Expert: Eliminate the deer, eliminate the ticks
As both Block Island and Jamestown wrestle with the problem of Lyme disease, the simple solution of killing deer has been greeted with skepticism. What if the ticks can survive without deer? What if they adapt to new hosts, as a letter writer to the Block Island Times asked this last week? If ticks also bite rodents, how can just killing the deer solve the problem?
For the answers we turned to Dr. Thomas N. Mather, University of Rhode Island professor and director of its Center for Vector-Borne Disease. Mather has been studying deer ticks for three decades, first at the Harvard School of Public Health and, since 1992, at URI. He says unequivocally that deer are a key linchpin in Lyme disease transmission.
Yes, ticks bite rodents, and that’s where they get Lyme disease. But only nymphal (baby) ticks bite rodents. In order to reproduce, adult ticks need a blood meal from deer. If you eliminate the deer, you will eliminate the adult ticks’ food supply, and in turn stop them from reproducing.
Anecdotally, there are accounts of places where deer culling seems to have virtually eliminated Lyme disease, such as on Monhegan Island in Maine, where five years after removing all of the island’s deer, the disease disappeared. The trick is in knowing what the minimum number is, Mather said. He studied Naushon Island in Massachusetts, where private landowners reduced the deer population from about 600 to 60, yet ticks were still prevalent.
“Very few deer can seed an enormous number of ticks,” he said. “There is going to be at some point a ‘deer threshold. ”
According to the National Institutes of Health, deer culling programs typically see a short-term resurgence of ticks. But, unable to reproduce, the ticks eventually die off.
The Center for Vector-Borne Disease has studied the Lyme disease problem from virtually every angle – testing tick pesticides and natural remedies, studying resistance to Lyme disease, building deer feeding stations coated with tick pesticides. So far, no study has yielded a simple answer to eradicating tick-borne diseases, which besides Lyme include ehrlichiosis and babesiosis.
The center is near the end of a five-year grant from the National Institutes of Health to develop an anti-Lyme vaccine. Mather’s team of researchers is studying a tick’s salivary antigens to develop a vaccine against infection and has observed resistance to infection in guinea pigs. If the project makes it to human trials, that would be beyond the scope of the center’s work. One idea is that if a response can be elicited in humans — that is, if you can make the site of a tick bite itch, so people know they’ve been bitten — you can reduce the amount of time the tick has to infect the human host.
With funding from the Centers for Disease Control, the URI center is testing natural products as alternatives to pesticides for killing ticks. Using a system developed by URI Professor Steven Alm for turf grass experiments, ticks are placed in small, contained “arenas” outdoors and exposed to a variety of products supposed to be lethal to them. It’s a preferred method to scattering ticks about a field, where they are difficult to measure and control.
“So it’s a very robust test, and you get answers right away,” Mather said. The only problem is that, so far, none of the natural products has shown much promise. Rosemary oil, for example, showed some lethality in one formulation, but researchers discovered that the formulation currently on the market did nothing to kill ticks.
Another intriguing possibility is the oil of the Alaskan yellow cedar, which does seem to have some effect on ticks. But the cedar oil is expensive and so has not been tested yet at URI. The problem, Mather said, is that some people have assumed that all cedar oils may be effective. URI tested products made from red cedar oils that are already on the market, and found they had no effect on the ticks at all.
“My job is to focus on these details, and then translate this so people understand it,” he said.
Mather also recently tested the question of whether this winter’s cold weather — from the so-called “polar vortex” — might be killing off ticks. After receiving questions about this on the center’s website, tickencounter.org, he set out to test the theory. He planted two vials of ticks under a light cover of snow on a night when the temperature fell to 3 degrees F. The next day, when the temperature had reached only 15 degrees, he dug the vials out of the snow and shook some ticks into his hand — where they began to crawl across his palm.
You can find a video of this experiment on tickencounter.org, where Mather and his researchers take a very public-centered approach to their discipline. They dubbed the latest video “Polar Vorticks!,” and it’s punctuated with a catchy sound track and graphics. The site also includes FAQs, a blog, and plenty of information about tick identification and Lyme disease prevention.
“I still think there’s hope that tick-borne diseases all can be prevented,” Mather said. “But it’s going to take an integrated effort and it’s going to take a degree of public responsibility.”
For places like Block Island and Jamestown, that might mean accepting that to get rid of the ticks, the deer have to go, too.
“There was once a time on your island when you didn’t have deer and probably didn’t have deer ticks,” Mather said, when asked what he would say to islanders on the subject. “It may be that you would choose to go back to that time. You lived without deer then. You can live without deer now.”