An old pest recently made news in November when the United States suspended Mexican cattle imports because a case of New World screwworm was identified in the southern part of Mexico. 

This pest used to be common in the southern United States, but starting in the 1950s, the USDA, with the support of various cattlemen’s associations, worked to eradicate the fly from the United States. Then with partnerships with other countries, to push south through Mexico, Central America and finally to Panama. The screwworm fly is still widespread in tropical areas of the Caribbean and South America. 

A fly goes through four life stages: an adult fly lays hundreds of eggs, the eggs hatch into larvae (commonly called maggots), then they enter the pupa stage (cocoon-like form) where they grow into adult flies. 

For most flies, the adults lay eggs in fresh or decaying manure, decaying plant material or occasionally decaying animal flesh (such as a wound with dead tissue). The screwworm is different, as the fly will lay eggs in any opening in otherwise healthy skin (such as the navel of newborn calves or a scratch or fresh wound), and the larvae (or maggots) will feed on the healthy tissue — causing severe damage or even death.  

Before the fly was eradicated from the United States, it caused millions of dollars of losses, and the damage extended beyond cattle and other livestock to any warm-blooded animal including wildlife, pets and even people. 

The eradication of this parasite is an impressive success story that required years of effort and investment by the USDA and other federal agencies as well as cooperation with Mexico and other Central American and Caribbean countries. But, in order for the work and investment involved in the eradication effort to pay off, a unique weakness in the screwworm fly lifecycle had to be exploited. 

A constant battle

Unlike other flies, a screwworm female only mates once. If she mates with a sterile male, she will never produce live offspring. Scientists at USDA used this information to begin looking for a method to sterilize male screwworms and found that a specific dose of radiation at a specific time in the pupa stage would sterilize the males while still allowing them to be able to mate. 

By raising and sterilizing many millions of screwworm flies and then releasing them across the southern United States starting in the late 1950s, the screwworm was eradicated by 1966. However, keeping the screwworm fly out of the country is a constant battle, and several isolated outbreaks have occurred, most recently in 2016 in the Florida Keys. In order to reduce the likelihood of reexposure, the United States has worked with Mexico and into Central America to use sterile flies to eradicate screwworm flies all the way to a biologic barrier established in Panama. 

Although the strategy to keep screwworms south of Panama has been mostly successful, the effort requires: continual release of sterile flies, restriction of animal movement and rigorous monitoring for signs of screwworm infestation. The only current screwworm sterilization facility is in Panama, and it produces about 20 million pupae per week. 

Because of the recent discovery of screwworms as far north as southern Mexico, the USDA and other agencies are working with Mexico and other countries in the area to reestablish an effective barrier in Panama.

Editor’s note: Robert L. Larson is a professor of production medicine and executive director of Veterinary Medicine Continuing Education at Kansas State University in Manhattan, Kan.