Do Opossums Really Eat Thousands of Ticks? What the Research Shows

Originally published 2020-04-20.

Probably not. The popular claim that a single opossum eats about 5,500 ticks a week, and that opossums could be "the answer to Lyme disease", traces back to one small laboratory study from 2009. Later research looked inside the stomachs of wild opossums and found no ticks at all. Opossums are fascinating, mostly harmless and useful animals, but inviting them into your garden will not protect you from ticks. Checking yourself after time outdoors still matters far more.

Here is where the famous number came from, what newer studies found, and some genuinely remarkable facts about North America's only marsupial.

Meet the Virginia opossum

Opossums are marsupials, mammals whose young finish developing in a pouch, like kangaroos and koalas. They are not close relatives of those Australian animals, though; opossums belong to their own American order, Didelphimorphia, which includes about 126 species, most of them in Central and South America. The name comes from the Powhatan language of Virginia and was first written down by English colonists in the early 1600s. Australia's "possums" are a different group, named for their resemblance.

The Virginia opossum is the species most people in the United States and Canada know. It is the only marsupial found north of Mexico and the most northerly marsupial in the world, ranging from southern Canada to Costa Rica.

A mother opossum carries her young on her back. She has 13 teats in her pouch, litters average eight or nine babies, and once the young outgrow the pouch they ride around clinging to her fur. Only around one in ten survives to adulthood.

A few more facts that surprise people:

• They have 50 teeth, more than any other North American land mammal.
• "Playing possum" is real. When badly frightened, an opossum may collapse, go limp with its mouth open and heart rate halved, and stay that way for hours. It is an involuntary reaction rather than a clever act.
• They are rarely rabid. Opossums are naturally resistant to rabies, though a few cases are found each year, so a wild opossum should still never be handled.
• They resist snake venom, thanks partly to a protein in their blood that neutralises toxins.
• They live fast. In the wild most live only about two years; in captivity around four.

Opossums are solitary and nocturnal, and they eat almost anything: insects and other invertebrates make up most of their animal food, along with fruit, carrion, eggs, small rodents and the occasional snake. Their grasping tail helps them climb, and they are comfortable both on the ground and in trees.

Lyme disease in brief

Lyme disease is caused by Borrelia bacteria and is spread to people by the bites of infected blacklegged ticks, often called deer ticks. People do not catch it from deer. Deer matter because adult ticks feed and mate on them, which keeps tick numbers high, while small mammals such as white-footed mice are the main source of infection for young ticks.

It is far from rare. More than 80,000 cases are reported to the US Centers for Disease Control and Prevention each year, and insurance data suggest around 476,000 Americans may be diagnosed and treated annually, although the CDC cautions that this estimate may be high. Caught early, Lyme disease is usually treated successfully with antibiotics. Left untreated, the infection can spread to the joints, heart and nervous system, causing arthritis, facial palsy or heart-rhythm problems. Anyone who develops a rash, fever or joint pain after a tick bite should talk to a doctor.

Where the 5,500-ticks figure came from

In 2009 ecologist Felicia Keesing and colleagues, including Richard Ostfeld of the Cary Institute of Ecosystem Studies, published a study in Proceedings of the Royal Society B on how different animals handle ticks. In one part of it, five captive Virginia opossums were each loaded with 100 tick larvae. The researchers collected the ticks that dropped off over four days and found very few; they assumed the rest had been groomed off and eaten. The animals were released without being checked for remaining ticks.

From that result the study estimated that opossums kill most of the ticks that try to feed on them, and Ostfeld described them as "net destroyers of ticks". Popular articles then multiplied the lab figures out to a wild opossum's week and arrived at roughly 5,500 ticks. The number spread widely online and on wildlife-charity posters, often turned into "opossums eat 95% of ticks" or "opossums stop Lyme disease".

The study had a narrower and more reliable point. Opossums appear to be poor hosts for the Lyme bacterium, so ticks that feed on them are less likely to become infected than ticks that feed on mice. That is a genuine contribution of opossums to tick ecology, but it is different from them hoovering up thousands of ticks.

What later research found

In 2021 Cecilia Hennessy and Kaitlyn Hild of Eureka College in Illinois tested the claim directly. They reviewed 23 published studies of opossum diets and found no evidence that opossums eat tick nymphs or adult ticks. They then examined the stomach contents of 32 opossums from central Illinois. Ticks were attached to the bodies, yet no tick parts were found in any of the stomachs.

The researchers also pointed out weaknesses in the 2009 work: only five animals, artificial laboratory conditions, and no check for ticks still attached when the animals were released. Ticks feeding on opossums take from three to nine days to finish their blood meal, so a four-day count could easily miss many of them. As of 2023, no published study had reported ticks in opossum stomach contents.

So the honest summary is this: opossums may remove some ticks while grooming and may dilute Lyme infection in tick populations, but there is no good evidence that they eat ticks in large numbers, and they themselves can carry ticks and tick-borne infections.

Should you welcome opossums in your yard?

If an opossum visits your garden, there is usually no need to worry. It will eat insects, snails, fallen fruit and the odd mouse, and move on. What University of Illinois Extension advises against is trying to attract them deliberately with food. Pet food or scraps left outside also draw raccoons, rats, mice and feral cats, and mice are among the most important hosts for Lyme-carrying ticks.

Practical tick protection looks the same with or without opossums:

• Keep grass short and clear leaf litter and brush near the house.
• Wear long trousers tucked into socks in long grass and woodland, and consider an approved insect repellent.
• Check yourself, your children and your pets for ticks after time outdoors, paying attention to the scalp, behind the ears, armpits and groin.
• Remove ticks promptly with fine-tipped tweezers, pulling steadily close to the skin.
• Talk to a doctor if a rash or flu-like illness follows a bite.

Opossums deserve their growing fan club. They are gentle, rarely carry rabies and clean up plenty of garden pests. They just are not a tick-control service.

Photo credits: goodpeoplenews.com, littlestsimonsisland.com, rentokil.com.

Sources: University of Illinois Extension, June 2024: Reassessing the opossum-tick relationship; Wikipedia: Virginia opossum; Wikipedia: Opossum; CDC: Lyme disease facts and statistics.