Q. Every fall, I see bot flies terrorizing my horses. I have tried to find a way to get rid of them or to even cut their numbers down, but I have not been successful. I have cows in the next pasture, and someone told me cows attract bot flies. Is this true?

Tracy, Oroville, Calif.

A. Bots are an inevitable aspect of equine ownership, but they are probably more bothersome to you than to your horse.

Bots are the equine representatives of a specialized family of flies (Oestridae) that employs a common strategy of spending the cold winter months within the body of a mammalian host. "Bots" (Gasterophilus spp.) winter in the stomach of horses, and "grubs" or "warbles" live in various tissues or under the skin of cattle (Hypoderma spp.) or rodents (Cuterebra spp.).

Adult female bot flies glue their eggs to the hair shafts of specific body parts of the horse. The eggs on the hair coat are stimulated to hatch by a combination of warmth, moisture, and carbon dioxide. All of these are supplied whenever a horse nuzzles its lower legs or grooms the coat of a herd mate. Small, spiny larvae emerge rapidly from the eggs and attach to the lips or tongue of the horse, and they embed themselves within fissures in the tongue.

After a molt to the second larval stage, the juveniles are found within pockets between the gums and cheek teeth. Ultimately, the second-stage larvae are swallowed and carried to the stomach and intestine.

When warm weather returns in the spring, bot larvae detach from the gastrointestinal tract and pass into the environment with the horse's manure.

Although bot larvae attach firmly to the gut wall by means of large, anterior (situated near the head) hooks, tissue damage from this stage is inconsequential. The egg-laying activities of adult female flies might be the most aggravating aspect of bot infestation.

The only dewormers that are effective against larval bots are those that contain ivermectin or moxidectin. Bot treatments should be scheduled between late autumn and early spring. An ideal application would be scheduled after the adult flies have stopped laying eggs for the year (usually after the first hard frost).

Some horse owners also attempt to decrease bot infestations by removing eggs from the hair coat before they can hatch. Eggs can be removed with fine combs, razors, rough bot-removal blocks, or sandpaper, and some people report success after scrubbing with warm water or coating the eggs with petroleum jelly. All of these practices are labor- intensive, must be repeated frequently, and are not likely to be entirely successful. Preventive products that might discourage female flies from depositing eggs on the hair coat have not been developed.

Regardless of how conscientious your bot control efforts, the level of control achieved is only as good as that of all the farms within several miles of yours. Because adult flies migrate actively by flight, or are carried passively by the wind, they can emigrate from other premises and attack your horses anew each season.

Although cattle have problems with their own species of Oestrid flies, grazing cows near horses does not particularly attract equine bot flies.

About the Author

Craig R. Reinemeyer, DVM, PhD

Craig R. Reinemeyer, DVM, PhD, is president of East Tennessee Clinical Research, Inc., an independent business in Knoxville, Tenn., that conducts clinical pharmaceutical research for animal health companies.

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