Threat to U.S. Cattle Industry from New World Screwworm Fly

The Screwworm Fly Infestation in Texas

The New World screwworm fly poses a significant threat to the U.S. cattle industry, valued at $113 billion. This marks the first time in over fifty years that this pest has been detected in south Texas. The infestation was discovered in La Pryor, Texas, in a 3-week-old calf, near the U.S.-Mexico border. Texas, home to $17 billion worth of cattle, has been a focus for federal and state efforts to prevent the parasite’s spread.

The screwworm fly has a history of affecting cattle ranchers in the U.S., being a notorious problem from the 1930s to the 1960s. Its eradication was achieved through a program that involved breeding sterile male flies and releasing them from planes to mate with wild females. This method successfully ended the fly’s threat in Texas by 1966, and the recent case is the first since that year.

Why Screwworm Flies Are Unusual

Both the New World screwworm fly and its Old World counterpart are unique because their larvae consume live flesh and fluids. Females lay eggs in open wounds and mucous membranes after mating once in their life. Livestock, pets, wildlife, and even humans can be infested, though cattle are particularly vulnerable due to handling practices that can create wounds.

Even small injuries, such as tick bites, can lead to infestations. Untreated infestations result in death, although there are approved treatments available. Historically, ranchers experienced significant losses from the fly, but the current situation does not affect food supply. U.S. Agriculture Secretary Brooke Rollins notes it’s unlikely to impact beef production, easing concerns amid high consumer prices.

Efforts to Address the Fly’s Spread

Officials have been alerting the public to the fly’s movement since confirmed cases appeared in southern Mexico in 2024. The joint U.S.-Panama program, initiated in 1994, was designed to control the pest, although it saw an outbreak in Panama in 2023. Entomologist Edward Burgess highlights rapid reproduction and transportation by host species like deer as challenges in combating the fly.

Outside the U.S., the parasite impacted over 171,700 animals and 2,000 people, resulting in ten deaths. Rollins closed border entries to livestock in 2025 to delay the fly’s entry into Texas, although climate change has enhanced the fly’s habitat expansion.

Texas Response to the Infestation

Texas State Veterinarian Bud Dinges instituted a quarantine zone in Zavala County to prevent further spread, with animal movement restricted. Local ranchers fear potential spread in wildlife, similar to past outbreaks like the 2016 Florida Keys incident.

Preventive measures are underway, including injections to avoid infestation and close monitoring for signs of infection. The USDA has been releasing sterile flies in Texas, utilizing a proven strategy to reduce fly populations. Plans are in place for new breeding facilities in southern Mexico and Texas, funded with a $21 million investment and $750 million respectively. These initiatives are expected to enhance control measures significantly.