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Tracking the mouse in the house

Deer mice are mainly responsible for transmitting hantavirus   

May 25, 1999
Web posted at: 2:10 PM EDT

Ask Rick Douglass, a man who has handled deer mice about 30,000 times, why the mouse enters the house.

For more than two years, Douglass, a biologist at Montana Tech, has been examining what brings deer mice into Montana houses, barns and outbuildings, where the likelihood of spreading hantavirus to humans is far greater than in the field.

Characterized by flu-like symptoms, hantavirus is a rare and highly fatal respiratory disease that can be avoided. Humans can catch hantavirus from deer mice urine, droppings, saliva or nesting materials. Since the first recorded U.S. outbreak in 1993, Douglass estimates only 200 people have been infected with the disease at a mortality rate of 40 percent.

"UV rays kill the virus outdoors. If you're going to get sick, you're going to get sick in a building," Douglass explains. "Most people who get hantavirus have been in dusty old buildings."

That's why Douglass and his colleagues, microbiologist Cliff Bond and postdoctoral researcher Amy Kuenzi, started the "Mouse in the House" study. In what he facetiously refers to as "hotel experiment," Douglass set up three identical modular buildings at ranches near Butte and Cascade, Mont. 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.

"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."

-- Rick Douglass, Montana Tech biologist

One building is baited with peanut butter, one has cotton-nesting material and the third is empty.

Douglass reports that deer mice will enter a building regardless of what is in store. "Even if there are traces of Lysol, and the building is clean as can be, the mouse will still go in."

"It's clear that they visit buildings for food, much more so than for bedding or shelter."

Once inside, the mice don't move around much, Douglass reports. 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. "We assume that the more they are inside the building, the more virus they are likely to deposit."

The mice urinate and defecate in close quarters. That, combined with denser populations, means it's more likely mice will encounter others' 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.

Douglass explains that it is easy to reduce one's chances of catching hantavirus. "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."

Since transmission usually occurs through inhalation, it is easiest for a human being to contract hantavirus within a contained environment, where the virus-infected particles are not thoroughly dispersed. Being in a small house, a crawl space, or a barn where rodents can be found poses elevated risks for contracting hantavirus. The environments that provide the greatest risk are unoccupied buildings, such as an abandoned house, a cabin, or the tool shed in your back yard. Rodents can thrive in such places, especially in cold weather.

Out of the 2,400 deer mice that Douglass has captured throughout his study, 23 percent have carried hantavirus. Those mice that never went in the house had a lower infection rate of 18 percent, Douglass said.

His findings also suggest that male mice are more likely to carry the virus, sometimes at a rate as high as 40 percent.

Douglass notes that hantavirus is not a major health concern, especially when compared with diseases like HIV/AIDS. "Look how many people have died in auto accidents in the United States," he said.

Nevertheless, Douglass suggests, "there are other emerging viruses like Ebola in third world countries that are hard to study because there is no money and no infrastructure. Here we have a situation with a dangerous disease that occurs in a country with funding for research, in an animal we know well. [Hantavirus research] could be a model for predictions in other emerging diseases."

The Mouse in the House Study will continue for the next six months.

For more information, contact Rick Douglass at (406)496-4450 or by e-mail at

Copyright 1999, Environmental News Network, All Rights Reserved

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