ATLANTA - A previously unknown infectious agent is being blamed for an outbreak of encephalitis that has killed 111 people in Asia.
Preliminary data from epidemiological and laboratory investigations indicate a previously unrecognized paramyxovirus related to, but distinct from, the Australian Hendra virus is responsible for a total of 240 cases of encephalitic illnesses and two cases of respiratory illness. The virus has since been identified as a "cousin" to the Hendra virus and has been named the Nipah virus after the village in Malaysia where it was first isolated, according to Tom Skinner, spokesman for the Centers for Disease Control and Prevention (CDC).
Most of the cases have occurred in men working on pig farms in the Malaysian states of Perak and Negri Sembilan, so investigators said close contact with pigs appears to be necessary for human infection. No pediatric cases have been reported.
"It is assumed that the workers were infected with the virus through contact with pigs," Skinner added, "but it is unclear exactly how they were infected."
Illness has been characterized by three to 14 days of fever and headache, followed by drowsiness and disorientation that can progress to coma within 24-48 hours. No cases have been reported among health care workers caring for these patients, according to a recent report from the CDC.
"It's likely that is has been around for a while, but for whatever circumstances has now surfaced and has been identified. Now we are trying to learn as much as we can about it."
Skinner said the CDC has sent a team of eight epidemiologists to Malaysia and two in Singapore conducting case-control studies. Health care workers are also being evaluated to identify potential human-to-human transmission. "There doesn't appear to be any at this point," he said.
Three clusters of illnesses have been identified in Malaysia. A case of suspected illness was defined as fever, severe headache, myalgia and signs of encephalitis or meningitis.
The cases have primarily been limited to two states within Malaysia. The first cluster began in late September 1998 near the city of Ipoh, Perak, and continued to occur in the region until early February. The second cluster occurred near the city of Sikamat, Negri Sembilan in December 1998 and into January. The third, and largest, cluster began near Bukit Pelandok, also in Negri Sembilan, in December 1998. Two additional cases have occurred in the state of Selangor.
Cases have occurred primarily among adult men with histories of close contact with swine. Concurrent with human cases, illness and death occurred among swine from the same regions, according to the CDC. Japanese encephalitis (JE) virus was identified as the probable etiologic agent for the outbreak, and specimens from some patients tested positive for JE; however, the predominance of cases in men who had close contact with swine suggested the possibility of another agent.
The 11 case patients in Singapore all had handled swine imported from Malaysia, including the one person who died. Serologic testing at CDC confirmed recent Hendra-like virus infection in these 11 workers and limited nucleotide sequence studies of the virus from the patient who died suggest it is identical to that from the Malaysia outbreak.
Laboratory testing in both Malaysia and at the CDC demonstrated virus-like structures consistent with a paramyxovirus, and immunofluorescence tests of cells infected with this virus suggested a virus related to Hendra virus.
The studies also demonstrated Hendra-virus IgM antibodies in serum specimens of 23 (88%) of 26 cases tested; in addition, Hendra-like antigens were detected in central nervous system tissue from four of five case-patients and from lung and kidney tissues of no case-patient tested. Tissues from swine from affected farms also test positive for Hendra-like antigens, according to the CDC report.
Hendra virus was first isolated in September 1994 after a respiratory illness occurred among 20 horses and two humans in Hendra, Queensland, Australia; 13 horses and one person died.
Studies are currently underway to confirm the source of human infection and to determine risk, if any, for human-to-human transmission among health care workers and family members of the victims, in addition to active surveillance for encephalitis cases. Investigators are also trying to define specific risk factors associated with exposures to pigs and tissue from infected animals and to determine the case-to-infection ratio and the epidemiology of the infection in pigs.
Malaysian authorities have banned the transport of pigs within the country to prevent further outbreaks. Malaysian health officials have recommended those in the affected areas who have exposure to pigs to use protective equipment and clothing.
Singapore and Thailand have banned importation of pigs from Malaysia, and Singapore has also banned horses returning from Malaysia.
For more information:
- CDC. Outbreak of Hendra-like virus - Malaysia and Singapore, 1998-9999. MMWR 1999;48(13):265-269.
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