Have Lyme disease-infected ticks found a new host?
A recent study suggested a link between an increase in black bear populations and a higher prevalence of black-legged ticks, known colloquially as the deer tick, which spread Lyme disease. Penn State researchers studied this surge in Pennsylvania, which coincided with black bear population growth.
Here in the Hudson Valley, too, the black bear population has risen sharply in recent decades.
Jeremy Hurst, a wildlife biologist for the New York State Department of Environmental Conservation (DEC), says the DEC tracks estimates of the black bear population in New York based on how many bear hunters “take away” in a hunting season. The data shows that from the mid-1990s to 2015, the number of bears hunted by hunters in the southern part of the state, which includes the Hudson Valley, increased from under 200 to over 1,000 per hunting season, which is indicates a sharp increase in the total number of bear populations.
More bears roaming where more people live (like the suburbs) can lead to other conflicts, like bears getting caught in trash, birdseed and car crashes.
In order to better manage the bear population and alleviate the subsequent problems, the DEC opened bear hunting nationwide from 2014. “The bear population seems to have stabilized,” says Hurst of the measure.
Ticks against bears, deer and mice
Given that the bear population growth has been so dramatic that intervention was required, where is the tick prevalence? Should we start calling the deer tick, commonly associated with the white-tailed deer it feeds on, the “bear tick”?
“I think the sustained impact on tick frequency is a real challenge,” says Hurst of black bears. “The density of bears is so much lower than the density of other potential tick hosts, mainly white-tailed deer.”
The DEC estimates that 6,000 to 8,000 black bears are currently roaming the state in areas open to bear hunting, such as the Adirondacks and the Catskills. In contrast, the DEC estimates that 220,000 white-tailed deer are killed by hunters nationwide each year – just part of the total game population, estimated at between 1 and 1.2 million, according to Hurst.
Aside from the fact that there are fewer bears in New York, Dr. Richard S. Ostfeld, disease ecologist and senior scientist at the Cary Institute, found that bears harbor far fewer ticks per animal than other animals such as white-tailed deer and mice. Ostfeld cites two studies, one in Pennsylvania and one in New Jersey, that observed the prevalence of ticks in black bears. The Pennsylvania study is cited by some reports claiming bears could cause tick populations to increase.
Ostfeld disagrees, however, saying the results of the studies show a relatively low number of ticks in black bears. “In the New Jersey study, they found 29 black-legged ticks out of a total of 11 bears, less than three per bear,” he says. “So you get a handful of ticks per bear.”
Although this study found a higher prevalence of dog ticks in bears, dog ticks are not known to transmit Lyme disease. And while the Pennsylvania study found only black-legged ticks in its bears, Ostfield says the numbers are still not mind-boggling – fewer than 10 ticks per bear.
However, white-tailed deer can harbor tens to sometimes hundreds of adult ticks per deer, and small mammals harbor hundreds of immature ticks, Ostfield says.
“If you compare bears with a lot of other wild animals, they just don’t harbor very many ticks.”
It is the small mammals like mice that are actually the greatest predictor of Lyme disease, says Ostfeld. When Lyme disease was discovered, scientists mistakenly identified the tick it was carrying as a new tick and gave it the common name “deer tick”. In reality, it was a tick with the common name “black-legged tick,” discovered 200 years earlier, he explains. Although scientists today use the proper name Schwarzbeinige Zecke, “Hirschzecke” was easier to say and caught on in large parts of the public. According to Ostfeld, this has led to deer being disproportionately focused on ticks, while mice are far more important.
“In our studies in Millbrook, and we’ve been doing it for 30 years, we find that when you look at the number of [blacklegged] For ticks, it correlates statistically much more strongly with mice than with deer, ”says Ostfeld. “So there are some misleading conclusions one can draw from calling the tick the deer tick. At least the public thinks the deer is the only game in town. “
Whether Lyme disease affects bears has not been studied, but it’s unlikely, says Kevin Hynes, wildlife biologist at DEC Wildlife Resources Center in Delmar. “Bears are quite disease-resistant animals,” he says, pointing out that the biggest disease-related problem for bears is mange.
Ostfeld says if Lyme bears were infested, they would still likely not return it to non-Lyme infected ticks. “If animals infect the ticks, which then feed, these ticks can make other animals or people sick,” says Ostfeld. “Black bears don’t seem to play that role.”
Ostfeld says, however, that more research is needed to conclusively understand the relationship between black bears and ticks, adding that he finds the discovery of relatively low numbers of ticks in bears “curious” as compared to other animals. He says skin density might play a role, but no research has been done to draw a conclusion.
“If you compare bears to a lot of other wildlife, they just don’t harbor a lot of ticks, and I don’t know why or why,” he says. “Adult ticks like to climb up to about thighs off the ground, and this is a good place to encounter bears, deer, and other medium-sized mammals. They just don’t feed on bears in large numbers. And I’m not sure why. “
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