The April issue of the newsletter of the Maine Woodland Owners includes an important article on deer, ticks, and Lyme disease, written by Peter Rand, MD, and Charles Lubelczyk, Vector Ecologist. The writers are associated with the Vector-borne Disease Laboratory of the Maine Medical Center Research Institute in Scarborough.
Among the interesting information presented was this sobering conclusion: “As unpopular as it may be, deer control by lethal means remains a primary tool in what should be an integrated approach to reducing the threat. Where deer can be excluded or maintained well below 10 per square mile, little else may be needed to control deer ticks.”
I encourage you to read this entire article. We all need to know more about deer ticks and Lyme disease, particularly because this is a significant issue as a new 15-year deer management plan is created.
Last year, despite the summer drought that should have depleted their numbers, deer ticks, which are now transmitting the agents of five human diseases in Maine, thrived, particularly in the state’s southern coastal counties. More than 1,400 cases of Lyme disease, the highest yet, were confirmed by the Maine Centers for Disease Control, which, due to underreporting, may represent one tenth of actual cases.
This follows 2015, when Maine had the second highest per capita incidence of Lyme disease in the nation. Reported incidence of two emerging, deer tick-transmitted diseases, anaplasmosis and babesiosis, have increased over seven-fold in the last five years. That’s a lot of suffering.
So what do deer have to do with it? Because white-tails are by far the major source of the blood that female deer ticks use to develop eggs, just about everything. Several studies have shown a decrease in ticks following deer reduction or exclusion.
An extreme example in Maine was the singular opportunity on Monhegan Island to document the virtual elimination of deer ticks and Lyme disease following the complete and permanent removal of deer. The reduction of ticks and elimination of Lyme dates to the late 1990s, and persists.
It’s true that female deer ticks can feed on other mid-size and larger animals, including dogs, cats, coyotes and horses, but the contribution of these hosts to the overall tick population is minimal, because deer ticks are specifically attracted to deer, and because the availability of alternative hosts, in comparison with white-tails, is minimal.
But where deer are present, their abundance, with respect to Lyme and other tick-borne diseases, is only half the story, since it is by feeding on infected small mammals and birds that deer tick larvae and nymphs obtain and multiply the infectious agents – as nymphs in summer and adults in the fall and spring. Mice, chipmunks, squirrels and birds, such as robins, are reservoir hosts.
Moles, voles, and other small mammals also harbor pathogens, while infected songbirds provide transportation for disease-carrying ticks from distant places where these diseases are established. So, while mice and birds control the pathogens, deer control the size of the tick population. And without the bite of an infected tick, there’s no way for humans to acquire Lyme disease.
If deer are critical to maintaining deer ticks, can effective deer management lower the risk? These are relevant questions, with the Maine Department of Inland Fisheries and Wildlife undertaking its comprehensive 10-year deer management plan. Some studies have estimated that herd densities of 10-15 per square mile will reduce human cases.
In a three-year project involving 29 square miles at eight tick-infested sites from Rockland to York, we directly compared deer pellet group counts with the number of adult deer ticks by flagging vegetation in the autumn from 1998-2000. The relationship between deer numbers and the abundance of ticks was statistically highly significant. With deer reductions, tick numbers dropped to zero. Reducing deer to zero, however, shouldn’t be needed to break the tick’s life cycle.
While our study supports current IF&W thinking that a target of no more than 15 deer per square mile will be needed to effectively reduce the threat of tick-transmitted diseases, it also demonstrates a very important corollary: the more deer, the more ticks (See chart above.)
Some non-lethal means of controlling deer have been tried, though neither trapping nor immunocontraception are feasible, the first because of unacceptable stress and mortality and the second is impractical with free-ranging deer.
A device called a “4 poster”(http://www.crdaniels.com/4poster) has been developed to coat the deer topically with the acaricide permethrin as they reach through treated rollers to feed. While initial trials achieved 60-82% tick reduction within three years, there are problems. Where natural feed is limited, the devices may need refilling three times weekly. And where natural feed is abundant, deer don’t visit the devices. Filling and maintaining them is highly labor-intensive, they are expensive – an initial package, with accessories, costs $3,600 – and they must be deployed in perpetuity.
In a five-year study in which the devices were deployed at what was considered a maximum practical density (1/150 acres rather than the recommended 1/50 acres) tick density was reduced only 8%.
As unpopular as it may be, deer control by lethal means remains a primary tool in what should be an integrated approach to reducing the threat. Where deer can be excluded or maintained well below 10 per square mile, little else may be needed to control deer ticks. Equally unpopular may be professional application of extremely effective acaricide sprays, today of far less concern than previously. We can kill ticks; the decision to do so is entirely up to the owner of infested property.