Multi-Colored Mice: No, You’re Not Hallucinating

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If you see a fluorescent green or pink mouse running around in the deserts of western Utah, don't be alarmed: Scientists are merely trying to find out which mice are mating and fighting together, in an effort to track the spread of the deadly hantavirus.

"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," explained Denise Dearing, a professor of biology at the University of Utah, in a release announcing the findings of the study. Added Christy Clay, who managed the study for her University of Utah Ph.D. thesis under Dearing's supervision: "You knew when they got lucky."

The study, published online in the British journal Proceedings of the Royal Society B, is the first to show that the so-called "20-80" rule - which holds that about 20 percent of a population accounts for about 80 percent of disease transmission - applies even to a disease that is transmitted among a single species. The study found that bigger, older mice comprised the small element of the disease-transmitting culprits.

"The biggest individuals are most likely to be the ones with the largest foraging range because they have to get more food," Clay said. "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."

The researchers are not proposing that larger mice should be exterminated because they are more likely to spread hantavirus, but suggest that a population study could lead to the development of a "risk map." The map would show humans in rural areas where the biggest and oldest mice lived so people could take special care, for instance, in not breathing in while sweeping up mouse droppings.

The Centers for Disease Control report that from the first reported outbreak of hantavirus in 1993 through March 2007, the United States has seen 465 cases of the pulmonary syndrome, resulting in 35 deaths. Experts have proved that the disease spread from mice to people, often when mouse droppings were inhaled.

The Utah biologists used toothbrushes to apply colored, fluorescent powders to five mice at each of 12 sites. The five mice within each group were powdered with five different colors and released just before sunset. The next day, all mice recaptured in special traps were held under a black light so that researchers could closely examine the most likely areas of what they termed "aggressive contact": the head, ears, mouth, feel and tail. "It took hours to screen every mouse that was caught," Dearing said.

The results of the new study and previous research suggest that low turnover in a deer mouse population - usually in areas where mice grow older and bigger because of better food, cover or nesting availability - often is associated with a high rate of hantavirus spread among mice.