Flesh-eating screwworm returns to US cattle for first time since 1960s, six cases confirmed

The outbreak

Federal officials have confirmed six cases of New World screwworm in the United States, marking the parasite's return to the country after a decades-long absence. The first case was confirmed on June 3 in a 3-week-old calf in Zavala County, Texas. The US Department of Agriculture subsequently reported additional cases, including a goat in Gillespie County, Texas, and a dog in New Mexico. The dog's infection was reported in Texas, and the animal may have recently traveled to Mexico, where the flies are also spreading. Four of the cases were in calves, with two in Zavala County and two in La Salle County.

USDA confirmed new cases on Monday in a goat in Gillespie County, Texas.

A correction published by The New York Times on June 10 clarified that these are the first cases found in cattle in the United States since the 1960s, not the first cases overall.

Containment and response

Federal and state teams are leaning on the same method that eradicated the screwworm in the 1960s: releasing sterile male flies so wild females produce no offspring. The USDA says it is working toward producing roughly 500 million sterile flies per week in response to the detections. Surveillance, testing, and trapping have been expanded in affected areas.

Communication between USDA and the Arkansas Department of Agriculture has increased significantly regarding NWS response activities. Our primary concern at this time is the movement of animals that could be carrying NWS larvae.

Quarantine zones are being enforced around each confirmed case, restricting the movement of animals in and out of those areas.

Economic stakes

The USDA has estimated that if the flies stage a comeback rivaling isolated outbreaks of the past, they could cost Texas producers $732 million per year and the Texas economy $1.8 billion. Arkansas' beef industry is smaller, with about 1.5 million head of cattle in the state in 2024 and cash receipts of $687 million in 2022.

Anytime there is a case, USDA enforces a quarantine zone around that animal, which restricts the movement in and out, and so the less movement of cattle selling and buying it creates a supply chain issue.

Major beef packers, including Tyson Foods, Cargill, JBS Foods, and National Beef Packing Co., did not immediately respond to inquiries about the outbreak.

Human health risk

While livestock are the easiest and costliest prey, humans are also at risk. Human cases are far less frequent than those in livestock, but when they occur, they are just as severe. The fly's larvae can destroy muscle, cartilage, and bone if they aren't caught in time, and can even break through a human skull. The flies are attracted to the smell of wounds, mucous membranes, and orifices of warm-blooded animals. Females can lay up to 3,000 eggs in their 10-30-day lifespan, and the resulting larvae begin boring into living flesh within a day of hatching.

My initial thinking is it's more of a burden on the cattle producer than it will be on the consumer or the restaurant chains.

Ranchers and animal owners have been urged to monitor animals closely and immediately report any suspected infestations. The USDA has reiterated that screwworm does not compromise meat, and the food supply remains safe.