Agricultural Disaster Preparedness and Recovery December 18, 2013 Print Friendly and PDF

Sterile male screwworm fly marked with a numbered tag to study fly dispersal, behavior, and longevity. Photo by Peggy Greb.

What Is Screwworm and Why Should I Care?

Screwworms are larvae of the New World Screwworm fly (Cochliomyia hominivorax) and Old World Screwworm fly (Chrysomya bezziana). 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.

Where Is Screwworm Found?

New World Screwworm flies are found in parts of Central and South America, and in certain Caribbean Islands. The United States, Mexico, and Canada are considered free of screwworm. Old World Screwwrom flies are found in Africa, the Indian subcontinent, and southeast Asia.

Can Screwworm Affect People?

Yes, 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.

What Are Signs of Screwworm?

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

How Is Screwworm Treated?

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.

How Can Screwworm Be Prevented?

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

Where Can I Find Additional Information?

Print Resources

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

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.