How climate perturbations can plague us.

Two years ago, a deadly respiratory syndrome emerged in New Mexico and radiated to 19 other states. The culprit--a new strain of hantavirus spread by symptomfree rodents--eventually claimed the lives of nearly half of the 94 people stricken.

Now, researchers report that this deadly outbreak appears to have been triggered by climate irregularities associated with the most recent El Nino, that occasional warming of waters in the tropical Pacific.

If true, says Paul R. Epstein of Harvard Medical School in Boston, the episode illustrates the infectious fallout that extreme regional climate perturbations can spawn.

In fact, it may provide a useful analog of the sort of health consequences that can be anticipated under any global warming, Epstein and other scientists argued last week at the Conference on Human Health and Global Climate Change. The Institute of Medicine and the President's National Science and Technology Council sponsored the meeting in Washington, D.C.

Until 1993, hantaviruses around the world had been linked to the development of hemorrhagic fever. But the variant that rose to infamy in the Four Corners area of New Mexico, Arizona, Colorado, and Utah provoked a new disease with quickly debilitating flulike symptoms, then respiratory failure.

This hantavirus undoubtedly resided in deer mice, its primary carrier, for a long time--and probably even caused isolated cases of human disease, suspects virologist Steven Morse of Rockefeller University in New York City. It went unrecognized until May 1993, he says, "because there hadn't been a critical mass of cases."

But early that year, area mice were experiencing a population boomlet, notes ecologist Robert R. Parmenter. He now suspects that spring cleaning may have jump-started the outbreak by exposing humans to urine-laced dust and droppings from infected mice that had spent the winter indoors.

Parmenter and his coworkers at the University of New Mexico in Albuquerque had been counting populations of rodents at Sevilleta National Wildlife Refuge, some 50 miles south of Albuquerque, since 1989. In the fall of 1992, deer mouse populations exploded. Where normally researchers would have found just one to three mice per hectare, they now saw up to 30. "Certainly," Parmenter told Science News, "we saw a typical expansion in all our sites of at least 10-fold."

Moreover, capture rates the next spring showed that rodent populations had not experienced a typical winter dieback.

Why was 1993 unusually mousy?

Parmenter says that mouse populations tend to boom after El Ninos and the abundant spring rains they can bring. In 1992, the El Nino's influence brought heavy, unseasonable rains months before the monsoon season began in July.

"So we had an incredibly green year in the desert," he says. With extra food, rodent populations took off. By the summer of 1993, those populations began declining--perhaps with a resurgence of predators. In seeming lockstep, the number of new hantavirus cases also began to fall.

Since the Four Corners also experienced a rainy spring, anecdotes of heavy mouse infestations there make sense, Parmenter says. In contrast, rodent populations stagnated in Utah's Canyonlands National Park, an area 300 miles away that the El Nino rains bypassed.

"So while we can't prove a causal effect, the relationship with these El Ninos looks pretty good," Parmenter says. His team hopes to publish data on this association soon.

It's hard to predict precisely where or when El Nino's climate variations will deliver their characteristically erratic patterns of severe rains, notes J. Michael Hall, director of global programs for the National Oceanic and Atmospheric Administration in Silver Spring, Md. But by mapping such altered precipitation patterns worldwide, he says, climate researchers should learn where to alert local individuals--be they physicians, government officials, or missionaries--to look for outbreaks of disease.

Currently, however, disease analysts "are not well coordinated," Morse says, and globally "our surveillance capabilities have severe limitations." But a 2-year-old nongovernmental organization that he chairs, with members in 80 nations, hopes to change that. Called ProMED, it collects and disseminates word of such outbreaks on the Internet. Interested parties can subscribe to these reports by E-mail ([e-mail]). Morse also encourages people worldwide to report outbreaks by E-mail ([e-mail]).