The work has attracted the interest of the Centers for Disease Control and Prevention, whose Division of Vector-Borne Diseases has given the group a five-year grant. “The preliminary work looked good at the proof-of-concept stage,” says Lars Eisen, a research entomologist in the Vector-Borne Diseases Division. “This is additional funding to do a larger-scale field trial in Connecticut, in an inland setting, and an island setting in Maine.”
The project contains complexities. A 1990s attempt to feed the original formulation of ivermectin to deer foundered on the multiple roles that deer play in the landscape. They are not only free-living wildlife, and not only suburban infringers, either charming or invasive depending on whether they pose in your garden or munch on it. They are also the much-desired target of sport hunters, who spend billions of dollars each year for access to them. Ivermectin carried a regulatory restriction, called a withdrawal period, that forbade consuming meat from a treated animal within 48 days. For hunters, that made the proposal a nonstarter.
That led to the best current method of tick control for deer, a device developed by the US Department of Agriculture called a “4-Poster” for its loose resemblance to that style of bed. A four-poster holds a bin of treated corn, two integrated troughs that the corn falls into, and—this is the poster part—two upright paint rollers, saturated with tick-killing chemicals, on either side of either trough. To get to the corn, the deer have to shove their faces between the rollers. That paints their cheeks and ears with the chemicals, which eventually coat the rest of their bodies—a messy field version of the tick-killing solutions that dog and cat owners squirt onto their pets’ necks.
Four-posters seemed like a solution to hunters’ objections, because the chemicals stayed on the surface of the animals’ bodies. And because deer can transport ticks as they wander, the topical treatment offered the possibility of controlling ticks throughout an area or neighborhood. But scientific reviews have found mixed effectiveness, from huge reductions in ticks to not much impact at all. And in some areas, the devices are effectively illegal: Multiple states either discourage or formally ban group feeding to prevent chronic wasting disease—and now, Covid—from circulating through herds.
Plus, they’re labor-intensive and expensive to maintain. “The four-poster method requires dumping a bushel of corn into one of these devices every two weeks, and removing all of the not-eaten corn at the same time, because it gets moldy,” says Sam Telford III, a professor at Tufts University’s Cummings School of Veterinary Medicine with a career-long specialty in tick ecology. In one positive USDA trial, “they required one every six hectares [almost 15 acres] in order to be effective,” he adds. “That’s essentially a full-time person’s job, just to go around and service the four-posters.”
The new method solves the drug-residue problem because moxidectin, the newer version of ivermectin, has a zero-day withdrawal, at least when used in cattle. It retains the neighborhood-wide influence of the four-poster devices. And it may elide concerns about congregate feeding, because multiple deer aren’t sticking their faces into one device; instead, they’re grazing on grain sprayed over a radius of several yards. That also ought to prevent the dominant bucks from monopolizing the food source, and encourage more members of a herd to consume the drug.