Donkeys hold the key to natural tick repellents

It started with a puzzle on a quiet farmyard: why did the donkeys never seem to get ticks? While deer, dogs, and humans all battled the tiny parasites, the donkeys stayed mostly untouched. Now, scientists think they’ve found out why, and their discovery could change how we protect ourselves from tick bites.

If you’ve ever pulled one of those stubborn bloodsuckers off your leg after a hike, you know just how unsettling they can be. Beyond being gross, ticks are also dangerous; they spread Lyme disease, babesiosis, and a growing list of other infections. In the United States, the blacklegged tick (scientific name Ixodes scapularis) is the number one culprit for human bites, especially in the Northeast and Midwest.

For decades, the go-to protection has been DEET, a synthetic chemical invented in the 1940s. It works, but it isn’t perfect. Some people dislike its smell or feel, ticks and mosquitoes can sometimes tolerate it, and concerns about overuse have fueled interest in alternatives. What if, instead of inventing new chemicals in the lab, we borrowed tricks from nature itself?

That’s exactly what a team of researchers from the University of Massachusetts Amherst and the U.S. Department of Agriculture explored in their study, published in PLOS ONE, which takes inspiration from an unlikely source: donkeys.

Donkeys, it turns out, secrete a natural oily substance on their skin that ticks don’t like. One of the key ingredients in this oily “sebum” is a chemical called (E)-oct-2-enal. Scientists call it an “allomone”, a chemical a species gives off that repels other creatures. For ticks, it’s like a warning sign: don’t bother trying to feed here.

This natural form of “chemical aposematism” (a fancy word for warning signals in the animal world) is common in insects, but until now, no one had shown it could work against blacklegged ticks.

The research team, consisting of Stephen Rich, Eric Siegel, Sophia Goodnow, Lucy Thompson, Sarah Nicolson, Elizabeth MacLeod, Andrew Li, and Guang Xu, decided to put (E)-oct-2-enal to the test against Ixodes scapularis in the lab.

To put ticks to the test, the scientists placed them onto filter paper treated with either: ethanol (a neutral control, basically no repellent), DEET (the gold standard), or (E)-oct-2-enal (the donkey-inspired candidate). They then watched how the ticks behaved, using tracking software to measure their movement, hesitation, and willingness to cross treated zones.

Here’s what they found. Ticks hated both DEET and (E)-oct-2-enal. They slowed down, wandered in circles, and often refused to cross the treated area. Males were especially sensitive. Male ticks exposed to (E)-oct-2-enal were even more repelled than when exposed to DEET. Females were tougher. Both repellents worked on them, but the effects were less dramatic. The donkey chemical sometimes lasted longer. In some trials, ticks stayed repelled for over 8 minutes (the full experiment window), while ethanol-treated ticks walked right across in seconds. Most importantly, this was the first demonstration that a natural vertebrate-emitted chemical can repel blacklegged ticks.

This research can inspire the creation of new tools for tick prevention. DEET isn’t going anywhere, but having new repellents could give people more options, especially natural compounds that might feel safer for daily use. Imagine bug sprays, treated clothing, or pet collars infused with (E)-oct-2-enal. The study could also provide insights on how to offer protection for animals. In fact, ticks don’t just bother humans; they plague livestock, pets, and wildlife. A donkey-inspired repellent could help keep ticks off deer, dogs, or cattle, cutting down the chances they spread disease. Also, learning from “non-host” signals is important to uncover what attracts ticks. Some animals, like donkeys or certain dog breeds, just don’t get as many ticks. Understanding and copying their chemical defenses could lead to a new generation of repellents designed around nature’s own warning systems. More broadly, relying on natural chemicals offers a path toward greener pest control. Instead of spraying more synthetic insecticides, which ticks can evolve resistance to, semiochemicals like (E)-oct-2-enal could offer a more eco-friendly alternative.

As this was a lab study, we don’t yet know how well (E)-oct-2-enal would work outdoors, on skin, or over hours instead of minutes. Ticks are stubborn, and in the wild, they follow strong host cues like carbon dioxide and heat. Would they ignore a donkey-scented signal if a juicy human was nearby? That’s a question future studies need to answer. But the early results are promising, and they show just how much we can still learn by paying attention to the subtle chemical conversations happening between species.

We don't know whether, in the future, hikers might spray on a donkey-inspired tick repellent before heading into the woods; however, this study is the first step in that direction. The idea that we can borrow natural “do not disturb” signals from animals to keep ourselves safe is both elegant and hopeful.

As the authors point out, much more research is needed, especially on how these chemicals work, whether they can be formulated into safe products, and how different life stages of ticks (like nymphs, the main spreaders of Lyme disease) respond. Still, the takeaway is simple: sometimes, nature already has the answers. We just have to notice them, and in this case, the humble donkey may have handed us a powerful new tool in the fight against ticks.

If you want to learn more, read the original article titled "Exploiting a natural instance of vertebrate-posed chemical aposematism for tick bite prevention: Repellency of Ixodes scapularis with (E)-Oct-2-enal" on PLOS One at http://dx.doi.org/10.1371/journal.pone.0317975.