Background
 
Nipah virus (NiV) first emerged in Malaysia in 1998, infecting 265 people and resulting in more than 100 deaths in a single outbreak. This lethal virus (case fatality rate >40%) was first identified in pigs and pig farmers. It spread quickly as infected pigs were bought and sold, eventually infecting abattoir workers in Singapore. The Malaysian government was forced to stop pig exports and culled almost half of its pig population, costing the industry millions of dollars but arresting the outbreak.

Joint investigations suggest a complex ecology for Nipah virus. There is evidence that dogs, cats, horses, and goats were also infected. The virus's sequence is closely related to that of Hendra virus, which emerged in Australia in 1994. Both Hendra and Nipah have been described as members of a new genus of paramyxovirus: Henipavirus. Researchers found that the four Australian mainland species of fruit bats carried antibodies to Hendra virus, and one carried virus in uterine tissue. Similar surveys of Malaysian bats have found Nipah virus antibodies in at least five species, including two that roosted near infected piggeries. Drs. Lam and Chua of the University of Malaya isolated the virus from the urine of one species, Pteropus hypomelanus, and from partially chewed fruit.

Outbreaks of Nipah-like viruses have continued to occur over the past 4 years in South Asia resulting in the loss of human lives. The most recent outbreak occurred in Bangladesh in January, 2004 with a case fatality rate of nearly 80% (ICDDR,B 2003). Conditions for the outbreak in Bangladesh appear to be different from the Malaysian outbreak in that no intermediate animal host has been identified (e.g. pigs). Based on epidemiologic studies conducted by the World Health Organization and Bangladesh's Center for Health and Population Research (ICDDR,B), it appears likely that human to human transmission of the virus occurred (ICDDR,B 2003).

Current Research

Under a grant from the NIH Fogarty International Center's Emerging Infectious Disease Initiative, the Consortium has begun a major collaborative project on Nipah virus with researchers from the CDC, CSIRO, University of Malaya, Queensland DPI, Veterinary Research Institute, Princeton University, Harvard Medical School, and Institute of Zoology (London).

The Henipavirus Collaborative Research Group (HERG) is working to understand the ecological and anthropogenic factors that drive the emergence of henipaviruses, as well as the mechanism for transmission between their wildlife hosts and humans. We are testing three main hypotheses:

• Did anthropogenic pressure on fruit bat habitat and populations via deforestation and hunting alter the distribution and movement patterns of fruit bats, bringing a higher than usual concentration of infected bats to the index farm prior to the 1998-1999 outbreak?

• Did climatic factors including the 1997 El Niño Southern Oscillation and land-use change, including the expansion of fruit orchards, alter the distribution of food availability for flying foxes, causing them to aggregate near the site of Nipah virus emergence allowing for an emergence in pigs to occur?

• Did an expansion or intensification of pig farming in Malaysia provide the correct conditions for a change in host-pathogen dynamics that allowed a repeatedly introduced virus to become enzootic, then epidemic in pigs?

Recent research has shown that Pteropus vampyrus and Pteropus hypomelanus carry NiV-neutralizing antibodies at a significantly high prevalence (Johara et al 2001; Chua et al 2002). These pteropid bats are considered the probable reservoirs for Nipah virus in Malaysia. The CCM and its collaborators are currently using satellite telemetry and ground observations to describe the distribution and long-range movement patterns of P. vampyrus. We are also conducting disease distribution surveillance in P. vampyrus and both distributional and longitudinal disease surveillance in the Island flying fox, P. hypomelanus. Computer models are being used to analyze the dynamics of NiV emergence in domestic swine and its spread between pig farms in attempts to identify a threshold density at which the infection can sustain itself long enough for an outbreak to occur. Laboratory studies are also underway to determine the mechanisms of transmission of henipaviruses between pteropid bats and between pteropid bats and other species.

The findings will be of particular use to public health officials trying to minimize the incidence of spillovers and deter large outbreaks. By determining the actual risk of transmission from bats, the project will also help wildlife managers, pig farmers, and public health officials define their objectives in a broader, more accurate ecological and economic context.

For additional information visit www.henipavirus.org.

References

ICDDR,B. 2003. Outbreaks of Encephalitis Due to Nipah/Hendra-like Viruses, Western Bangladesh . Health and Science Bulletin . Vol. 1 No. 5. 1-6.

Johara, M.Y., H. Field, M.R. Azmin, et al. (2001) Serological evidence of infection with Nipah virus in bats (order Chiroptera) in Peninsular Malaysia. Emerging Infectious Diseases vol. 7. no.3 May-June, 2001.

Chua, KB, L.K. Chong, S.H. Poh,. et al, 2002. Isolation of Nipah virus from Malaysian Island Flying-foxes. Microbes and Infection . 4 (2002) 145-151.