Relatively little has been documented regarding the role of female horse flies and female deer flies in the transmission of disease agents. This is largely because it is very difficult to conduct carefully controlled scientific studies in a natural setting and because no one has been able to establish a laboratory colony of either horse flies or deer flies on which to conduct research. Nearly all projects have focused on checking for disease agents on the mouthparts or inside the digestive system of females captured in the field. This research has revealed the presence of numerous viruses, bacteria, and protozoa, but does not prove females are involved in transmission of disease agents to livestock and humans. For example, West Nile virus and the bacterium that causes Lyme disease have been isolated from female horse flies, but there is no evidence that they transmit these agents to humans.
There is evidence, however, that a species of deer fly in the western U.S. is a vector of the bacterium that causes a disease known by the following three names: "tularemia," "rabbit fever," and "deer fly fever." This role, however, is minor compared to transmission by ticks and other modes included in the following section.
A recent (2002) textbook by G. Mullen and L. Durden, Medical and Veterinary Entomology, that has an excellent chapter devoted to horse flies and deer flies covering biology, behavior, medical and veterinary risk, and general comments on control.
The following website provides additional information on horse and deer fly biology and control.