New World screwworm poses limited threat to a prepared Pennsylvania, veterinarians say

by Natalie Javitt, Pennsylvania Capital-Star
June 24, 2026

Natalie Javitt is a 2026 Pennsylvania Legislative Correspondents’ Association intern. 

A flesh-eating parasite that hasn’t been found in the U.S. for nearly 60 years was detected in the South in early June. 

Sixteen confirmed cases of New World screwworm, a parasitic blowfly that feeds on the tissue of warm-blooded animals, are causing states to roll out protection plans. 

In Pennsylvania, Gov. Josh Shapiro’s office issued a quarantine order on June 9 that restricts animal movement and requires veterinary inspections to limit risks of infestation. But state and federal veterinarians worked to assure Pennsylvania lawmakers that the parasite poses limited threats in the commonwealth at a House Agriculture and Rural Affairs information meeting on Tuesday. 

While no screwworm cases have been reported in Pennsylvania, 15 cases have been confirmed in Texas, where it was first detected on June 3, and one is in New Mexico. 

Erin Luley, the assistant state veterinarian for the Pennsylvania Department of Agriculture, said the largest risk would come from animals that are imported to the state. She noted the nature of the screwworm and how the infection is transmitted mean it’s both detectable and treatable. 

“This is not like (the bird flu) where we will need to cull a large number of animals,” said Kellie Hough, a veterinarian and Pennsylvania’s district emergency coordinator for the U.S. Department of Agriculture. “We can treat this disease early enough.” 

What is the New World screwworm?

Screwworms lay eggs in open wounds where their larvae then infest. Hough said an infestation doesn’t spread from animal to animal and relies wholly on a screwworm laying its eggs, differentiating it from a contagious virus or bacterial infection. 

Because one animal can’t infect another, this also means the screwworm poses no risk to the food supply chain. 

Luley said a key sign of infestation is a malodorous scent that will often attract other screwworms.

“Because of the severe tissue damage and secondary infection and pain and other systemic effects, infested animals will typically die within one to two weeks if this is not treated,” she said. 

The screwworm itself is about the size of a common housefly and lives for up to three weeks. It can’t survive temperatures below 50 degrees Fahrenheit, which Luley said means any risks in Pennsylvania would be mitigated by the winter. 

Luley also said the screwworm can fly for up to 12 miles. This helps establish boundaries of an infected zone that are used in current control measures. 

History and treatment 

The screwworm was first detected in southern states in the 1930, but was completely eradicated from the U.S. by 1966. It was also eradicated from Mexico in 1991 via confinement strategies. 

For almost 60 years, the screwworm was only found in Central America and South America. But it broke through the Darién Gap in Panama in 2022, allowing it to reenter the U.S. where it’s continued to spread northward this past year.  

Luley said the most common eradication tactic is “the sterile insect technique,” where a large number of sterile male flies are released in infested areas. This breaks the screwworm’s lifecycle because females only mate once and reduces the frequency of infections. 

The federal Agriculture Department  has been releasing sterile flies in Texas since February to mitigate the risk, Hough said. Since detection this June, two million sterile flies have been released in Texas twice each week.

There is currently only one facility in Panama that produces sterile flies, which Hough said is a limiting factor in screwworm containment. 

“The reason (the screwworm) broke through the Darién Gap, the narrowest part of the Panamanian Peninsula, which is where we’ve been able to contain it for the longest period of time, is at the moment we’re just not able to keep up with the amount of sterile flies that we need to because of the blood production facility,” Hough said. 

She said there is another facility being constructed in Mexico that should be complete by the end of June, and another in Texas by December 2027. 

Outbreaks can cause agricultural costs from loss of animals, decreased production from infested animals or other treatment costs. Luley said South America loses $3.6 billion a year from screwworm infestations, and there’s an estimated annual cost of $540 million to livestock producers and consumers in the U.S should the outbreak worsen.

Next steps 

The state Department of Agriculture is relying on the federal New World Screwworm Response Plan and tailored state-specific actions, as well as interstate and international movement restrictions. 

“Animals that are in infested zones require a seven-day inspection by a veterinarian prior to movement,” Luley said. “Animals from currently infected premises are not allowed to move from Pennsylvania until that case has been successfully resolved.”

Lawmakers questioned whether the state and U.S. are equipped to handle a possible outbreak given federal funding reductions. Congressional Democrats have raised concerns as to whether President Donald Trump’s funding and staff cuts to the Department of Agriculture are inhibiting the ability to respond to a screwworm threat. 

Hough assured state lawmakers on Tuesday that proper resources are in place to respond to an outbreak. 

Luley said her only concern would be if screwworm infestations in Pennsylvania coincided with an outbreak of highly pathogenic avian influenza, or the bird flu, as it would be challenging on the state’s resources. 

Reporting cases to veterinarians, and the state or federal Department of Agriculture, is the first step Luley said someone should take if they spot an infestation so that animals can get proper treatment. 

“We need eyes on the ground to catch the pest early if it does come to Pennsylvania,” Hough said. “We’re hoping that is not the case, but there are not enough of us out there to inspect every animal, so we are asking for our producers, pet owners and veterinarians’ help in this fight.”

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