Hantavirus causes a severe respiratory disease called hantavirus pulmonary syndrome. In its initial stages, the disease has flu-like symptoms, but people who have it quickly become seriously ill. Three Montanans have died from hantavirus since it was discovered in the United States in 1993.

Since Montana’s cases have been associated with deer mice, scientists focused on deer mice in a project called "Mouse in the House." Between October 1996 and August 1999, the researchers trapped 2,185 deer mice, said Amy Kuenzi, assistant research professor at Montana Tech in Butte. Of those, 2,003 mice were tested for hantavirus antibodies and 490 were found positive. That means that 24.5 percent of the tested mice were once infected with hantavirus.

"That’s pretty high," commented Kuenzi, one of three scientists involved in the study.

Other researchers were Rick Douglass and Clifford W. Bond. Douglass, who received his Ph.D. from Montana State University-Bozeman in 1973, is a biologist at Montana Tech. Bond is a microbiologist at MSU-Bozeman.

The researchers can’t explain the high incidence of hantavirus in the recent study, but they said they hope to build on their work and come closer to understanding hantavirus with future studies. They are applying now for a grant from the National Institutes of Health (NIH). The Mouse in the House project was funded by the national Centers for Disease Control and Prevention.

In the meantime, Kuenzi said the scientists were surprised by other findings in the study. Montana, for one, has three deer mice populations. Some deer mice live outdoors all the time even though they have access to barns and other outbuildings. Other deer mice always live inside. A third group lives in both places.

"That’s interesting. That’s significant that there are those three designations," Douglass said.

Surprisingly, the highest rate of infection occurred in the mice that alternated between living indoors and outdoors, Douglass added. Mice that stayed indoors without the benefit of wind and sunshine had the second highest rate of infection. Least likely to be infected were the deer mice that always lived outdoors.

He doesn’t know why the rankings turned out as they did, but more males than females tend to live both outdoors and indoors, Douglass said. Also, more males than females had hantavirus in this study. And since males are more aggressive than females, they may be spreading the virus by biting each other.

"Animals with sores have higher infection rates," explained Douglass who saw many mice with torn ears and scarred backs.

Bond and Douglass started the Mouse in the House project in 1996 to determine whether buildings are a special environment to deer mice and what attracts mice to certain buildings. The study is related to another Montana project that began in 1994. That study will run another five years and deals with deer mice that live in fields.

"People don’t generally contract the disease out in the field," Bond said, explaining the rationale behind the Mouse in the House study. "They contract it more in buildings."

Fieldwork for the Mouse in the House project was done on two cattle ranches near Butte and one near Cascade. The researchers used live traps to capture the deer mice living in and around buildings. Then they attached radio collars and ear tags and took blood samples.

They also placed three identical "hotels," or wooden boxes, on the ranches. The buildings had holes in each side, a grid on the floor and fluorescent dust at every opening. One hotel contained peanut butter. Another had cotton bedding. The third was empty.

The hotel with food had "tremendously" more activity than the other two buildings, Douglass said. But when the weather turned cold, the mice didn’t move anywhere. They didn’t seek bedding. They didn’t pig out on peanut butter.

Hantavirus can spread to people who are exposed to deer mice urine, droppings, saliva or nesting materials. To protect themselves, Douglass suggested they mouse-proof their houses.

"Inspect everything," he said. "If you can put your finger into a hole, a deer mouse can get in."

To fill the hole, he advised pounding it full of steel wool. Styrofoam won’t do the job, he said.

Outbuildings can’t be mouse-proofed, so "Just be careful," Douglass added. However, the chances of getting hantavirus can be greatly reduced by wearing respirators, rubber gloves, overalls, and cleaning with a mixture of bleach and water, he said.