LEXINGTON, Ky.--Horn flies are blood feeders that primarily attack pastured cattle. Bulls are more attractive to horn flies than are steers or cows. Individual flies pierce the skin with their short, tube-like mouthparts 20 to 30 times per day to ingest a small amount of blood. Their feeding activity is painful and annoys the animals, as well as causing some blood loss. This stress is probably the cause of reduced weight gain during the summer. Horn flies can cause a 12 to 20 pound lighter calf at the end of the summer, as well as, greater weight loss per nursing cow.
In addition, horn flies have been implicated in the spread of summer mastitis, a suppurative disease of non-lactating mammary glands. Incidence of this disease is highest during the fly season and decreases in response to effective fly control. Horn fly feeding lesions on teats may serve as entry sites for the pathogen and these, and other pasture flies, may carry the disease organism on their bodies.
Source: Lee Townsend, University of Kentucky Extension