History of the Screwworm

December, 2016
Lauren Butler, Okeechobee County Cooperative Extension Service, Okeechobee

Most ranchers today do not have a first hand memory of the devastating effect the screwworm epidemic had on Florida’s ranches. Screwworms were eradicated from Florida ranches in 1959, but that time they were active on Florida Cattle ranches and cost the industry $20 million annually. Thankfully, due to innovative radiation techniques, the screwworm is no longer inhabiting the United States. A trip down memory lane may serve us well to appreciate the science that freed us from those awful flies!

Screwworms are an invasive species brought to Georgia from the Southwestern United States in the 1930’s. They get their name because the maggots actually burrow into living tissue. The screwworm infected warm blooded animals through any open wound. Brandings, cuts, castrations, dehornings, and navels of newborn calves were all susceptible to infestation. The fly laid her larval eggs in the wounds and they proceeded to eat live flesh, not dead tissue. Once the larvae developed into pupae, they fell to the ground. At that point it was only a matter of time before they developed into adult flies and began the cycle all over again.

When screwworms were at their worst, cattlemen lost all hope of eradicating them. The pest not only affected their cattle herd, but it also impacted their wildlife. Wildlife played a significant role in the cattle industry as most cattlemen relied on hunting leases to supplement their income. Cattlemen were desperate to control the fly. The only methods available were benzol to kill active larvae and putting tar on open wounds. Having a calf crop in the spring and separating infected animals from non-infected animals reduced the losses from screwworm, but these methods were not enough to eradicate the threat.

In order to eliminate the screwworm fly, scientists used radiation to sterilize the screwworm males and release them into the general population. Sterile males would mate with fertile females. The resulting non-viable eggs disrupted the life cycle and reduced the number of offspring. This project worked and, with the help of a cold winter, by 1959 Florida was free of the screwworm.

The same tactics used in the 1950’s are being used today in the Florida Keys where screwworms are again making an appearance. Sterile flies are being produced and released. The zone of infestation has remained small and localized. If diagnosed early, experts are able to treat infested key deer. Their current method of treatment includes sedation and administration of medication. In appropriate cases euthanasia has been used.

Look for these indicators if you believe an animal has been infected: eggs are creamy and white, laid in a shingle-like manner on or near the edges of a wound. The maggots are cylindrical with a blunt end and one pointed end. Screwworm flies are larger than a housefly and also have a dark bluish green body. Contact your local veterinarian and local extension agent for help.


Baumhover, A.H. 1997."A Personal Account of Programs to Eradicate the Screwworm, Cochliomyia hominivorax, in the United States and Mexico with Special Emphasis on the Florida Program."

Florida Department of Agriculture. 2016. “New World Screwworm” (2016) Retrieved from http://www.freshfromflorida.com/Divisions-Offices/Animal-Industry/Consumer-Resources/Reportable-Animal-Diseases/New-World- U.S. Fish and Wildlife Service. 2016. “Key Deer Response Efforts” (2016) retrieved from https://www.fws.gov/nwrs/threecolumn.aspx?id=2147595381

Bruce. W.G. and Sheely W.J. 1936. “Screwworms in Florida” Agricultural Extension Service, University of Florida.

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