Bot flies (Cuterebra) infest dogs accidentally, and while no product is specifically labeled to prevent them, a combination of environmental management, seasonal awareness, and parasite preventatives can significantly reduce your dog’s risk. These flies are native to North America and most active from June through September, with infestations in dogs most commonly discovered in early September.
How Dogs Pick Up Bot Flies
Dogs are not the intended host. Cuterebra flies normally target rodents and rabbits. The adult females lay clusters of 5 to 15 eggs on grass stems, wood chips, and bark along narrow trails or near the openings of rodent burrows. Some species even enter the burrows to deposit eggs directly inside. The eggs sit dormant for about a week, then wait for a sudden increase in temperature, like the body heat of an animal passing by, to trigger hatching within seconds.
Once hatched, the tiny larvae (just 2 to 4 millimeters long) crawl into a dog’s fur and enter through any natural body opening: the mouth, nose, eyes, or other areas. They do not penetrate intact skin. From there, larvae migrate under the skin and form a visible lump with a small breathing hole, called a warble. This is the stage most owners notice. But in some cases, larvae take a wrong turn and migrate into the brain, eyes, or respiratory system instead, which is far more dangerous.
When and Where the Risk Is Highest
Bot fly season runs from June through September across most of North America. In cooler, temperate regions there is only one flight season per year, while warmer southern areas may see two, with adults active in late spring and again in summer. The first week of September is a particularly common time for dog owners to discover infestations.
Any property with active rodent or rabbit populations carries risk. Bot flies concentrate their eggs around rabbit runs, squirrel paths, chipmunk burrows, and mouse nests. Wooded edges, overgrown fields, and yards bordering wild areas are prime habitat. Cases have been documented across the U.S. and Canada, with reports from states as varied as Colorado and Texas. Small-breed dogs appear to be especially vulnerable to serious complications. Anecdotally, breeds like West Highland White Terriers and Yorkshire Terriers show up disproportionately in case reports of severe infestations.
Reduce Rodent and Rabbit Habitat in Your Yard
Since bot flies lay eggs exclusively around rodent and rabbit activity, reducing those populations near your home is the most direct form of prevention. Clear brush piles, woodpiles, and dense undergrowth where rodents nest. Mow tall grass along fence lines and property edges where rabbits create trails. Seal gaps under sheds, decks, and outbuildings that attract burrowing animals. If you have bird feeders, consider relocating them away from areas your dog frequents, since spilled seed draws mice and chipmunks.
You don’t need to eliminate every rodent on your property, but reducing the density of burrows and trails in areas your dog uses regularly means fewer egg-laying sites within reach.
Limit Exposure During Peak Season
From June through September, keep your dog away from known rabbit warrens and rodent burrows. If your dog likes to explore wooded edges, stick to open, well-maintained paths rather than letting them nose through underbrush. Dogs that dig around rodent holes or stick their faces into burrows are at the highest risk, since larvae enter through the nose, mouth, and eyes.
Supervised outdoor time matters more during these months than at any other time of year. If you live in an area with heavy wildlife activity, shorter leash walks through maintained areas are safer than off-leash exploration through tall grass and brush.
Use Parasite Preventatives Year-Round
No commercial product is specifically approved to prevent Cuterebra in dogs. However, two categories of preventatives may help. Topical parasite products containing ingredients like fipronil or imidacloprid could offer some deterrent effect against larvae on the skin. Monthly heartworm preventatives, which belong to a drug class called macrocyclic lactones, may also help control migrating larvae before they can establish themselves under the skin.
Keeping your dog on both a topical preventative and a monthly heartworm medication during bot fly season gives you the best pharmacological safety net available. These products serve double duty by also protecting against fleas, ticks, and heartworm, so there’s no downside to maintaining them consistently.
Check Your Dog After Outdoor Excursions
After walks through wooded or grassy areas during summer and early fall, run your hands thoroughly over your dog’s entire body. Pay close attention to the face, around the nose and eyes, and under the chin. Bot fly eggs are tiny and can be difficult to spot, but a thorough hands-on check can catch larvae before they have a chance to enter through a body opening. Brushing or combing your dog after returning from high-risk areas helps dislodge anything clinging to the fur.
Bathing your dog after outings in heavily wooded or rodent-dense areas during peak season adds another layer of protection. Warm water and light scrubbing can wash away eggs or newly hatched larvae that haven’t yet found an entry point.
Signs Your Dog May Already Be Infested
The most obvious sign is a firm, marble-sized lump under the skin with a small hole at the center. You might see the larva’s breathing tube poking through, or notice fluid draining from the opening. Dogs often lick or scratch at the spot. These warbles most commonly appear on the head, neck, or trunk.
More concerning are signs of abnormal larval migration. If larvae travel to the brain or nervous system, your dog may show lethargy, loss of coordination, circling, head tilt, behavior changes, blindness, or seizures. Respiratory signs can develop if larvae reach the lungs. Eye involvement can affect both the front and back chambers of the eye or the eye socket itself. Small-breed dogs are at particular risk for a severe inflammatory response that can become life-threatening.
Why You Should Never Remove a Larva Yourself
If you find a warble on your dog, do not squeeze it or try to pull the larva out. Rupturing the larva under the skin can trigger a serious allergic reaction. Veterinarians typically pre-treat with medications to reduce the risk of a hypersensitivity response before carefully extracting the intact larva. The extraction itself is usually straightforward when done properly: the vet enlarges the breathing hole slightly and eases the larva out whole. Crushing it or leaving fragments behind creates complications that are entirely avoidable with professional removal.
For cases where larvae have migrated internally, systemic antiparasitic medication like ivermectin can sometimes be used to treat the infestation, though outcomes depend on where the larva has traveled and how much damage has already occurred.