The New World screwworm fly is again threatening the U.S. cattle industry after more than five decades without a major Texas outbreak, prompting urgent federal and state action to contain the parasite before it spreads further. The U.S. Department of Agriculture announced two new confirmed cases on Monday, including a dog in Lea County, New Mexico, just across the Texas border, and a calf in La Salle County, Texas, bringing the total number of reported cases to four. The first two Texas cases were found in young calves in La Pryor and Zavala County, near the U.S.-Mexico border.
The outbreak is being watched closely because Texas is the nation’s largest cattle-producing state, with about $17 billion worth of cattle. The broader U.S. cattle industry is valued at about $113 billion, making any spread of the parasite a serious economic concern. Screwworm flies were once a regular warm-weather threat to ranchers from the 1930s through the 1960s, until the United States largely eliminated them by releasing sterile male flies from aircraft to mate with wild females and interrupt reproduction.
The New World screwworm is unusual because its larvae feed on live flesh rather than dead tissue. Female flies lay eggs in open wounds and mucous membranes after mating only once in their lives. Once hatched, the maggots burrow into the wound and can cause severe damage or death if not treated. Although the pest is primarily a danger to livestock, it can also infest wildlife, pets and, in rare cases, humans. Ranchers say even minor injuries, such as tick bites or scrapes from handling, can give the flies an opening.
Officials have been warning for nearly two years that the parasite was moving north through Mexico after being detected there in late 2024. The fly had previously been held near the southern edge of Panama, but outbreaks later spread through Central America and into Mexico. According to the CDC, as of June 3, more than 171,700 animals and 2,070 people had been affected across Central America and Mexico, with 10 human deaths reported.
Texas officials have imposed a 12-mile quarantine zone in parts of Zavala and Uvalde counties, restricting animal movement unless inspected. Ranchers are stepping up wound treatments and preventive shots, while the USDA is releasing millions of sterile flies in south Texas each week to suppress the population. The agency is also building new breeding facilities in Mexico and Texas to expand production. Officials say the goal is to stop the fly from becoming established again in the United States and to protect cattle, wildlife and the wider food supply.
