The U.S Department of Agriculture (USDA) has shut down animal imports at the southern border following an outbreak of a deadly flesh-eating parasite, with concerns of the return of the eradicated pest in the U.S.
The new world screwworm (NWS), or Cochliomyia hominivorax, is a parasitic fly that resembles a common house fly. The USDA is particularly concerned with the species’ larval stage.
Female flies lay their eggs on or near open wounds or body openings — sometimes as small as a scratch. Once hatched, the larvae burrow in the host, eating away at its live healthy tissue.
The fly primarily infests livestock, pets and other wildlife, but can occasionally infect birds and in rare cases, humans.
READ MORE: How The Screwworm's Sex Life Saved Your Steaks
USDA Secretary Brooke Rollins officially suspended live imports on May 11, after confirmed NWS cases were reported about 700 miles south of the U.S.-Mexico border. Surveillance efforts have increased following reports of NWS in Mexico and neighboring countries in 2024 and 2025.
Although the parasite was eradicated in the U.S. in 1966, experts warn it still poses a serious threat to the country’s livestock industry.
“When new world screwworm was endemic in this country in the 1950s, producers just in the Southwestern U.S. lost between $50 and $100 million annually,” said Dr. Sally DeNotta, Clinical Associate Professor in large animal clinical sciences at the University of Florida.
“When eradication was deemed successful, the cost of that [program] was $32 million — that's about $345 million today.”
DeNotta added that the larvae create and propagate large wounds in the host. When left untreated, they can be fatal.
“When you multiply that out over herds of animals, particularly livestock, they can have a substantial deleterious effect on the industry of food producing animals or recreational animals like horses.”
In the 1950s, experiments revealed the success of the Sterile Insect Technique (SIT) for NWS outbreaks. The process involves releasing sterile male flies into affected areas so they mate with native, fertile females.
“We know that the female flies only mate once. If they pair up and mate with a sterile male fly, then that is the end of their life cycle. They will not propagate on or reproduce. It [is] this ingenious way to capture a biological lifecycle and use it to our advantage,” DeNotta said, emphasizing that SIT is a costly and complex process.
The Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG), is a joint program between International Services and Panama’s Ministry of Agriculture that works to implement this strategy using advanced surveillance and diplomacy to implement SIT and halt spread of the parasite.
In 2016, this method was successfully used for an outbreak of NWS in Florida’s endangered Key Deer population. According to the USDA, over 188 million sterile flies were released across 35 different sites from the Keys to Homestead. The response cost the agency $3.2 million.
With the threat of NWS looming at the Southern border, the U.S. government has announced plans to open a sterile fly breeding factory in Texas, near the Mexico border, to further combat the risk of infestation.
“That's one of the reasons for this closure of the border,” DeNotta explained. “To give [the] USDA some time to try to identify where these cases [are] coming from, where the areas of interest [are], how close to the border [they are]. To get those areas cleaned up with this sterile insect technique before we have animals reach the border and actually get established in the US.”
There are currently no reports of NWS in the US. Still, experts urge pet owners and producers to keep an eye on their animals — especially those who may have recently entered the country through the southern border.
The situation at the border is being monitored and will remain active on a month to month basis.
If you suspect that your pet or animal has been infected with NWS, DeNotta recommends contacting your local veterinarian.
