Nipah virus--information from the World Health Organization.

Overview

Nipah virus is a newly recognized zoonotic virus. The virus was 'discovered' in 1999. It has caused disease in animals--and in humans, through contact with infectious animals. The virus is named after the location where it was first detected in Malaysia. It is closely related to another zoonotic virus that was recognized in 1994, the Hendra virus, named after the town where it first appeared, in Australia. Both Nipah and Hendra are members of the virus family Paramyxoviridae. Although members of this group of viruses have caused only a few local outbreaks, their biological properties--the ability to infect a wide range of hosts and to produce a disease causing significant mortality in humans--has made them an emerging public-heath concern.

Natural Hosts

It is currently believed that certain species of fruit bats are the natural hosts of both Nipah and Hendra viruses. These bats are distributed across an area encompassing northern, eastern and southeastern areas of Australia, Indonesia, Malaysia, the Philippines, and some of the Pacific Islands. The bats appear to be susceptible to infection, but do not themselves become ill.

Transmission

It is not known how the virus is transmitted from bats to animals. The mode of transmission from animal to animal, and from animal to human, is uncertain, but appears to require close contact with contaminated tissue or with body fluids from infected animals. Nipah antibodies have been detected in pigs and other domestic and wild animals. The role of species other than pigs in transmitting infection to other animals has not yet been determined.

It is unlikely that Nipah virus is easily transmitted to humans, although previous outbreak reports suggest that it is transmitted more readily than Hendra virus. Despite frequent contact between fruit bats and humans, there is no serological evidence of human infection among people who work with bats. Pigs were the apparent source of infection among most human cases in the Malaysian outbreak of Nipah, but other sources, such as infected dogs and cats, cannot be excluded. Human-to-human transmission of Nipah virus has not been reported.

Clinical Features

The incubation period is between four and 18 days. In many cases, the infection is mild or not apparent (subclinical). In symptomatic cases, the onset usually takes the form of influenza-like symptoms, with high fever and muscle pains (myalgia). The disease may progress to inflammation of the brain (encephalitis) with drowsiness, disorientation, convulsions, and coma. Fifty percent of victims with clinically apparent cases die. Treatment: No drug therapies have yet been proven to be effective in treating Nipah infection. Treatment relies on providing intensive supportive care. There is some evidence that early treatment with the antiviral drug, ribavirin, can reduce both the duration of feverish illness and the severity of disease. However, the efficacy of this treatment in curing disease or improving survival is still uncertain.

Protection of Health Care Professionals

The risk of transmission from sick animals to humans is thought to be low, and transmission from person to person has not yet been documented, even in the context of a large outbreak. Therefore, the risk of transmission to health care workers is thought to be low. Nevertheless, transmission without percutaneous exposure is theoretically possible, as respiratory secretions contain the virus. For this reason, Nipah virus has been categorized as a biohazard that should be managed in a high-level biosecurity laboratory. It is recommended that close contact with body fluids and infected tissues be avoided if Nipah infection is suspected.

Outbreaks of Nipah and Hendra Viruses

From September 1998 to April 1999, there was a large outbreak of encephalitis in Malaysia. During the investigation of this outbreak, Nipah virus, a previously unrecognized virus, was identified as the causal agent. A total of 265 people were infected, of whom 105 died. Ninety-three percent of victims had had occupational exposure to pigs. An associated outbreak among abattoir workers in Singapore during March 1999 involved 11 cases and one death. These workers had been handling pigs that had been imported from the outbreak areas in Malaysia.

There have been three recognized outbreaks of Hendra virus in Australia, in 1994, 1995, and 1999. Three human cases, leading to two deaths, were recorded in the 1994 and 1995 outbreaks; the precise mode of virus transmission is not fully understood. All three victims appear to have acquired their infections as a result of close contact with horses that were ill and later died.