Screwworms are larvae of the Cochliomyia hominivorax (New World Screwworm) and Chrysomya bezziana (Old World Screwworm) flies. Screwworm flies lay eggs in open wounds on animals and people. One female can lay up to 400 eggs at a time, which can hatch within 12 hours. After hatching, larvae feed on exposed tissue. As larvae feed, they make the wound larger and deeper. Screwworm infestation is fatal if not treated.

Endemic Areas

Parts of South America, India, Southeast Asia, parts of Africa, and the Middle East. The United States and Canada are considered free of screwworm.

Clinical Signs

• Blood-tinged fluid with a foul odor seeping from wounds
• Separation from the herd
• Decline in milk production
• Anorexia (poor appetite)
• Discomfort
• Maggots around or in open wounds

Treatment

Treatment for Screwworm involves killing and removing larvae. The wound should be treated with an effective and approved pesticide. Antibiotics may be given to treat secondary infection. Any animal suspected of having a Screwworm infestation should be reported to the State Veterinarians or USDA Area Veterinarian in Charge immediately.

Prevention

All animals entering the United States from Screwworm endemic areas must be thoroughly inspected for the presence of larvae.

Public Health

People can develop Screwworm infestation through open wounds. Open wounds should be cleaned and treated immediately to deter C. hominivorax or C. bezziana flies. Any suspected Screwworm infestation should be referred to a doctor immediately for appropriate treatment.

Links

Screwworm (USDA-APHIS)

Screwworm Eradication Collection (National Agricultural Library)

Screwworm (Iowa State University)

Screwworm Photos (University of California - Davis)

Print Resources

Alexander, JL. Screwworms. JAVMA. 2006;228(3):357-366