An Ecological Solution to a Medical Mystery
When young, otherwise healthy people in the remote Four Corners area of Arizona and New Mexico began dying of a mysterious acute respiratory disease in the spring of 1993, people were scared. Those who caught the disease got very sick, very quickly. Eventually twenty people died. At the time, some wondered if the disease was a biological warfare agent, a military experiment gone bad.
The Atlanta-based U.S. Centers for Disease Control and Prevention (CDC) sent scientists to the region to investigate. Tests of the victims' blood yielded a surprising result: the people had become infected with a previously undetected kind of hantavirus. Named after the Hantaan River in Korea, hantaviruses were known to spread from rodents to humans but until the Four Corners outbreak, the microbes had only been seen in Asia and Europe. Moving quickly, CDC investigators asked biologists at the University of New Mexico for help in collecting rodents and insects around the homes of people who had gotten sick. A likely suspect soon appeared when the infection popped up in one particular kind of mouse.
"The CDC called us and asked, 'What mouse is this?'," says University of New Mexico mammologist and museum curator Terry Yates, who also serves as co-principal investigator at the NSF-funded Sevilleta LTER siteso-called because the site's 230,000 acres are located within the Sevilleta National Wildlife Refuge, about an hour south of Albuquerque. Yates told the CDC that the infected animal was a deer mouse, a close relative of the type of Old World mice that also carry hantaviruses and that transmit the disease through their droppings and urine.
Now the CDC knew what the disease was and how it was transmitted. But the investigators still didn't know why a disease carried by a common animal like the deer mouse seemed to be cropping up for the first time in North America. For answers, the CDC turned to what Sevilleta researcher Robert Parmenter calls "a bunch of rat trappers" who had
been working on matters entirely unrelated to medical science at Sevilleta even before the site was admitted to NSF's LTER network in 1988.
The major research question at the Sevilleta LTER site was this: How do the Sevilleta's four major ecosystems (grassland, woodland, desert, and shrub steppe) respond to short-term and long-term fluctuations in climate? One way to address that question was to measure the population fluctuations of plants and animals. Climate changes affect vegetation, which in turn affect the amount and kind of food available to animals. Keeping track of the rodent populations was just one part of a multi-investigator projectbut it turned out to be a crucial part of the CDC investigation.
Parmenter, who directs the Sevilleta Field Research Station, recalls being told by the CDC that "I could take all the time I wanted so long as [the rodent report] was ready by next Tuesday." He and his team of students and fellow professors "were gung-ho excitedworking up the data, doing the analyses just as fast at we could."
Their conclusion? The hantavirus outbreak could be blamed on El Niño, a periodic pattern of change in the global circulation of oceans and atmosphere. Parmenter's team saw in their long-term data that massive rains associated with the 1991-1992 El Niño had substantially boosted plant productivity in the Sevilleta after several years of drought. A banner year for plants was followed by a banner year for rodents. Rodent populations during the fall of 1992 and spring of 1993 surged as much as twenty times higher in some places as compared to previous years. The same phenomenon likely occurred in the nearby Four Corners region. More mice meant that more humans stood a greater chance of exposure to infected rodents as the people moved among their barns and outhouses and did their spring cleaning of cabins and trailers.
Data from the Sevilleta also helped to determine that the deadly hantavirus wasn't new to New Mexico. Yates and his colleagues tested tissue samples collected from rodents prior to 1993 and detected evidence of hantavirus. In other words, the virus had been in rodents all alongit was the change in climatic conditions that triggered the fatal outbreak in humans. Such knowledge may have helped save lives in 1998, when a particularly active El Niño event prompted health authorities to warn residents of the American southwest to be careful when entering areas favored by mice. The events of 1993 continue to be felt directly at the Sevilleta LTER, which now counts among its studies one that aims to identify the ways in which hantavirus is spread from rodent to rodent.
Yates says, "This is a classic example of basic research done for totally different reasons coming to the rescue when a new problem arises."