In February the Nipah virus reemerged, killing 35 people in Bangladesh in two outbreaks. Although the number of victims is small, the deaths have health officials worried. Unlike its first appearance in Malaysia in September 1998, the virus in Bangladesh may have jumped from person to person, raising concern about its ability to spread farther and faster.

Nipah is a henipavirus, a family named after its two only known members, Hendra and Nipah (both take their appellations from the places they first struck). Distant relatives of measles, henipaviruses appear to reside naturally in flying foxes, the world's largest bats. The virus spreads through bodily fluids such as saliva or urine.

Flying foxes live across the Pacific lands and Africa. Roughly a third of those in Malaysia and Australia harbor antibodies against the infections, suggesting that the bats and viruses evolved together. The bats, which are critical to rain-forest ecology as pollinators and seed dispersers, apparently do not get sick from the viruses, ¿which makes them particularly good carriers,¿ says veterinary epidemiologist Jon Epstein of the Wildlife Trust's Consortium for Conservation Medicine in Palisades, N.Y. Intrusions on and fragmentation of the bats' natural habitat as the result of logging and other human activities help to create the conditions ¿for a spillover disease event from animals to humans,¿ he explains.

When Nipah reappeared this past winter, it came deadlier than ever. In contrast to its original outbreak in Malaysia, which claimed nearly 40 percent of those infected, the mortality rate of the Bangladeshi outbreaks was 74 percent. ¿That's approaching Ebola levels,¿ Epstein observes. (SARS has a mortality of about 9 percent.) It remains unclear whether the Bangladeshi strain is inherently more deadly than the Malaysia strain or whether it proved more lethal because of poorer access to health care.

Equally unclear is how the Bangladeshi patients became infected. Previously Hendra and Nipah leaped from bats to humans via intermediate animal hosts. Victims in the first Nipah outbreaks, for instance, caught the sickness from pigs. The bats frequented mango trees that grew directly over pigpens, leading medical researchers to suspect that the bats either dropped saliva-tainted fruit or excreted waste into the pens. The sickened pigs coughed and wheezed--and amplified the virus population in their bodies to levels far greater than those in bats.

Many victims in Bangladesh, however, had no direct contact with animals, and no infected domestic animals were seen. Reports suggest that children picked fruit from trees before dawn, perhaps roughly the same time bats finished nightly feeding. But without an identifiable intermediate vector, transmission among people cannot be ruled out. A human vector would allow henipaviruses to expand beyond the natural range of the bats, says Peter Daszak, director of the Consortium for Conservation Medicine.

Human-to-human transmission would also make henipaviruses even more desirable to bioterrorists. ¿If you want to cause serious human disease and even more serious animal biowarfare, Nipah's your guy,¿ comments virologist Chris Broder of the Uniformed Services University of the Health Sciences. ¿These viruses in fruit bats can be isolated with just rudimentary skills in microbiology.¿ The Centers for Disease Control and Prevention ranks them in the same bioterrorism class as the hantavirus (category C), though not as high as anthrax or cholera.

To prevent outbreaks, Malaysia now screens pig blood samples for Nipah. For Bangladesh, the recommendations include precautions among health care workers such as wearing goggles, masks and gloves, along with improved local hygiene that includes washing of fruit and hands.

Two international collaborations--one taking place between Malaysia and France, the other between Australia and Broder's U.S. team--are experimenting with vaccines. One therapeutic approach being pursued by Broder and his colleagues are proteins that inhibit Nipah's fusion with cells in the body. Antibody therapies to neutralize the viruses are also possible but prohibitively expensive, Broder says.

Controlling new diseases also means good surveillance; to date, groups such as the Food and Agriculture Organization of the United Nations and the World Organization for Animal Health monitor diseases in internationally traded animals, but no global group yet exists to keep track of wildlife diseases. ¿Seventy-five percent of all emerging infectious diseases are known to come from animals, and wildlife is part of that equation,¿ Epstein notes. ¿The more we can identify the natural reservoirs of a disease and understand the conditions that allow them to emerge,¿ he says, ¿the more we can predict and ultimately prevent diseases.¿

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