Several species of blow flies (Diptera, Calliphoridae) infest soiled wool on sheep and are collectively known as “wool maggots”. In the United States, the most important species are the black blow fly, Phormia regina, and two species of green bottle flies, Lucilia sericata and Lucilia cuprina. Occasionally, other species including the secondary screwworm, Cochliomyia macellaria, may be involved as well. All the wool maggot species normally develop in carrion, but occasionally will infest wool contaminated with urine, feces or blood, as well as wounds on living animals. Wool maggots feed on dead, necrotic tissues and normally do not invade healthy tissues as do primary screwworms, Cochliomyia hominivorax. However, maggots feeding next to the skin cause the wool to loosen in large patches and the skin becomes red and raw. Eventually, the skin becomes necrotic and the maggots invade the tissue, often resulting in death of the host. Most cases of wool maggots occur in the rump regions of ewes shortly after lambing. Other infestation sites include around the horns and in the genital regions. Infestations are most frequent when the weather is cool and damp. Wool in the infested area appears bluish green in color and often is loose and falling out in patches. Wool maggot infestations produce a strong, offensive odor. Treatment for wool maggots includes shearing wool in the infested and surrounding areas and treatment with an appropriate insecticide following manufacturer’s recommendations.

House Fly Sheep

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