Larger, Older Deer Mice Spread Hantavirus

Humans can become infected with hantavirus pulmonary syndrome when they come into contact with infected rodents or their urine and droppings. Hantavirus was first recognized in 1993 in the Four Corners region where the boundaries of Utah, Arizona, New Mexico and Colorado meet. It has since has been identified in 30 states, including most of the western half of the United States.

University of Utah scientists used microchip transmitters and five colors of fluorescent powder to document how often and how long deer mice came into contact with other deer mice.

They dusted wild deer mice with fluorescent pink, blue, green, yellow and orange talcum powders to show which rodents most often fought or mated with others and so were most likely to spread hantavirus. The study identified bigger, older mice as the culprits.

A University of Utah researcher uses a toothbrush and a plastic bag to coat a wild deer mouse in pink fluorescent powder. (Photo by Denise Dearing courtesy U. of Utah)

"If mice were in contact with a powdered mouse, you'd see the colored bite mark on their ear or tail, or color on their genitals," says Denise Dearing, a University of Utah professor of biology and senior author of the study that will be published online tomorrow in the British journal "Proceedings of the Royal Society B."

Radio transmitters were implanted in other mice to track their contacts during the study, which is the first to show that the so-called "20-80 rule" applies to a disease that is directly transmitted among members of a single species of wildlife.

The rule says that roughly 20 percent of a population accounts for about 80 percent of disease transmission.

The University of Utah scientists worked with a specific hantavirus known as the Sin Nombre virus.

"Because Sin Nombre virus is transmitted through direct contacts, the results suggest that larger-bodied individuals are responsible for maintaining hantavirus in deer mice populations," says Dearing.

"You knew when they got lucky," said Christy Clay, who ran the study as part of her University of Utah Ph.D. thesis under Dearing's supervision.

"The biggest individuals are most likely to be the ones with the largest foraging range because they have to get more food," says Clay. "Or they could be territorial, so they are defending a nest or their food resource. If they are bigger, they are older, so they may have more experience in defending their territory," than younger mice.

Dearing says earlier research found that hantavirus-infected mice tend to be larger than uninfected mice. "But they didn't examine contacts between individuals," she says. Those studies "imply these [heavier] animals likely have more contacts, and that's why they are infected. Now we have shown what others suspected."

Clay says the 20-80 rule never before has been seen in a disease transmitted by direct contact from a member of any wildlife species to another member of the same species.

It has been seen in sexually transmitted diseases in humans, including AIDS, and in severe acute respiratory syndrome, or SARS, in which few people are responsible for transmitting the virus to others, she says. The rule also holds true for humans with measles, monkeypox and smallpox.

The 20-80 rule also has been shown in indirectly transmitted diseases such as the West Nile virus, in which a few birds are responsible for infecting many of the mosquitoes that feed on bird blood and then spread the virus to humans and other animals, says Clay.

The rule also has been shown in tick-borne encephalitis - fatter, older mice host the most ticks, which feed on their blood and spread the disease to other mice and humans.

"We are not proposing you exterminate larger mice although they are most likely to spread hantavirus," Clay says, noting the wildlife food chain depends on mice.

But she suggests that if places where the animals are older and heavier could be identified, then a risk map could be drawn to show people which are the areas where they should take precautions such as avoiding inhaling dust when sweeping up mouse droppings.

Dearing and Clay, who is now an assistant professor of biology at Westminster College in Salt Lake City, did the study with University of Utah postdoctoral researchers Erin Lehmer and Andrea Previtali, and virologist Stephen St. Jeor at the University of Nevada, Reno.

Copyright Environment News Service (ENS) 2009. All rights reserved.