Expanding predator populations could explain an uptick in infections

By Susan Milius

Web edition: June 21, 2012

Coyotes expanding into new territories across North America may be driving a surge in Lyme disease.

It’s often deer that municipalities blame for raising the risk of human infection with the tick-transmitted Lyme bacteria. Yet records from the past three decades link rising numbers of Lyme cases not with booming deer populations but with spreading coyotes, says ecologist Taal Levi, now at the Cary Institute of Ecosystem Studies in Millbrook, N.Y.

As coyotes have trotted into new ranges, red foxes have retreated, Levi and his colleagues find. Coyotes don’t pack a landscape as tightly or kill and cache rodents in flush times as foxes do. So a red fox fade-out allows more little rodents to survive, including white-footed mice and others known as hospitable hosts for the Lyme pathogen and the ticks that spread it. This scenario — coyotes in, foxes out, small rodent numbers up — could be fueling the spread of Lyme disease, the researchers suggest online June 18 in Proceedings of the National Academy of Sciences.

Connecting coyotes and foxes with the disease “is quite a fresh idea,” says Jean Tsao of Michigan State University in East Lansing. Some ecologists have put forth the more general idea that changing predators might let the small rodents boom and the disease flare up, but not with these specific animals.

Lyme disease has become the most commonly reported disease that is spread by an animal in the United States. Probable and confirmed cases totaled about 30,000 in 2010. Most of these cases showed up in 12 states, in the upper Midwest or Northeast, and Levi reports that some of these states have seen dramatic flare-ups of Lyme disease between 1997 and 2007: a 380 percent increase in Minnesota and 1,300 percent in Virginia.

Lyme infections cause flulike symptoms and can be successfully treated when caught early. But some infections can linger or repeatedly reappear over the course of years, causing pain and other problems that disrupt everyday life.

The disease doesn’t spread directly from person to person. Instead a small Ixodes tick passes along the bacterial pathogen. Sometimes misleadingly called deer ticks, these little menaces actually bite a wide range of creatures.

To test the idea that predators of small rodents might influence the disease increase, Levi and his colleagues pieced together state information on wildlife sightings and hunting from Minnesota, Wisconsin, Virginia, Pennsylvania and New York. In the first four of those states, Lyme cases climbed as coyote populations rose and red foxes declined, hunter records suggested. Deer records, however, didn’t show any consistent pattern.

Researchers also zoomed in on regions within a state where possible. A decade of wildlife sightings recorded in five regions of Wisconsin, for example, suggests red fox populations dwindled as Lyme incidence climbed.

The pattern of coyotes replacing red foxes fits with other research showing that the foxes won’t make dens when coyotes move into the neighborhood. 

Next, Tsao says, she’d like to know whether rodent populations actually did increase. Also, she would want to see whether experiments, and not just observational data, support the link between fox declines and disease increase.

The new study adds to evidence suggesting that culling deer in the Northeast is unlikely to reduce Lyme disease in most places, says Lyme disease ecologist Richard Ostfeld, also of the Cary Institute, who was not involved in the new study. Deer kills may lower risk on islands and in enclosures, but free-ranging populations with lots of alternative animals to bite are different. “Reducing the herd by even as much as 60 or 70 percent likely won’t do squat to the tick population,” Ostfeld says.