The flies are potential vectors of such diseases as anthrax, tularemia, anaplasmosis, hog cholera, equine infectious anemia, and filariasis. Biting deer flies frequently attack humans along summer beaches, near streams, and at the edges of moist, wooded areas. Some people, when bitten, suffer severe lesions, high fever, and even general disability. Symptoms are allergic reactions to hemorrhagic saliva poured into the wound to prevent clotting while the fly is feeding. A person can become increasingly sensitive to repeated bites.
Smaller species are brown, black or gray, and often have brilliant green eyes.
The eyes are sometimes crossed with reddish-gold bands that disappear when the fly dies. Larger species are brown to black and may be slightly striped. Horse flies have their antennae divided into three parts, the third being long and composed of five to eight rings. Horse flies usually have clear wings whereas deer flies have wings with dark markings. Deer flies are slightly larger than house flies, and mostly yellow or black with darker stripes on the abdomen and dark markings or patterns on the wings.