Utah biologists say bigger, older mice are spreading hantavirus

U. study » Fluorescent powders reveal who's biting whom

Salt Lake Tribune
Jan. 6, 2009
By Brian Maffly

By applying fluorescent powder to deer mice, University of Utah biologists have determined older, larger rodents are most likely to spread hantavirus, which causes a deadly pulmonary disease in humans when they inhale viral particles in ground droppings from infected mice.

The study, published online Tuesday in the British science journal Proceedings of the Royal Society B , can help manage disease risk if data are used to compile maps indicating areas with the greatest concentrations of infected mice. The virus has sickened at least 465 people, killing about one-third of them, since its 1993 emergence in the Southwest.

"We are not proposing you exterminate larger mice," co-author Christy Clay said in a statement. "But if you could identify places where the animals are older and heavier, then ostensibly you could make a risk map" to show where precautions are a good idea.

Now an assistant professor at Westminster College, Clay used the study for her doctoral thesis under the direction of U. biology professor Denise Dearing. Wild deer mice infect each other with hantavirus through physical contact, mostly in the form of the fighting and mating where saliva is swapped. But prior to this study no one had studied contact rates among these tiny nocturnal creatures, according to Dearing.

"These animals are solitary and are very aggressive; they defend territories," said Dearing, a small-mammal specialist who has been supervising mouse-behavior studies near Utah's Tintic Mountains since 2002.

The new study found that 17.5 percent of the mice accounted for 75.4 percent of the contacts observed, roughly following the so-called "20-80 rule," a general notion that 20 percent of individuals in a population account for 80 percent of disease transmissions. The high-contact mice were nearly 10 percent heavier than the average.

Infection rates of her study population range from zero to almost 50 percent, depending on the year. Dearing and her funders at the National Science Foundation want to know why.

"We can't predict prevalence of hantavirus from the population density of mice. It's more complicated than that," she said. In the current study, funded by NSF's Ecology of Infectious Diseases program, Clay gathered data at the Tintic sites in 2005.

Her research team used toothbrushes and plastic bags to coat captured mice with colored powder. Researchers released five powdered mice -- one each tinged pink, green, yellow, orange or blue --in locations surrounded by dozens of traps, which captured mice through the night. In the morning, each captured mouse was inspected under a black light for signs of fluorescent powder on their heads and genitals, evidence of contact with one of the powdered mice.

"That was the worst job. Because of the risk of hantavirus, the screener had to wear a mask and gloves and stick her head in a box covered in a black material," Dearing said. "She would have her head in the box for hours looking for the powder on the mice, say a touch of pink on its ear," Dearing said.

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