Nasal Bot Flies
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Nasal bot flies — Oestrus ovis — are a worldwide parasite mainly affecting sheep and goats (along with deer and occasionally horses, dogs, cats, and even humans). They do not mature in domestic species other than sheep and goats.
Nasal bot flies are “obligate” parasites, meaning they cannot complete their life cycle without parasitizing their hosts. They reproduce by depositing about 50 larvae — not eggs, but larvae — directly into the nostrils of the host animal. (The females are larviparous, meaning they do not lay eggs but deposit already-hatched larvae.) Females can deposit the larvae in and around the nostrils without alighting. Visualize the flies “squirting” larvae in rapid succession while on the move. Each female can produce up to 500 larvae, but she will only deposit smaller batches inside the nostrils of each victim.
These first-stage larvae crawl up the mucous membrane of the animal’s nasal cavity into the frontal sinuses. Here they pass through two molts (into second- and third-stage larvae), which takes anywhere from two to eight weeks. The mature larvae can be giant, up to 3 centimeters (over an inch) in length.
Once mature, the larvae crawl out of the sinus cavity, and the host animal presents with sneezing and nasal discharge. Imagine trying to sneeze dozens of inch-long, moving maggots out of your nose. Goats sneeze the larvae onto the ground, where larvae bury themselves and pupate within 24 hours. Depending on the temperature, the pupal stage may last anywhere from one to two months. From there, they develop into adult flies. Adult flies do not feed and live only for two to four weeks — long enough to mate and reproduce.
The timing of infestations varies regionally. The flies are late spring and summer pests in areas with cold winters, and in warmer climates, they can infest all year.
Sheep are the most common domestic host of nasal bot larvae, and goats are generally only afflicted with the parasite when exposed to sheep harboring the larvae. Sheep do not transmit the larvae to the goats; the flies select goats as an inferior host if they’ve used up all the sheep.
Animals recognize the presence of nasal bot flies by their characteristic buzzing. When the flies are active, host animals may attempt to avoid the insects by running with their heads down and pushing their noses into corners. Goats fear nasal bot flies and will hide in dark places when the flies are active. Farmers should watch for sneezing, nasal discharge, and the behavior of goats pushing their noses into trees, legs, or other surfaces to relieve the itching.
Despite the disturbing life cycle, the larvae usually do not cause significant problems, and caprine owners may be unaware of their presence. However, the larvae still have an impact. In their attempt to escape, the regular grazing and herd behavior of afflicted animals is interrupted, resulting in malnutrition, weight loss, and poorer conditions that affect productivity (milk, meat, etc.). As the larvae crawl into the nasal cavity, the resulting irritation can result in excessive mucous discharge, swelling, sneezing, and breathing difficulties.
A large infestation can lead to secondary bacterial infections that can be debilitating. Young or weak animals can die from nasal bot larvae infestations. If some of the larvae fail to leave the nasal cavities, they will die inside the host, which can cause septic sinusitis, which in turn can lead to death through septicemia. Once in a while, a few larvae may even reach the host’s brain, which is generally fatal.
For these reasons — primarily factoring in the comfort of your animals — treatment is recommended. Unfortunately, no repellents prevent these flies, and no traps specifically catch nasal bot flies. Nor are there vaccines to protect animals by making them immune to the bots.
Most of the research on treatment for nasal bot larvae has been done on sheep (their most common domestic host). It involves several veterinary parasiticides applied either as injectables or as oral drenches. While several medications are registered as treatment in sheep (ivermectin, abamectin, moxidectin, closantel), only abamectin is registered for use in goats for treatment of nasal bot larvae. Use off-label use of other treatments (such as ivermectin, levamisole, moxidectin, etc.) within a trusted veterinary-client relationship to ensure animals receive the proper dosages.
Abamectin is part of a class of macrocyclic lactones — products or chemical derivatives of soil microorganisms belonging to the genus Streptomyces. Use this product in consultation with a veterinarian who can recommend the medication’s proper dosage, application, and timing. The vet can also recommend the correct withdrawal time before slaughtering, preliminary fasting requirements for the animals, lactation limitations, age cutoffs, and other specifics for maximum efficacy.
The parasitic larvae that infect a particular animal cannot be transmitted to other animals directly. And fortunately, these parasites are not contagious to humans.
If you see your animals running with their heads low, hiding nose-first in corners, sneezing, or with a runny nose, consider whether your goats have been parasitized by nasal bot flies and seek veterinary care as needed. Your goats will thank you. The nasal bot larvae? Not so much.
Pull quote: Sheep are the most common domestic host of nasal bot larvae. Sheep do not transmit the larvae to the goats; the flies select goats as an inferior host if they’ve used up all the sheep.
Originally published in the March/April 2023 issue of Goat Journal and regularly vetted for accuracy.