A rise in the deer population has been blamed for an explosion of Lyme disease cases, but foxes might actually be responsible, according to a recent study.
A rise in the deer population has 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 recent study.
Wisconsin saw a 280 percent jump in Lyme disease cases between 1997 and 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 Lyme 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 differ 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 grown 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 the Lyme 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. Hunting (and the hunters who were bitten), 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 fell.
Incidence of Lyme disease over the same period mirrored the rise of coyotes and the decline of foxes. Deer abundance and Lyme cases were not related in Wisconsin, debunking the belief that more deer equals more Lyme, according to the study, 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 what hunters were seeing. 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, a Lyme 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.
Based on their research, Levi and colleagues suggest a deer reduction strategy be combined with efforts to rehabilitate the red fox, to reduce tick abundance and ultimately to stall the spread of Lyme disease. In fact, this may be already happening organically, as wolves recover and cougars move eastward. These top predators may control the coyote population, helping foxes recover.