Ask the Vet

Hair Loss May Be Flies

Hypersensitivity to fly bites can cause hair loss in cattle, as well as other animals such as dogs.(Progressive Farmer photo by Jim Patrico)


We had a wet spring, and several of our cows and calves had red, circular areas of hair loss. I have seen ringworm and lice, but these did not look like that. They seem to have improved after we worked them and weaned the calves. Do you have any ideas what this was?


Over the years, we have seen similar problems here in the Southeast, caused by a hypersensitivity to insect bites. While flies are the most visible biting insects affecting livestock, biting gnats can also cause irritation. This condition is similar to dogs with flea allergy dermatitis. Some dogs have a hypersensitivity to the saliva in flea bites. These dogs will scratch and lose hair, and can develop severely infected skin. Other dogs in the same house with even more fleas may show minimal or no signs. This can also occur in cattle and other animals, including humans.

I bet when you worked your herd, you also treated them for flies. This could be why you saw the improvement at that time. Also, weather conditions and seasons change, which could also lead to improvement.

Certainly, there are other potential causes including photosensitization, herpes mammillitis virus, ringworm and dermatophilosis (rain rot or rain scald), to mention a few. If the problem recurs, a skin biopsy can give you a definitive diagnosis.