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By Alice Gibbs
Senior Reporter
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A flesh-eating parasite that can infect pets, wildlife and even humans is drawing renewed attention as officials work to contain its spread near the southern U.S. border and prevent it from becoming reestablished domestically.
The New World screwworm, a fly whose larvae feed on the living tissue of warm-blooded animals, was eradicated from the U.S. decades ago.
But a growing outbreak across Central America and Mexico—combined with a recent confirmed detection in a Texas calf—has sharpened focus on the risk of reintroduction.
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What Is the Current Screwworm Risk in the US?
Federal health agencies stress that there is no widespread presence of screwworm in the U.S., and for most Americans, the risk remains low.
However, the situation is evolving. The parasite has spread north through Central America since 2023 and has been detected increasingly close to the U.S. border, with thousands of cases reported in animals and some in people in affected regions.
That proximity—along with isolated detections and a recent Texas case under containment—means authorities are treating the threat seriously, particularly in southern states.
Sally DeNotta is a leading expert in infectious disease in equine internal medicine at the University of Florida. She told Newsweek: "The parasite itself, which is a fly larva that feeds on live tissue, can infect all mammals—including dogs, cats and rarely humans. All warm-blooded species are at risk of infection."
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How Can Screwworm Affect Pets?
Dogs and cats are among the animals that can become infested if exposed. Screwworm flies are attracted to even very small wounds, laying eggs that hatch into larvae within hours.
Pet owners should watch closely for any wound that appears to worsen rather than heal, particularly if it becomes painful, swollen or starts to smell.
Early signs of a potential screwworm infestation include persistent licking or biting at a specific spot, discharge or bleeding from a sore, and—more seriously—visible larvae or a sensation of movement inside the wound.
Because the parasite can develop quickly, vets say even small cuts or insect bites should be checked and treated promptly, especially in animals that have traveled to or come from higher-risk regions.
"Visual inspection is the primary way that infected animals are identified. The disease is very treatable, and the sooner affected animals are seen and treated by their veterinarians, the less the chance of further spread to additional animals," DeNotta said.
"Owners should inspect their animals and livestock for wounds that are not healing appropriately or appear to be infected with maggots.
"NWS-infected wounds are quite painful, and animals are often particularly bothered by the sites of infection. If any of these signs are noted, owners and ranchers should contact their veterinarian immediately."
If the parasite were to spread, pets could experience:
- Rapidly worsening wounds
- Severe pain and infection
- Tissue destruction that can become fatal without treatment
Infestations are most likely in animals that spend time outdoors or have untreated cuts or surgical wounds.
Despite this, the likelihood for pets inside the U.S. remains low at present, unless they have traveled to or been imported from affected regions.
CDC
How Can Screwworm Affect Humans?
While much rarer, screwworm can also infect humans. Cases typically occur when flies lay eggs in open wounds or body openings, leading to painful infestations known as myiasis.
Symptoms include:
- Painful, worsening sores
- Foul-smelling discharge
- Visible movement of larvae
U.S. officials note that recent human cases linked to the parasite have been travel-associated, and there is no immediate public health threat domestically.
Impact on Wildlife
Wildlife experts warn that screwworm could have significant ecological impacts if it became established again in the U.S.
The parasite can infect deer, birds, and other wild animals, often killing untreated hosts and reducing populations.
Historically, outbreaks have caused major economic losses in livestock, with untreated infestations capable of killing animals within one to two weeks.
Because it affects multiple species, federal agencies consider screwworm a multisector threat—spanning pets, agriculture and ecosystems.
"I am certain that we will eradicate New World screwworm again," DeNotta said, "similarly to when it was last eradicated back in the 1960s, but the time frame and cost to do so will depend on our collective response.
"While the USDA continues control efforts through dispersal of sterile flies, animal owners can monitor their animals and work with their veterinarians if infection is suspected."
Are New Rules Being Introduced for Pet Imports?
Existing and recently tightened federal import controls already target screwworm risk, particularly for animals coming from affected countries.
Under U.S. Department of Agriculture (USDA) rules, dogs entering the U.S. from screwworm-affected regions must have a government-certified health document confirming they are free of the parasite or were treated before travel.
Animals must also be inspected shortly before entry and may face additional checks at the border.
These requirements apply to pets brought in through travel, purchase, or adoption, including dogs originating from parts of Mexico.
Separately, broader restrictions and inspections have also been applied to livestock imports from Mexico in response to the outbreak.
While not a blanket ban on adoptions, the rules effectively mean stricter screening and certification for rescue animals entering the U.S. from higher-risk areas.
Update, 06/05/26 10:49 a.m. ET: This story was updated with comment from Sally DeNotta.
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