Bigger, older mice of both genders are the probable "super spreaders" of potentially deadly hantavirus, according to University of Utah research.

U. researchers tracked contact between deer mice by powdering them with green, orange, pink, yellow or blue fluorescent talcum or implanting microchips. People get the deadly infection by inhaling virus contained in droppings and urine from infected deer mice.

The answer the researchers in biology professor Denise Dearing's lab sought as part of Christy Clay's doctoral thesis was whether there is a subpopulation of deer mice most likely to spread the virus, probably those with the most contacts.

Their findings are being published Thursday in the British journal Proceedings of the Royal Society B.

Twice a year at 12 different desert sites in Juab County, researchers released one each of the powdered mice at dusk, then trapped dozens of deer mice the next morning, looking for traces of the fluorescent color under black light to see which had been bitten, or scratched or had mated with one of the powdered critters. They thus showed that the bigger, older mice are most apt to infect other deer mice with hantavirus; it's spread by biting and fighting.

Because of the limited number of fluorescent colors, Dearing said, they gained most of their data from mice they tagged with micro-chips. That showed the so-called 20/80 rule, the concept that about 20 percent of a population — in this case, deer mice — cause 80 percent of disease spread.

The 20 percent are "super spreaders," a concept true with SARS, AIDS and measles. Clay likens super spreaders to the sociable girl who sits by every other little girl, twirling her hair. "She's the lice super spreader. The big old mice are the hantavirus super spreaders."

The researchers could not initially see what the tagged animals were doing in the foraging arena, although they could see how long they stayed and who else was there. Later, using a video surveillance system, they could view contacts. "They differ. Some are submissive and won't interact, while others engage in full-on mouse tussles," Dearing said.

The researchers expected males to be the super spreaders because they are more likely to be infected, but found no gender difference. They did find evidence that female mice have a stronger immune response.

Clay said the study may indicate that the bigger mice have the largest foraging range, they may be more territorial or perhaps simply better at defending territory, all fodder for another study.