External parasites that affect sheep include keds, ticks, lice, mites, and flies. Mange (sheep scab) in sheep is rare and a reportable disease in the U.S.
Fly Strike
(blowflies, wool maggots, fleece worms, myiasis)
Fly strike is the infestation of the flesh of living sheep by blowfly maggots. Of all domestic animals, sheep are most often affected because of their wool, as particularly dirty wool attracts blowflies. Blowfly populations are greatest during the summer months.
Docking, shearing, and removal of dags (wool contaminated with feces) will help to prevent flystrike. Insecticides are another control measure. Hair sheep are less susceptible to fly strike due to their absence of wool. Blowflies are also attracted to wounds, foot rot, weeping eyes, or sweat around the base of the horns of rams.
Sheep Keds (or ticks)
Sheep keds are wingless, reddish brown biting flies that resemble, and are sometimes called, ticks. They use piercing- sucking mouthparts to feed on blood. High ked populations cause unthriftiness and emaciation and make animals more susceptible to diseases and other stresses. Sheep keds are readily controlled with insecticides. Treatment is recommended immediately after shearing. Keds can only survive off the animal for about a week. Keds do not thrive well on the short hair of hair sheep.
Lice
Lice are quite small, ranging from 1/20-inch to 1/10-inch long. They spend most of their time next to the skin, and are difficult to see within dense wool or hair. Three species of lice are found on sheep. The primary animal reaction to lice is itching. Severe infestations can cause anemia. Various insecticides can be used to control lice on sheep.
Nasal bots (bot flies, head bots)
The sheep bot fly is a fuzzy, yellowish-gray or brown fly that deposits tiny larvae on the muzzles or nostrils of sheep. The larvae migrate into the nostrils and head sinuses and develop. A snotty nose is the most common symptom. Animals will hold their heads down or in a corner to escape the flies. Weight reductions of up to 4 percent have been attributed to bot infestations in some studies. The highest bot levels are seen in November and December. A systemic insecticide formulation containing ivermectin is effective against larval stages of the nasal bot.
Scabies
(sheep scab, psoroptic mange, wet mange)
Sheep scab is a very contagious disease, caused by mites feeding on the surface layers of the sheep’s skin. Severe itching occurs, wool or hair falls out in patches, and the skin becomes reddened, crusted with scabs and sore. Positive diagnosis can be made only by scraping lesions and examining the scrapings microscopically for mites. The preferred method of treatment is dipping with insecticides. Scabies has been eradicated from the United States.