Parasites in pets in Valley are on the upswing
By Sarah Bahan, DVM
For The Weekly Sun
With October upon us, and everyone beginning to cuddle up by the fire to watch creepy movies, the image of an alien popping out of Sigourney Weaver’s chest seems particularly seasonally appropriate. In this case, it also provides an apt, if not slightly hyperbolic, analogy for describing the organism at the heart of our very own local alien invasion. I’m not talking about extraterrestrials but, rather, botfly larvae.
Botflies (also known as warble flies, gadflies, and other colorful names) are a family of about 150 species of insect whose larvae are internal parasites of mammals. While most species of botfly are fairly species specific, such as horse botflies that only lay their eggs on horses, the species that targets rabbits (Cuterebra sp.) is not as picky and can inadvertently end up on dogs or cats instead.
The typical Cuterebra life cycle consists of an adult fly laying eggs in the environment near where rabbits live and forage, such as around burrow entrances, along trails and on the bottom of fenceposts. The eggs are stimulated to hatch by the body heat of a passing mammal, at which point the larvae jump onto its fur. As the animal grooms itself later, it ingests the first-stage larvae. Those larvae then migrate through the animal’s body to the skin, where they create a walled-off space in the subcutaneous tissue that has a small breathing hole in the center, and mature into third-stage larvae. Sometimes fluid is discharged from the site and one can even see the larvae moving within if one looks closely. The larvae grow quickly in the subcutaneous tissue and, once large enough, exit through an enlarged breathing hole, usually about three to six weeks after initial infection. At this point they burrow into the soil where they eventually pupate and emerge as adults.
Cuterebra larvae are most often found in spots that the animal cannot easily lick or scratch, such as along its spine. Though botfly larvae don’t usually cause disease, they are uncomfortable for their host, not to mention pet owners. Rarely, an aberrant migration to an unintended part of the body, such as the eye or brain, can lead to a condition known as myiasis, however, and cause ocular or neurologic problems, for instance.
Dr. Mark Acker, of Sawtooth Animal Center in Bellevue, has only seen a couple of instances of botfly infestation in over 35 years of practice, yet dozens have been removed from dogs, cats, and rabbits in recent weeks. Why the sudden increase? As residents of Hailey and Bellevue have undoubtedly noted, the South Valley is now home to a booming population of feral domestic rabbits in yards and along the side of the road. In fact, it was a few of these feral rabbits that were brought in by Good Samaritans that was the first clue to the Cuterebra problem.
The Magic Valley Regional Office of the Idaho Department of Fish and Game has also noted a recent increase in wild cottontail and jackrabbit populations. The soaring rabbit populations are likely a direct result of decreased predator numbers in the Valley due to a recent mange outbreak that claimed many coyotes and foxes. After those populations were significantly reduced, the local rabbit species proliferated. Sightings of healthy foxes and coyotes are on the rise, however, so local predator numbers should continue to increase, probably abetted by the bountiful supply of rabbits.
Dr. Mark L. Drew, the Idaho Fish and Game state veterinarian, says this is a common cycle in which the number of wild canids will increase following a boom in rabbit numbers, leading to rising numbers of predators just as increasing predation pressure begins to thin rabbit numbers. Greater densities of predators and less abundant food sources make them more vulnerable to disease, such as sarcoptic mange, causing their numbers to fall.
As winter approaches, we’re unlikely to see continued botfly infestations continue.
Until then, if you find a botfly larvae on your animal, bring your pet to a veterinarian to have it removed with sterile medical instruments, the hole cleaned and flushed with antiseptic solution, and your pet put on antibiotics to prevent a subsequent infection.
Never squeeze the cyst or try to rip the larvae out. If the botfly larvae is damaged during removal, it could potentially cause anaphylactic shock in your animal or a severe infection. Rabbits are particularly sensitive.
Instead of pulling the larvae out directly, you can encourage the botfly to depart on its own by covering its breathing hole and suffocating it with thick lubricants, such as Vaseline or K-Y Jelly.
Dr. Sarah Bahan, a native of the Wood River Valley, is the newest addition to the veterinary team at the Sawtooth Animal Center in Bellevue.