Bot Grubs; The Monster Within

One of the clinic’s technicians came in this morning and asked me about a mouse she had found in her garden. The poor thing was not moving very rapidly and therefore was very easy to catch. After capturing the mouse she noticed that there was a “worm” underneath the skin, which was nearly as big as the mouse itself. Feeling sorry for the creature, she mercifully killed it with her shovel and after arriving at work she asked me if I had any idea what it was.

The most likely culprit is the rodent botfly Cuterebra spp.. Botflies are a diverse group which parasitizes a wide variety of species including; rodents, dogs, cats, wildlife, livestock and humans. After mating, botfly females must lay fertilized eggs (oviposit) where they will come in contact with a susceptible mammalian host. The rodent botfly often oviposit on vegetation near rodents nests. As vulnerable animals forage or walk through vegetation, the fly eggs hatch and then the larvae burrow underneath the skin1. After settling, usually on the back of the host, it chews an air hole in the skin with their pointed mouth hooks. It is through this hole that the grub respires and secretes fluids1. After about 3-10 weeks the mature larvae emerges from the host, falls out on the ground, pupates and eventually metamorphosis into a fly.

All botflies follow similar life cycles, but with some interesting variations.

In horses bots lays eggs on the hair of their legs. The eggs cause itching which stimulates the animal to lick at the site. The vibrations of licking and the moisture of the saliva stimulate the eggs to hatch. The larvae then migrate through the tongue and esophagus before entering the stomach. They remain in the stomach for roughly 9-12 months at which time the mature larvae detach from the stomach and are passed in manure. After reaching the ground the grub burrows into the soil, where they pupate and then undergoes metamorphosis into a mature fly.

Cattle are afflicted with two types of botflies, the Northern Cattle Grub and the Common Cattle Grub. Female flies of both species lay eggs on the hairs of the lower legs of cattle. Upon hatching Common Cattle Grub larvae migrate to the esophagus and the Northern Cattle Grub larvae migrate to the spinal cord. After spending the summer migrating in the aforementioned tissues, the grubs of both species move to the skin on the back where they remain until spring; at which time they emerge, drop to the ground, pupate, and metamorphosis into adult flies2.

Possibly the most interesting lifecycle is that of the human botfly Dermatobia hominis, which is native to the Americas from Mexico to northern Chile and Argentina3. D. hominis females have developed an ingenious relationship with blood feeding mosquitos and ticks. Female flies capture these arthropod vectors in order to oviposit. After being released they transport botfly eggs to a host while taking a blood meal3. The eggs hatch in response to the temperature change and burrow under the skin through hair follicles or the bite wound3. After approximately 12 weeks the larvae crawl out of the host, fall to the ground, borrow into the soil, pupate and metamorphosis 3. In addition to humans D. hominis prey upon “many warm-blooded animals including buffalo, cattle, cats, dogsâÂ?¦ monkeys, pigs, rabbits, and sheep3.”

I have had the opportunity to treat two cases of botfly infestations in my career, both in cats. In each case the owners had first noticed a lump on the back of their pets. After closer inspection however, they observed the breathing hole and movement of the larvae within the swelling. Realizing that these were no ordinary lumps, they called and set up a time for me to examine these patients in more detail. After making a diagnosis I was able to surgically remove the grub under a local anesthetic. Then I cleaned the wound, prescribed and antibiotics and pain medications. Both cats healed with no complications.

The next time you notice a lump on one of your pets, take a closer look. There may be more inside than you imagine.

1. Bot fly (diptera: cuterebridae) infestation of nest-bound infant eastern gray squirrels F. S Lansky and l. R. Kenyon. Scientific Notes page 369-371.

2. Cattle Grub Biology and Management West Virginia Extension Service Peggy K. Powell PhD. October 2005

3. Human Bot Fly, torsalo (Central America), moyocuil (Mexico), berne (Brasil), mucha (Colombia, mirunta (Peru), and ura (Argentina, Paraguay, and Uruguay), Dermatobia hominis (Linnaeus, Jr.) (Insecta: Diptera: (Oestridae) University of Florida Extension Service Stephanie K. Hill and C. Roxanne Connelly document EENY 440 (IN775), 2008

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