HANTA

A NEW DANGER IN THE WEATHER

The population of mice, rats and other rodents exploded. Unknown to humans at the time, some of the animals carried the fatal strain of hantavirus that later came to be called Sin Nombre virus. Otherwise healthy people who had come into close contact with the deer mouse, a small, white-bellied animal drawn to houses, sheds and cabins, contracted the deadly respiratory disease. After suffering from what first seemed like flu-like symptoms, they died quickly with severe lung damage, confounding doctors and scientists.

The frightening outbreak had the country's best disease detectives scrambling to figure out what they faced. By early summer they identified the disease's source as a strain of hantavirus, a type of rodent-related virus first identified decades earlier when soldiers in the Korean War fell ill with hemorrhagic fever with renal syndrome. Both diseases spread through exposure to infected rodent urine, droppings and saliva.

The numbers overall are modest when compared with other diseases that strike millions of people. What's startling about this hantavirus is that it appeared to come from nowhere. In reality, hantaviruses likely have been around a long time in rodents, maybe even causing sickness in people. Why, ask scientists, did this strain suddenly emerge to kill and how can we predict future outbreaks?

Many suspect weather could be the key. Sudden shifts might change the environment enough to open the door for disease where it otherwise might not have had much chance.

Bailey's pocket mice, white-throated wood rats, brush mice, Merriam's kangaroo rats, cactus mice, yellow-nosed cotton rats, southern grasshopper mice, white-footed mice, desert pocket mice, fulvous harvest mice and deer mice have all landed in traps at one time or another.

Their blood samples are tested for signs of hantavirus. Population numbers, weather information and notes about the health of surrounding plants are recorded. Funded and coordinated by the Centers for Disease Control and Prevention. They likely will be doing it for several more. It takes a lot of data to pin down ecological trends.

While preliminary, the information they have so far, recently reported in the journal Emerging Infectious Diseases, is intriguing.

"The evidence appears to show that when the rodent population is low only a few have hantavirus, but something happens when the numbers increase. Moreover, for some species many more males are infected than females, perhaps because they have more physical contact, fighting and injuring each other.

It may be that when populations grow the animals come into contact more frequently, raising the spread and prevalence of the virus. More rain and snow, more food, more mice, more hantavirus.

How disease outbreaks tie in to weather has only recently come under close scrutiny by scientists. While some were generally aware of the implications.

"The climate is exhibiting increasing instability and the greatest manifestation of climate change may be the increase in extreme weather events. "It's having profound consequences for health and for economies."

The southwest hasn't seen another outbreak of hantavirus such as that experienced in 1993. Now residents know, though, that there is a hidden enemy within rodents that can cast a shadow in seasons of plenty.