A proof-of-concept study shows the potential of feeding wild white-tailed deer corn treated with moxidectin, a derivative of ivermectin that eliminates ticks. Deer are key hosts in the lifecycle of ticks—such as the lone star tick (Amblyomma americanum) shown here—and systemic treatment could aid in reducing tick abundances. And, unlike ivermectin, moxidectin in deer meat may be safely consumed with no required delay. (Photo by Susan Ellis, USDA APHIS PPQ, Bugwood.org)

By Scott Williams, Ph.D.

Scott Williams, Ph.D.

Long before it was splashed across social media for its controversial off-label use for COVID-19 infection, ivermectin was and continues to be used for its intended, labeled purpose, as a highly effective anti-parasitic treatment. Discovered in the late-1970s from a single microorganism isolated from Japanese soil, ivermectin has been dubbed the “wonder drug” in its ability to impact internal and external parasites such as roundworms, lungworms, mites, lice, hornflies, and ticks. In fact, in 2015, the Nobel Prize in medicine was one-half jointly awarded to William C. Campbell, Ph.D., and Satoshi Ōmura, Ph.D., for their work using ivermectin against infections caused by roundworm parasites.

Long used in the cattle industry, ivermectin has also proved to be very effective at clearing white-tailed deer of disease-causing ticks. Decades ago, in the late 1980s and through the 1990s, scientists at the United States Department of Agriculture’s Agricultural Research Service in Kerrville, Texas, researched the efficacy of systemic treatment of white-tailed deer—i.e., feeding deer ivermectin-treated bait—to kill various species of parasitizing ticks (see Miller et al. 1989 and Pound et al. 1996). The impetus for their research was not only to reduce tick abundances to prevent ticks from crossing over, feeding upon, and infecting cattle with agents of the deadly cattle fever but also to reduce human infection with the pathogens that cause Lyme disease, anaplasmosis, and babesiosis, among others.

On the heels of that work, researchers from Maine tried the same strategy for the specific purpose of treating blacklegged ticks (Ixodes scapularis) on free-ranging deer to reduce host-seeking tick abundances across the landscape (see Rand et al. 2000).

Despite its effectiveness in deer, the use of systemic treatment with ivermectin fell out of favor, largely due to labeling restrictions that prevent human consumption of cattle for 48 days post-treatment—problematic in a species like deer for which there are regulated hunting seasons.

Instead, the 4-poster device was developed and commercialized. Four-poster devices entice deer to self-apply a topical formulation of permethrin to their head and necks when they feed on untreated corn in a trough adjacent to vertical, saturated paint rollers. After several decades of research, 4-posters were found to be effective in certain situations, but they are very expensive and labor intensive and they inject tens of thousands of additional calories into increasing the fitness of already overabundant tick reproductive hosts.

Contemporaneously, moxidectin, a more modern derivative of ivermectin, was discovered and, in 2018, was approved by the U.S. Food and Drug Administration for use in medicine in humans over 12 years old for curing river blindness caused by the parasitic worm Onchocerca volvulus. The pour-on formulation Cydectin (Bayer Animal Health) has moxidectin as its active ingredient with a labeled 0-day withdrawal period for human consumption of treated cattle; in other words, muscle tissue and milk can be consumed at any time post-treatment.

A proof-of-concept study shows the potential of feeding wild white-tailed deer corn treated with moxidectin, a derivative of ivermectin that eliminates ticks. Deer are key hosts in tick lifecycles, and systemic treatment could aid in reducing tick abundances. And, unlike ivermectin, moxidectin in deer meat may be safely consumed with no required delay. To test this strategy, free-ranging white-tailed deer were fed moxidectin-treated corn from automatic broadcast feeders in Norwalk, Connecticut, in a study conducted in 2021 and 2022. In both years, deer were captured to document how many parasitizing ticks they carried and determine moxidectin levels in their blood. (Photo by Rebecca Earnest, Yale School of Public Health)

A proof-of-concept study shows the potential of feeding wild white-tailed deer corn treated with moxidectin, a derivative of ivermectin that eliminates ticks. Deer are key hosts in tick lifecycles, and systemic treatment could aid in reducing tick abundances. And, unlike ivermectin, moxidectin in deer meat may be safely consumed with no required delay. To test this strategy, free-ranging white-tailed deer were fed moxidectin-treated corn from automatic broadcast feeders in Norwalk, Connecticut, in a study conducted in 2021 and 2022. In both years, deer were captured to document how many parasitizing ticks they carried and determine moxidectin levels in their blood. The efforts were successful in that moxidectin levels known to be effective at eliminating ticks (equal to or greater than 5-8 parts per billion) were detected in 24 of 29 (83 percent) deer captured when treated corn was available. (Photo by Rebecca Earnest, Yale School of Public Health)

With this new development, my colleagues and I at the Connecticut Agricultural Experiment Station and partners at White Buffalo, Inc., launched a proof-of-concept study funded by the U.S. Centers for Disease Control and Prevention (CDC) to determine if the oral delivery of Cydectin-treated corn to wild white-tailed deer for systemic tick management was even possible. Our findings are reported in a new article published in May in the Journal of Medical Entomology.

Through spring and early summer 2021 and 2022, when nymphal and adult lone star ticks (Amblyomma americanum) were actively host-seeking, a herd of free-ranging white-tailed deer were fed Cydectin-treated corn from automatic broadcast feeders in Norwalk, Connecticut. In both years, deer were captured to document how many parasitizing ticks they carried and determine moxidectin levels in their blood.

These efforts were successful in that moxidectin levels known to be effective at eliminating ticks (equal to or greater than 5-8 parts per billion) were detected in 24 of 29 (83 percent) deer captured when treated corn was available. Additionally, there was no difference in abundances of parasitizing ticks on deer with varying levels of moxidectin, which was expected as systemic acaricide treatment does not deter ticks from feeding but rather eliminates ticks that have ingested blood containing sufficient avermectin levels. However, fewer engorged or partially engorged A. americanum were found on deer with increased moxidectin levels, as parasitizing ticks need to ingest treated blood to expose themselves to the acaricide found within.

The approach of oral delivery of systemic acaricides has the potential to be effective at not only eliminating parasitizing ticks on individual animals but also reducing abundances of host-seeking ticks across the landscape. Along these lines, we recently received a research grant from the CDC to investigate the combined impacts of treatment of white-tailed deer and Peromyscus mice on host-seeking blacklegged ticks (Ixodes scapularis) and their associated pathogens.

In the meantime, more detail on this proof-of-concept systemic treatment of white-tailed deer with moxidectin can be found in our recent article in the Journal of Medical Entomology.

Scott Williams, Ph.D., is Chief Scientist and Head of the Department of Environmental Science and Forestry at the Connecticut Agricultural Experiment Station in New Haven, Connecticut. Email: scott.williams@ct.gov.