The Weekly Newsmagazine of Science
Volume 156, Number 3 (July 17, 1999)
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By R. Monastersky
In late 1997, heavy rains in East Africa unleashed a viral epidemic called Rift Valley fever that killed tens of thousands of livestock and hundreds of people before the outbreak faded. Next time, however, Kenya and its neighbors won't be caught off guard.
Scientists report in the July 16 Science that they have developed a strategy for predicting outbreaks of Rift Valley fever several months in advance. Data going back to 1950 indicate that water temperatures in the Indian and Pacific Oceans start rising long before the extreme rains wash over East Africa and trigger the disease. Carried by infected mosquitoes, the virus spreads first to wild animals and livestock and then to people.
"It's potentially possible that if you can predict an outbreak, all of the [health] organizations could be mobilized to curtail or somewhat lessen an outbreak," says study leader Kenneth J. Linthicum of the Water Reed Army Institute of Research in Washington, D.C.
A medical entomologist, Linthicum studied Rift Valley fever in Kenya in the 1980s and returned to the country during the recent outbreak, in late 1997 and 1998. The epidemic coincided with the record-breaking El Niño warming in the Pacific, raising the prospect that scientists could use ocean temperatures as a prediction tool. Linthicum teamed up with Kenyan geographer Assaf Anyamba of NASA's Goddard Space Flight Center in Greenbelt, Md., and others to test this concept.
The researchers studied records of viral attacks and ocean temperatures going back to 1950. Pacific temperatures on their own did not reliably indicate when Rift Valley fever would erupt. The researchers, however, found a consistent pattern when they considered the Indian and Pacific Oceans together: If water temperatures in both areas surged, a viral outbreak followed in 2 to 5 months.
Satellite measurements can also help by pinpointing which areas face the greatest risk, they report. Instruments on weather satellites routinely track changes in vegetation color, providing a way to sense where particularly intense rains are falling in East Africa.
Nations in that region could take a number of steps if scientists forecast the appearance of Rift Valley fever, says Linthicum. He and his coworkers have experimented with spreading insecticides in mosquito-breeding sites before the arrival of rains, a tactic that could stem the spread of the disease. An effective livestock vaccine exists that must be given a month before exposure to the virus. The U.S. military is now testing an experimental human vaccine.
While disease specialists applaud the new study, some wonder about its utility. "I'm a little skeptical about what you could do with the information," says Thomas G. Ksaizek of the Centers for Disease Control and Prevention in Atlanta, who worked in Kenya and Tanzania tracking the recent epidemic.
The disease spread across such a broad region that it would be difficult to apply enough insecticide to prevent such an outbreak, he says. Relatively wealthy livestock owners could vaccinate their large herds, says Ksaizek, but he holds less hope for reaching small rural communities and nomadic herders.
Others see more possibilities arising from forecasts of Rift Valley fever. "This technique could permit us to become proactive," says David L. Heymann of the World Health Organization in Geneva, noting that it would take a month to obtain the resources for combating a predicted epidemic.
"It would be extremely helpful," agrees Paul R. Epstein, who studies tropical public health at Harvard Medical School in Boston.
The recent fever outbreak harmed the economies of East Africa because other nations blocked livestock exports from affected areas, says Epstein. Kenya and neighboring nations, therefore, have a strong motivation to combat the disease.
The latest El Niño sparked disease outbreaks across the world, including cholera, encephalitis, malaria, and Dengue fever. This episode may provide a foretaste of the future, says Epstein. If climatic disruptions increase as expected, the changes will often benefit the insects, rodents, and other animals that spread such diseases.
From Science News, Vol. 156, No. 3, July 17, 1999, p. 36. Copyright © 1999, Science Service.
Copyright © 1999 Science Service