Nipah virus

Nipah virus, first identified in Malaysia in 1999, is a zoonotic virus that can cause disease in humans and animals; it is an overlap agent under the Select Agent Program due to high lethality and a wide range of susceptible hosts. It is in CDC Bioterrorism Agents-Disease list category C.

Several zoonotic and vectorborne viral diseases have emerged, since the mid-1990s, in Southeast Asia and the Western Pacific, among them Nipah virus, which caused an outbreak of fatal pneumonia in pigs and encephalitis in humans in the Malay Peninsula.

Ecology

The virus is named after the location where it was first detected in Malaysia.

Reservoir

It is currently believed that certain species of fruit bats are the natural hosts of both Nipah and Hendra viruses. They are distributed across an area encompassing northern, eastern and south-eastern areas of Australia, Indonesia, Malaysia, the Philippines and some of the Pacific Islands. The bats appear to be susceptible to infection with these viruses, but do not themselves become ill. It is not known how the virus is transmitted from bats to animals.

Transmission

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 body fluids from infected animals. Nipah antibodies have been detected in pigs, other domestic and wild animals. The role of species other than pigs in transmitting infection to other animals has not yet been determined.

Taxonomy and genetics

Hendra and Nipah viruses have been placed into a new genus of the family Paramyxoviridae, Henipavirus.

They are single-stranded RNA viruses, approximately 15000 nucleotides long.

Formerly in the genus Morbillivirus, now classified in a new genus along with the similar Hendra virus (70% to 88% nucleotide homology, 67% to 92% amino acid homology): Henipavirus (see References: Bossart 2002, Mackenzie 2001)

Laboratory presentation

Morphology

• Helical
• Enveloped with distinct surface projections
• 150 to 200 nm in diameter; 10,000 to 10,040 nm long
• Spherical or filamentous, but pleomorphic forms occur

Clinical features

It is unlikely that Nipah virus is easily transmitted to man, although previous outbreak reports suggest that Nipah virus is transmitted from animals to humans more readily than Hendra virus. Despite frequent contact between fruit bats and humans there is no serological evidence of human infection among bat carers. 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.

The incubation period is between 4 and 18 days. In many cases the infection is mild or inapparent (sub-clinical). In symptomatic cases, the onset is usually with "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 clinically apparent cases die.