May 18, 1999 BOZEMAN - Ask Rick Douglass, a man who has handled deer mice about 30,000 times, why the mouse goes into the house.
Douglass, a biologist at Montana Tech in Butte, wants to know what brings the rodents into Montana houses, barns and outbuildings, where the likelihood of spreading hantavirus to humans is far greater than in the field.
Hantavirus is a respiratory disease characterized by flu-like symptoms. Humans can catch the virus from deer mice urine, droppings, saliva or nesting materials. Ten Montanans have had the disease since it was first discovered in 1993. Three have died.
Most people see deer mice in the field or as road kill, but "you're not going to get the virus driving down the road," Douglass said during a recent lecture at Montana State University in Bozeman. "If you're going to get sick, you're going to get sick in a building."
That's why Douglass and MSU microbiologist Cliff Bond started the Mouse in the House study two years ago. In what he calls the "hotel experiment," Douglass set up three identical modular buildings at ranches near Butte and Cascade. The buildings have holes in each side, a grid on the floor and fluorescent dust at every entry. The dust coats the rodents so researchers can count the squares the mice visit.
One building is baited with peanut butter, one has cotton nesting material, and the third is empty.
"It's clear that they visit buildings for food, much more so than for bedding or shelter," Douglass said. The ones that go in the empty buildings may be drawn by the scent of a mate, a theory he plans to test in the future.
What's astounding, he added, is how quickly the mice find the buildings--within 15 minutes after dark.
"If you leave your deck door open, you're going to have deer mice going in and out and your cat's not going to see them," he said.
Once inside, the mice don't move around much, he found. In the winter they may stay in one small area for three or four days.
That behavior may explain why mice inside the buildings have a higher infection rate than those captured outside, Douglass said. Blood samples showed that at one site the infection rate of inside mice was nearly double what it was outside.
The mice urinate and defecate in close quarters. That, combined with denser populations, means it's more likely mice will encounter other's urine and feces. Biting is probably more common, which is another way the disease is spread from rodent to rodent. What's more, the dust is finer, making virus particles more likely to become airborne.
People need to be cautious when cleaning areas that have mouse droppings. Sweeping can stir up virus-laden dust particles. Instead, the area should be sprayed with a mix of bleach and water (nine parts water to one part bleach) and wet mopped.
In a related study of deer mice at six outdoor sites in Montana, Douglass and Bond discovered that most of the infected mice are adults. Most of the non-infected ones are juveniles and subadults. The infected group also has a high number of breeding animals. Those findings suggests that breeding plays a role in transmission, perhaps through bites during fights. A study in New Mexico, in fact, found a correlation between bite scars and infection, Douglass said.
The Mouse in the House project ends this October, but the related study continues. Both have been funded by the Centers for Disease Control.
Douglass said he hopes to use the Mouse in the House study to develop a way of predicting mouse populations in buildings based on some easily measured environmental factor, like precipitation at a particular time of year. Then he would test that model in Patagonia while on sabbatical next year to see how well it will work in different places. Patagonia has a strain of hantavirus called Andes virus, which is spread by the long-tailed mouse.
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