Debunking the Myth: Opossums Don’t Eat Ticks

You’ve probably seen the meme: it features a cute picture of a Virginia opossum, with text that implores the reader to appreciate opossums because they eat ticks (see Figure 1 for an example). Among the many variations of this meme that I’ve encountered, the rate of tick-ingestion varies from thousands per week to thousands per season, but the intent of the message doesn’t change: if you hate ticks (and who doesn’t?), then you should love opossums. There’s just one problem with this sentiment: it’s not true.

Student Poses A Question and the Research Begins

I was teaching a wildlife course at Eureka College when one of my students, Kaitlyn Hild, asked about this meme. Based on my experience live-trapping and handling hundreds of opossums as a PhD student, I opined that the claim seemed dubious, and encouraged Hild to investigate the source of the claim. She came to the next class armed with an answer: it originated from a scientific paper, titled, Hosts as ecological traps for vectors of Lyme disease¹. We read the paper, which raised more questions than it answered. We drew up a research plan and dove in. I applied for a scientific collection permit from the Illinois Department of Natural Resources and started collecting road-killed opossums for stomach analyses. After three seasons of pulling over and snatching opossum carcasses off the roads, and only getting 11, we decided we needed a better source. I reached out to a local nuisance trapper and explained our project. He called me about two weeks later to let me know he had a freezer full of carcasses, all humanely euthanized per his licensure with the state. I surveyed each opossum for ticks before dissecting and removing its stomach. Then came the fun part. Using a dissecting microscope, Hild searched the entire contents of 33 stomachs, bit by bit, identifying the source type (see pie chart). No one can accuse us of having weak stomachs!

Meanwhile, I searched the scientific literature for results on the diet of opossums. With help from our research librarians, I tracked down 23 published accounts. The earliest research (1851) was by John James Audubon², which described the stomach contents of an opossum that he had shot and gutted in the woods. Subsequent research was more comprehensive, especially a paper by Bill Hamilton³ that described the diet items of 186 opossums much in the same manner we were doing: recording the amount of each type of insect, flower and vertebrate species found. In total, the literature presented the results of diet analysis of over 1,280 opossums from across their range in the United States. Not a single paper reported ticks being found in the stomachs, or in any other section of the gastrointestinal tract, of Virginia opossums.

Hild, meanwhile, was becoming more surprised that she wasn’t finding any ticks in the stomach contents. She found worms, a French fry, broken glass, and a mostly undigested shrew, but no ticks. She found three intact fleas (see Figure 3), which may be some indication of grooming, but no ticks. I admit I was surprised. I figured we’d find a few ticks, that it’s hard to imagine how an animal can groom off fleas but not ticks, or even that they might eat a tick accidentally while eating another animal that has ticks on it.

We wrote up our results (you can read our paper in the International Journal of Ticks and Tick-Based Diseases⁴) and the findings were clear: there were no ticks in the stomachs and there were no published reports of ticks in the diets of Virginia opossums. So where did this fantastical “fact” about opossums eating ticks originate?

The paper that started this investigation¹ was written by scientists in New York state who were investigating the potential for various vertebrate species to harbor blacklegged ticks (Ixodes scapularis), as this species is a carrier of Lyme disease. They live-trapped four opossums and kenneled them in a lab, where they put 100 larval blacklegged ticks on each individual. Other species were undergoing the same treatment, including chipmunks, squirrels, mice, veeries and catbirds. The animals were observed for four days, at which point the researchers counted the ticks that had obtained a bloodmeal and fell off.

You may be lucky enough not to know that when ticks feed, their bodies expand and they get rounder and rounder (see Figure 4) until they can’t consume another drop and they drop off, a process called ‘feeding to repletion.’ Unlike the kennels of the other species, the bottoms of the opossum kennels were surprisingly devoid of ticks. The opossums were set free in the woods whence they came, without being combed or checked for ticks that might have still been in their fur. The researchers concluded that all the ticks that hadn’t dropped off in the kennels must have been eaten by the opossums. Thus, was born the myth of the tick-hungry marsupial.

Physiological Facts About Opossums

Their claim doesn’t take into consideration some interesting physiological facts about opossums. Opossums have a lower metabolism than the other species tested, which contributes to their lower body temperature and lower blood pressure. Previous research shows that ticks feed more slowly at lower temperatures and on animals with lower blood pressure. After four days, it’s likely the ticks were still on the opossums, slowly obtaining their bloodmeal. The researchers could have easily checked this by combing the opossums before release, but they didn’t. They obviously didn’t check the stomach contents of the opossums to confirm tick consumption, and they also didn’t poke through the scat left behind for tick remains (tough tick parts are identifiable after passing through the intestines).

The extraordinary numbers that appeared in the memes of tick-eating opossum came from the scientists’ creative extrapolation of the actual numbers of ticks involved in the experiment. Math-lovers pay attention. On average, only 3.5 ticks fell from the four opossums during their stay in the lab, or about 3 percent of the ticks placed on the opossums. Other studies of opossums in New York State⁴ have found that they carry an average of 199 (±90) larval ticks during tick season. The scientists worked backwards to conclude that opossums must therefore host more than 5,500 larval ticks: theorizing that 97 percent are groomed off and eaten, so only 199 remained to be counted in examinations of wild animals. Therefore, opossums would be eating 5,301 (97 percent) ticks every tick season. There is no evidence that opossums are harboring that many larval ticks per tick season and certainly no evidence that they are eating any at all. This mathematical sleight of hand gives a whole new meaning to fuzzy math.

Peer review is a process whereby scientific papers are scrutinized by other experts in the field before being accepted for publication into a scientific journal. That these claims made it past peer review is a good reminder that one must always maintain a healthy level of skepticism, especially when the conclusions are as extraordinary as these are.

The Value of Opossums

In the end, what does it matter? What if the memes give pause to someone who might abuse opossums because they think they’re ugly? After all, what species could use some good PR more than the lowly opossum? The problem is, people have begun attracting opossums to their residences on purpose, believing that the opossums will come for the table scraps but eat a thousand or so ticks from their yard for dessert. You don’t need to be a wildlife scientist to know that putting out food to attract opossums will also attract raccoons, feral cats, rats, foxes and coyotes.

This causes two problems: First, wildlife will associate humans with a food source. This is a common precursor to acting aggressively towards humans, which will typically lead to the removal (i.e., death) of the human-conditioned animals. Most documented coyote attacks, for example, occurred in suburbs where residents had been purposefully feeding coyotes, conditioning them to associate humans with food. Secondly, attracting wildlife to a food source increases the risk of disease contagion among the animals themselves and from the animals to humans (Figure 5). Ironically, the researchers were researching opossums as a means to understand how humans can reduce their risk to Lyme disease exposure, but the end result might actually increase human risk to wildlife-based diseases, thanks to humans setting out food ‘for opossums.’

I love opossums. I do think they’re cute, even when they’re baring their teeth; even though they smell like garbage. To be fair, they’re not very fast or strong, so their best defenses are being ugly, stinky and playing dead. Opossums do not need to validate their existence by eating ticks like some sort of Pac-Man of the forest. They do not need to earn their keep. They are an important part of our ecosystems for other reasons, like scavenging, seed dispersal and as prey for other species.

Opossums are the only native marsupial in Illinois, but they’re not the only marsupial in North America, as Mexico alone has nine species of marsupial. Opossums have prehensile tails, but they don’t hang on branches by their tails on purpose; if you ever see one doing so, it’s because a person set them up that way. If left that way, they will get tired and fall to the ground (ouch!). They actually use their tails to carry nesting material to their dens. Opossums are amazing, all on their own. They don’t need to be superheroes to earn our respect.

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Cecilia Hennessy, PhD, studied wildlife and genetics starting in 2002, focusing on issues of population connectivity and wildlife disease. She was a biology professor at Eureka College in Eureka, Illinois until a family move to Milwaukee, Wisconsin in 2021. She can be reached at cecilia.hennessy@gmail.com.

Resources

1. Keesing, F., Brunner, J., Duerr, S., Killilea, M., LoGiudice, K., Schmidt, K., Vuong, H., Ostfeld, R.S., 2009. Hosts as ecological traps for the vector of Lyme disease. Proc. R. Soc. B 276, 3911-3919.
2. Audubon, J.J., Bachman, J., 1851. The Viviparous Quadrupeds of North America, Vol. 2. V.G. Audubon, New York, New York, USA, pp. 107-125.
3. Hamilton, Jr., W.J., 1951. The food of the opossum in New York State. Journal Wildlife Management. 15, 258-264.
4. Hennessy, C., & Hild, K. 2021. Are Virginia opossums really ecological traps for ticks? Groundtruthing laboratory observations. Ticks and Tick-borne Diseases, 12(5), 101780.