MILWAUKEE — Increases in the deer population have been blamed for the explosion of Lyme disease cases in recent years, but changes in the numbers of foxes and coyotes — and what they eat — may actually be responsible, according to a study published Monday.
This could have implications for how wildlife is managed, and shed light on the complex ecosystems underlying the rise of Lyme and other tick-borne diseases.
Wisconsin saw a 280 percent jump in Lyme disease cases in the decade from 1997 to 2007, with a total of 2,376 cases statewide just last year. Other states in the Midwest and the East Coast have seen even greater increases. The bacterial infection that starts with a distinctive bull’s-eye rash can require extensive antibiotic treatment and may lead to arthritic and nervous system complications.
But what do small predators such as foxes and coyotes have to do with a disease spread by the deer tick?
The answer lies not only in the life cycle of the Lyme bacterium, Borrelia burgdorferi, but also in the ecological changes of all the animals with which it comes in contact. Normally, small mammals get infected by the bacteria, ticks get infected by feeding on the mammals, and then ticks feed and lay their eggs on deer. Foxes disrupt the chain by feeding on the small mammals.
“It was thought that deer were the only game in town for ticks,” said Taal Levi, lead author of the new study and a research fellow at the Cary Institute of Ecosystem Studies in New York.
Foxes have hunting habits that are different from those of coyotes: They will kill many small mammals at once, stashing the kill for later. Coyotes, on the other hand, especially those that have crossbred with wolves, will eat deer, rabbits, or even foxes, and are not efficient predators of small mammals the way foxes are. As coyotes have expanded in numbers and range, the new study suggests, they interfere with the important role served by foxes: to suppress Lyme disease rodent hosts, especially around human habitation.
The chain of events that leads to Lyme disease starts small, with a larval tick biting, say, a white-footed mouse that carries Borrelia bacteria. The tick matures into a nymph that can infect other animals each time it feeds. The life cycle of the tick typically ends with deer, on which they prefer to feed and lay their eggs. The unlucky outdoorsman or hunter may intrude at any stage and be bitten. Hunting, it turns out, was key to understanding the spread of Lyme.
Using harvest records from 1982 to the present, the researchers tracked the number of deer, coyotes and foxes in Wisconsin, Minnesota, Pennsylvania and Virginia. In all four states, coyote hunter harvests were up over the 30-year period, while fox harvests decreased.
Incidence of Lyme disease over the same time period mirrored the rise of coyotes and the decline of foxes. Deer abundance and Lyme cases were not related in Wisconsin, debunking the common belief that more deer equals more Lyme, according to the study, which was published in the Proceedings of the National Academy of Sciences.
There also was no consistent increase in Lyme with deer numbers in the other states. In fact, an area with a high fox population in western New York was notably devoid of Lyme.
A new picture was emerging, where Lyme appeared to be more closely associated with changes in predators rather than deer. Local survey data from the Wisconsin Department of Natural Resources helped to corroborate the hunter data. Deer observations were stable, even somewhat declining, from 1999 to 2009. The initial emergence of Lyme in the state may be linked to a deer boom in the 1980s and ‘90s. In the past 15 years, however, deer have waned while Lyme has continued its relentless spread. This more recent burst in Lyme prevalence appears to be linked to the statewide rise of coyotes and fall in foxes. Foxes don’t build dens where coyotes are present, and they may even be killed by coyotes. As a result, the small animals that host infected ticks are left to multiply freely.
Infectious disease emerging from altered predator-prey dynamics is nothing new; Levi points to bubonic plague and hantavirus as diseases whose spread also depends heavily on rodents and other common prey species. For Lyme research, shifting the focus from deer alone to the ecosystem underlying the disease has been challenging, says Levi, and perhaps overdue.
Jennifer Coburn, Borrelia researcher at the Medical College of Wisconsin, agrees: “These ideas fill an important hole in our knowledge of why Lyme disease is continuing to emerge. It’s not all accounted for by deer, who are dead-end hosts (for the bacteria).”
TICK PREVENTION TIPS
Use bug repellents containing DEET.
Wear long pants tucked into socks and long-sleeved shirts when walking in the woods or grassy areas.
Check skin and clothes for ticks after being in the woods.
Remove tick by grasping it as close to skin as possible with tweezers and gently pulling until it releases grip. Clean bite with antiseptic.
If you live near woods, remove brush and litter and put wood chips or gravel path between your yard and the woods as a buffer zone to minimize tick exposure.
—CDC, Wisconsin Department of Health Services