Abstract
During the 1990s two novel, closely-related, paramyxoviruses emerged in Australia and Malaysia which resulted in the significant loss of human life.1,2 Hendra virus (HeV) and Nipah virus (NiV) each moved from natural fruit bat reservoirs into intermediate domestic animal hosts (horses and pigs respectively) and then into humans, with fatal consequences. These two pathogens have been described as members of a new genus of paramyxovirus: Henipavirus. While the number of human cases of HeV was limited, (3 cases, 2 were fatal) in Malaysia there were 265 cases of NiV with a near 40% case fatality rate. Recurrent neurologic infection has affected approximately 7.5% (n=160) of those who survived NiV infection.3 Outbreaks of novel Nipah-like viruses have occurred within the past 4 yr in South Asia, resulting in the loss of human lives, with the most recent outbreak occurring in Bangladesh in January, 2004.4 The Henipavirus Collaborative Research Group, funded through the NIH Fogarty International Center, 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 in this study:
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 1998–9 outbreak?
Did climatic factors including the 1997 El Nino 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 and allow 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?
Pteropus vampyrus and Pteropus hypomelanus have been found to carry NiV-neutralizing antibodies at a significantly high prevalence, with virus having been isolated from P. hypomelanus.5,6 These pteropid bats are considered the probable reservoirs for Nipah virus in Malaysia. We are using satellite telemetry (Microwave Technologies, Maryland) combined with ground-truthing to describe the distribution and long-range movement patterns of P. vampyrus, which has been located at the point of emergence of Nipah virus. We are also conducting disease distribution surveillance in P. vampyrus and both distributional and longitudinal disease surveillance in the island flying fox, P. hypomelanus. To date, approximately 26.3% of P. vampyrus (n=38) and 20% of P. hypomelanus (n=157) have carried serum neutralizing antibodies to Nipah virus.5,7 Computer models are being used to analyze the dynamics of NiV emergence in domestic swine and its spread between pig farms, with the goal being 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 bats and other species.
Ultimately, our goal is to be able to prevent future outbreaks of known pathogens such as Hendra, Nipah, and other Nipah-like viruses, and by improving our understanding of the factors that drove their emergence, we also hope to prevent the emergence of new, potentially more lethal, paramyxoviruses.
Acknowledgments
This study is funded by an NIH/NSF “Ecology of Infectious Diseases” award via the John E. Fogarty International Center of NIH, TW05869 and by core funding to the Consortium for Conservation Medicine from the V. Kann Rasmussen Foundation. We would like to thank the consultants to our group, Tom Ksiazek (CDC, Atlanta) and Sai Kit Lam (Univ. Malaya).
Literature Cited
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6. Chua KB, LK Chong, SH Poh, et al. 2002. Isolation of Nipah virus from Malaysian island flying-foxes. Microbes and Infection. 4: 145–151.
7. Sohayati Abdul Rahman. Unpublished.