Explore climate's role in spread of diseases

September 19, 2002|By Rita R. Colwell

WASHINGTON - Infectious diseases are the leading cause of death worldwide and are on the rise.

Take West Nile virus. As mosquitoes buzz around our back yards, parks and other public places, residents and health officials in Southern, Eastern and Midwest states are poised to retreat under protective netting. For fear of the lethal virus, citizens are being urged to drain flower pots, wash out birdbaths and eliminate standing water where the insects can breed.

And now the virus has marched onto football fields.

The Jackson, Miss., high school football team faces a potentially deadly foe: night-feeding mosquitoes that carry the virus. Players are being sprayed with insecticide before games and at halftime. More than 1,500 cases of the disease have been reported nationwide, and 71 people have died.

Other dreaded diseases of centuries past, such as bubonic plague, are happening here and now.

Plague, for example, is on the rise in the Southwestern United States. Biologists conducting long-term ecological research have found that human plague cases in New Mexico are occurring after wetter-than-average winter-to-spring periods. In fact, during much wetter than normal periods from October to May, there has been a nearly 60 percent rise in the number of cases. Yersinia pestis, the plague bacterium, has survived the Dark Ages and is with us yet.

And now researchers have discovered that a bumper year for acorns may result in a bad season for Lyme disease. This intriguing link is the result of a long-term study of the ecological relationships in oak forests of the Eastern United States. In those acorn-rich years when deer gather in oak forests to feed, large numbers of adult deer ticks drop from their hosts onto the forest floor and lay their eggs in the leaf litter.

An abundance of acorns also encourages growth of white-footed mice, which in turn infect the ticks with the Lyme disease bacterium (ticks latch onto mice, which carry large populations of these microorganisms in their blood).

After a bumper-crop acorn year, then, numbers of ticks carrying Lyme disease skyrocket, and the incidence of Lyme disease follows suit.

What causes outbreaks of these diseases? Surprisingly, the answer may lie in the relationship of infectious diseases to our planet's climate.

Recent average temperature increases and changes in precipitation amounts and patterns may be to blame. The combination of hot days and warm nights where once there were warm days and cool or cold nights has introduced changes in ecosystems.

Insects and microorganisms that are disease carriers previously couldn't survive in certain localities but now thrive there.

Federal agencies have noted these environment and health interactions. A program jointly administered by the National Science Foundation, the U.S. Geological Survey and the National Institutes of Health is encouraging collaborative research among climatologists, ecologists and biomedical scientists.

A better understanding of how climatic factors affect the spread of infectious diseases will enhance our ability to predict the health effects of both seasonal weather changes and large-scale disturbances such as El NiM-qo. For example, in the Southwest, outbreaks of hantavirus, which can result in severe respiratory problems in humans, have been traced to the heavy rainfall associated with El NiM-qo.

Much of the current information we have comes from "brush-fire biology," however, in which researchers have focused their attention on short-term and emerging epidemics.

We must undertake research in diverse fields of science to make clear the links between broad-scale environmental and climatic factors and diseases such as West Nile virus.

Otherwise, in 50 years, when Earth's temperatures may have risen, we won't be able to predict, or prevent, the spread of infectious diseases that are climate-responsive.

We need an answer to a problem that, while erupting in isolated pockets now, might be epidemic in another half-century.

Rita R. Colwell is director of the National Science Foundation.

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