External Parasites

The external parasites of horses of significant importance are bot flies, biting flies, mosquitoes, lice, horse mange and ticks.

Bot flies

Bot flies have four life stages as referred to in the section in internal parasites. The main problem of bot flies is the annoyance factor. On warm sunny days when bot flies are active, horses will fight the flies from morning until late afternoon. When the bot flies are present, horses will walk, run, bob their heads and seek shelter in shaded areas. As referred to earlier, the larvae stages of the bot can do extensive damage to the horse system.

Biting flies

There are several kinds of flies that suck blood from horses. These may include: horn flies, horse flies and deer flies, stable flies and in some eases, black flies. The main damage that these flies do is annoyance to the horse.


Mosquitoes breed in water and become a pest in any area during rainy seasons or in areas where water is prevalent. Some species breed in pot holes and become a pest in dry seasons. Females of all species suck blood. They will converge on horses in large numbers during periods of abundance.


Two types of lice are carried in our area. Biting louse and the blood sucking louse are the two. The blood sucking lice are more injurious because heavy infections mean a heavy loss of blood that may seriously weaken animals.

Lice occur in largest numbers in the winter months when the hair is long. Infections spread from animal to animal In adjacent stalls around managers.

Sucking lice are usually found on the head, neck, back and inner surface of the thighs. Biting lice may be found any where on the body, although they seem to occur in greatest numbers around the withers and the base of the tail. The biting lice feed on the hair and scales from the skin.

Horse Mange

Mites cause skin diseases known as mange, barn itch, scab and scabies. Transmission is by direct contact between healthy and infested animals, or by means of contaminated equipment.

Horses seek relief by rubbing affected areas of the body against any available object. The constant rubbing will cause skin to become swollen and enflamed and sometimes cause skin to rupture.


In some areas ticks are also a problem. Ticks feed on horses by sucking blood. Ticks are only of minor importance in Montana.

For current control measures of these external parasites contact your local veterinarian or extension entomologist.