Back to the study. The Connecticut group chose a site where they could monitor a small, contained deer herd: a peninsula jutting into Long Island Sound where the property surrounding a decommissioned power plant is returning to a wild state. They deployed a feeder that would regularly scatter a small amount of corn—enough to dope the animals, but not to fatten them up—and dosed the bait with moxidectin, a second-generation ivermectin that comes in a veterinary formula. Then, with the help of the wildlife-management nonprofit White Buffalo, they drugged and captured deer periodically through 2021 and 2022, tagging them, taking blood samples, and counting the ticks on their bodies.
They focused on the lone star tick, Amblyomma americanum, which can transmit ehrlichiosis, borreliosis, Bourbon and Heartland viruses, and the meat allergy known as alpha-gal syndrome. That tick prefers to feed on white-tailed deer, while the type that spreads Lyme disease also preys on rodents. (For both species, humans are opportunistic targets. Ticks don’t have good eyesight, but they detect exhaled carbon dioxide—so when we blunder by them as they perch on vegetation, they sense us and attack.)
In this first study round, the researchers found that the number of ticks crawling on an individual deer did not change with blood levels of the drug—which makes sense, because the ticks wouldn’t know before biting whether a deer was dosed or not. However, the amount of latched-on, blood-engorged ticks declined as the level of the drug rose in a deer’s blood. “As the serum level increases, ticks don’t have to consume as much before they get paralyzed and fall off,” Williams says. “You wouldn’t see ticks engorged and feeding on animals with higher serum levels, because it would impact them much quicker.”
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.