New World Screwworm Reemerges in U.S., Threatening Livestock and Industry
The New World screwworm (Cochliomyia hominivorax) has reappeared in the United States, with 15 confirmed cases in Texas and New Mexico as of June 21, according to the U.S. Department of Agriculture (USDA). The parasitic fly, native to South America, lays eggs in open wounds of mammals, including cattle, goats, sheep, and even a dog. Larvae feed on living tissue, causing severe pain and potential death if untreated. While human infections remain rare, the outbreak has triggered urgent containment efforts.
How Did the Screwworm Eradicate in the U.S. Before?
The U.S. eliminated the screwworm by the 1980s using the sterile insect technique (SIT). This method involves mass-rearing and releasing radiation-sterilized male flies to outcompete fertile ones, reducing populations over time. The approach was pivotal during a 1950s–60s outbreak, leading to the eradication of the pest from the U.S. and much of Central America.
Why Are Screwworms Spreading Again?
Experts point to climate change, which favors the fly’s tropical habitat, and gaps in SIT programs. A 2023 study suggested warming could expand the screwworm’s range in North America. Additionally, reduced production of sterile flies after the 1970s, coupled with illegal cattle movements across borders, may have allowed the pest to reestablish. The Panama–U.S. COPEG facility, which produces sterile flies, now faces a 20% shortfall in capacity to combat the outbreak.
What Are the Risks to Humans and Livestock?
Human infections are exceedingly rare, with no locally acquired cases in the U.S. since 1982, according to the USDA. However, Mexico and Central America reported 2,100 human cases in 2023–2024. For animals, the economic impact is severe: a 2024 USDA report estimated a potential $1.8 billion annual cost in Texas alone during a similar outbreak. Livestock treated promptly can recover, but untreated wounds lead to secondary infections and death.
How Is the U.S. Fighting the Outbreak?
Federal agencies are deploying new strategies, including the Novo Fly strain, a genetically modified sterile male fly that doubles production by eliminating female larvae. The USDA is also expanding facilities, such as a new dispersal site in Texas, and using meteorological data to target releases. Emergency treatments like Dectomax, an injectable insecticide, have been authorized for use in animals.
What’s the Cost of Containment?
The USDA has allocated $105 million to combat the outbreak, including $21 million for a Mexico facility and $8.5 million for Texas. Experts emphasize the investment is justified: “It’s worth every dollar to eliminate this fly,” said Phillip Kaufman, a veterinary entomologist at Texas A&M University. However, full eradication may take years, as the Edinburgh, Texas, facility is not yet operational.
Why Does This Matter for Public Health and Agriculture?
The outbreak underscores the vulnerability of U.S. agriculture to invasive pests, particularly amid climate shifts. The 1976 screwworm outbreak cost the Texas economy $1.8 billion annually, a figure experts warn could be repeated without swift action. While the food supply remains safe, the incident highlights the need for sustained investment in pest control programs like SIT.
What’s Next for Screwworm Control?
The USDA aims to increase sterile fly production to suppress the outbreak, but challenges persist. The Novo Fly strain requires EPA approval, and border surveillance must improve to prevent illegal cattle movements. Researchers stress that collaboration across borders is critical to stopping the spread and avoiding another economic crisis. “We’re going to be successful,” Kaufman said. “It’s just a matter of time.”