The most common external parasite is the sheep tick or ked. The ked is a wingless fly. It lives on the sheep and sucks blood from the skin. It causes irritation and restlessness, and tick residue can detract significantly from the character of the fleece, ultimately affecting its value. Additionally, ked infestation will cause cockles (raised blemishes) on the hide, resulting in a discounted hide or pelt value.

Three species of lice occasionally infest sheep. The usual symptoms of louse infestation are scratching and rubbing. This can affect the quality of the wool. Often the wool becomes matted and entangled because of excessive rubbing. Control measures are much the same for sheep ticks and sheep lice. These should be controlled by following a yearly spraying, dusting, or dipping program using only insecticides recommended and approved for that specific use. Currently, the pour-on treatments (synthetic pyrethroids) are the most popular and most effective for treatment of keds and lice.

Several species of flies attack sheep that have open sores, wool that is bloody from lambing, or wool soiled with urine and feces following grazing on green pastures. Adult blow flies lay eggs on damp or soiled wool. Maggots soon hatch and feed on the wet wool adjacent to the skin, causing the wool to loosen. Open sores infested by the maggots may become infected with bacteria.

Sheep should be tagged or crutched when favorable conditions for fly strike exist. To treat sheep for wool maggots, shear and treat the affected areas with a smear, spray, or aerosol product approved for fleece worm or wool maggot control. Approved insecticides change from year to year, so ask your county Extension agent or local veterinarian for the latest recommendations for approved insecticides.