In the 1950’s, Cochliomyia hominivorax, colloquially known as screwworm, was successfully eradicated from North and Central America and cordoned off in the Darién Gap. Screwworm is a flesh eating parasite that is attracted to open wounds on mammals. Once the female lays her eggs, larvae hatch and burrow their way into the mammal, “screwing” their way in to eat the host alive. This causes myiasis and in extreme cases, death.
The prior containment of screwworm was made possible by the sterile insect technique. This method, which involves the use of sterile male flies to render the population infertile, was published in a theoretical paper by Russian geneticist Alexander Serebrovsky and then developed by American entomologists Raymond Bushland and Edward Knipling. Continuous dropping of sterile male flies over infected sites in the 1950s and the subsequent suppression of sporadic outbreaks have kept the screwworm at bay, until now.
During the pandemic a “perfect storm” of conditions allowed the screwworm to rear its head again. Staffing for cattle inspectors at the Darién Gap screwworm border was impacted, sterile male fly production was down, and illegal cattle transport increased. In 2024, screwworm outbreaks began plaguing Central American cattle ranchers, who claim that higher temperatures caused by climate change, environmental destruction of the Darién Gap, and the reduction of sterile flies have all contributed to the resurgence of the parasite.
As the screwworm creeps closer to the U.S. Mexico Border, Texas cattle ranchers are worried about what this means for beef production in America. Screwworm infections cause billions in losses for the beef industry and containment without sterile flies requires hours of painstakingly removing “screwed in” larvae from infected cows. Whereas cattle can be monitored, wildlife populations aren’t so lucky. In 2016, a screwworm outbreak in Florida tore through and nearly decimated the population of endangered Key deer. Moreover, climate change driven temperature rises are giving the screwworm new territory where it has less natural predators.
The USDA is scrambling to contain the burgeoning screwworm outbreak before it hits critical mass. Political tension between the United States and Mexico is impeding efforts to collaborate on harm reduction and sterile male fly production in Texas has yet to begin. It is predicted that three times as many sterile males will need to be produced and dropped off in Mexico to keep the fly at bay, and perhaps 10-20 years of continued efforts to return to 1950s containment levels.
