Hospital, Kuala Lumpur; Seremban Hospital, Seremban; Ipoh Hospital, Ipoh; Institute of Veterinary Research, Veterinary Svc, Ministry of Agriculture, Malaysia. Primary Production Dept, Ministry of National Development; Quarantine and Epidemiology Dept, Ministry of the Environment, Singapore. Australian Animal Health Laboratory, Geelong, Queensland; Animal Research Institute, Queensland Dept of Primary Industries, Australia. Western Pacific Regional Office, World Health Organization, Manila, Philippines. Respiratory and Enterovirus Br, Special Pathogens Br, and Infectious Diseases Pathology Activity, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note

Editorial Note: The absence of new Nipah virus cases in Singapore in the month since abattoirs were closed and the decrease in new encephalitis cases in Malaysia following the institution of measures to limit human contact with pigs suggest that pigs are the primary source of Nipah virus among infected humans in this outbreak. Investigations continue to define risk factors for infection and disease in humans to determine the modes of Nipah virus transmission between animals and from animals to humans and to identify the primary reservoir of this virus.


  1. CDC. Outbreak of Hendra-like virus -- Malaysia and Singapore, 1998-1999. MMWR 1999;48:265-9.


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