Only the female fly feeds on blood, first inflicting a painful bite with knife-like mouthparts.
Male flies feed on nectar and pollen.
When the female bites, the wound inflicted often continues to bleed for several minutes because the fly s saliva contains an anticoagulant that prevents blood from clotting. A single animal may suffer significant blood loss if many flies attack. Horse flies also are potential carriers of livestock diseases such as anthrax, hog cholera and equine infectious anemia. Some research suggests that they might also transmit Lyme disease. The greatest deer and horse fly activity occurs on warm, sunny days when there is little or no wind.