The New World screwworm, a parasitic fly notorious for its flesh-eating larvae, has made an unwelcome return to the United States. Federal officials confirmed its presence in a 3-week-old calf in Zavala County, Southwest Texasmarking the first detection in the state since the early 1980s. This resurgence signals a new phase in the parasite’s northward spread through Central America and Mexico, which began in 2026.
While human infestations are rare in the US, the potential economic impact on ranchers and the animal agriculture industry is substantial. With beef prices already near record highs, the spread of screwworm could further strain the economy. Texas officials are now focused on determining whether this is an isolated case or indicative of a larger infestation.
The Immediate Response to the Screwworm Detection
The Texas Animal Health Commission (TAHC) has initiated a comprehensive response to contain the infestation. A 12-mile surveillance zone has been established around the detection site, restricting the movement of warm-blooded animals until they are inspected. The TAHC has been monitoring the Texas-Mexico border since July 2026, collecting over 54,000 suspicious flies without confirming any screwworm cases until now.
To combat the infestation, sterile flies are being deployed through ground release chambers and aerial dispersal. This strategy aims to flood the area with sterilized flies, preventing wild screwworms from reproducing. Texas has been conducting precautionary aerial sterile-fly drops over South Texas since late January, but these efforts are now being redirected to the response zone around Zavala County.
Understanding the New World Screwworm
The New World screwworm (Cochliomyia hominivorax) is a parasitic fly found in parts of South America, Central America, and Mexico. Unlike ordinary house flies, screwworm flies are attracted to fresh wounds on warm-blooded animals. Female flies lay up to 200 eggs at a time, which hatch within 12 to 24 hours. The larvae then burrow into the flesh, causing extreme pain and tissue damage.
Infestations can be treated if caught early, but missed cases can lead to rapid reproduction and spread. The larvae feed on living flesh for up to a week before pupating in the soil and emerging as adult flies. This cycle can lead to repeated infestations in the same animal, causing severe wounds, blood loss, and even death if left untreated.
The Historical and Current Efforts to Eradicate Screwworms
The US successfully eradicated screwworms in the mid-20th century using the sterile insect technique (SIT). This method involves rearing large numbers of screwworms in a lab, sterilizing the pupae through radiation, and releasing the sterile flies into the wild. The sterile males mate with female flies, preventing reproduction and eventually collapsing the population.
The SIT was initially used in the American Southeast and Southwest, costing roughly $42 million. By 1966, the fly was eradicated from the US. The technique was later adopted in Mexico and Central America, pushing the flies back to the Panama-Colombia border. However, in 2026, the firewall cracked due to cattle smuggling and climate shifts, leading to the current resurgence.
The USDA is now working to rebuild its capacity to combat the screwworm threat. This includes renovating a facility in Metapa, Mexico, and building new production capacity in Texas. Additionally, genetic tools like the male-only screwworm strain, Novofly, could make sterile-fly production more efficient. However, these tools require regulatory approval and field testing before deployment.
The next few weeks will be crucial in determining whether Texas is dealing with a contained incursion or a more serious outbreak. The response underway involves animal movement restrictions, fly and animal surveillance, and sterile-fly releases. If successful, these measures could prevent the screwworm from spreading further and protect the livestock industry from significant economic losses.
