The sheep ked, Melophagus ovinus, is one of the most widely distributed and important external parasites of sheep. There are also keds that parasitize deer in North America (Lipoptena depressa and Neolipoptena ferrisi).
Keds are wingless dipterans. The adult is ~7 mm long, brown or reddish in color, and covered with short, bristly hairs. The head is short and broad, and the legs are strong and armed with stout claws.
The female gives birth to a single, fully developed larva, which is cemented to the wool and pupates within 12 hr. A young ked emerges after ~22 days. Females live 100–120 days and produce ~10 larvae during this time; males live ~80 days. The entire life cycle is spent on the host. Keds that fall off the host usually survive <1 wk and present little danger of infestation to a flock. Ked numbers increase during the winter and early spring when they spread rapidly through a flock, particularly when sheep are assembled in close quarters for feeding or shelter.
To feed, sheep keds pierce the skin with their mouthparts and suck blood. They usually feed on the neck, breast, shoulder, flanks, and rump but not on the back where dust and other debris collect in the wool. Ked bites cause pruritus over much of the host’s body; sheep often bite, scratch, and rub themselves, thus damaging the wool. The fleece becomes thin, ragged, and dirty. The excrement of the keds causes a permanent brown discoloration, which is likely to reduce the value of the wool. Keds also cause a defect in hides called a cockle, which affects the grade and value of the sheep skin. Infested sheep, particularly lambs and pregnant ewes, may lose vitality and become unthrifty. Heavy infestations can considerably reduce the condition of the host and even cause anemia. Keds also transmit Trypanosoma melophagium, a nonpathogenic protozoan parasite of sheep.
Shearing removes many pupae and adults. Thus, shearing before lambing and subsequent treatment of the ewes with insecticides to control the remaining keds can greatly reduce the possibility of lambs becoming heavily infested. Sheep are usually treated after shearing, and best results are obtained if an insecticide that has a residual activity of ≥3–4 wk is used. By this means, the keds that emerge from the pupae are also killed. Modern treatments to control lice also control keds.
Dipping is an effective method of treatment. Completely submerging the sheep in vats ensures the destruction of all keds present but, in most instances, does not kill the pupated larvae; a long-acting insecticide is required to kill newly emerging keds. Large flocks of range sheep should be treated in a permanently constructed dipping vat. Smaller flocks and farm flocks may be successfully treated in portable, galvanized-iron dipping vats or in smaller tanks, tubs, or canvas dipping bags.
Spraying may be as effective as dipping and is more convenient in some areas. Pressures of 100–200 lb/sq in. (7–14 kg/cm2) for short wool and 300–350 lb/sq in. (21–28 kg/cm2) for long wool are commonly used.
Shower dipping is also sometimes used; the sheep are held in a special pen and showered from above and below until the fleece is saturated. The run-off is returned for recirculation, and the concentration of insecticide used is the same as for dipping. The concentration of the insecticide can drop rapidly and become ineffective if the instructions for replenishment are not followed explicitly.
Jetting involves the forceful application of the insecticide by means of a hand-held, multiple-jet comb drawn through the short fleece. Although a little slower and less effective than dips or sprays, jetting may be advantageous for smaller flocks because it is economical and does not require a permanent installation.
Spot-on or pour-on formulations of the newer pyrethroids are easy to apply and very effective.
Powder dusting fits well into management practices at shearing time. It is rapid, economical, and avoids wetting the animals. Various types of equipment for dusting are available commercially.