New World screwworm was eradicated from the U.S., Mexico and Central America decades ago through a sterile fly campaign. But the tropical parasite has crept its way north again in recent years.
The U.S. Department of Agriculture (USDA) announced the first confirmed state-side case on June 3 and 18 more as of June 24. The detections include cattle, sheep and goats in Texas and a dog in New Mexico.
Jeff Kaisand, Iowa’s state veterinarian, said New World screwworm is “very concerning.”
“Our goal is to contain and eradicate this,” Kaisand said during an Iowa State University Extension and Outreach webinar on Monday. “We want to make sure that we don’t let this move any farther north, and if it does, we catch it quickly so the fly population doesn’t get established.”
Adult females lay eggs around a wound, which can be as small as a tick bite or a scratch from a barbed wire fence, said Matthew Brewer, associate professor of veterinary medicine at Iowa State University.
The larvae then burrow — or screw — into the flesh. If left untreated, Brewer said wounds can turn into deep cavities and rapidly kill animals.
After five to seven days of feeding, the larvae drop to the ground, develop into pupa and hatch into flies. The complete life cycle can be as short as 21 days.
Adult flies can potentially fly 10-12 miles per day, Brewer said. But “human intervention” is likely the highest risk for spreading the parasite.
“Putting animals on trucks and taking them other places and not paying attention and not examining animals is more of a risk than these flies just flying by themselves long distance, even though it’s possible,” Brewer said.
Fighting screwworm with sterile flies and vigilance
USDA scientists developed a technique in the 1950s to control New World screwworm populations without insecticides. It involves radiating captive male flies during a specific developmental stage, which makes them sterile, and releasing them into infested regions.
Since screwworm flies only mate once, the population crashes when enough wild females lay unfertile eggs.
The USDA declared New World screwworm eradicated in 1966 and worked with governments in Mexico and Central America to do the same. A sterile fly facility in Panama created a barrier to populations in South America until a breach in 2023.
Mexico’s department of agriculture detected the parasite in late 2024 and has confirmed over 30,000 cases since then. More than 1,800 were active on June 22 this year.
Most of the cases in Mexico have been cattle, but dogs make up the second largest category. Alejandrina Da Silva, a veterinarian and graduate student at Iowa State University, said all but one of the reported swine cases were from backyard production systems.
Earlier this year, the USDA’s National Veterinary Services Laboratories in Ames confirmed larvae from a calf in Texas were New World screwworm.
The federal, state and tribal response to detections includes a targeted release of sterile flies and establishing a 20 km — or 12.4 miles — “infested zone” for testing and restricting the movement of animals.
“Before animals can move from infested zones, they need to be inspected for wounds and also be treated for something to control the larvae,” Kaisand said.
The Iowa Department of Agriculture and Land Stewardship issued guidance earlier this month for bringing animals into the state.
Kaisand emphasized livestock producers and pet owners should know where their animals are coming from and routinely inspect them for open wounds.
“If you see any maggots or anything that’s odd, report to your veterinarian,” Kaisand said.
Animals can recover with early detection and care, Brewer said. Treatment includes federally approved drugs and the physical removal of larvae. He explained they need to be placed in alcohol in a sealed container.
If the larvae are alive and fall to the ground, they could enter the pupae stage and become flies.
In high risk areas, Da Silva said it’s important to make sure everyone on the farm is trained to detect New World screwworm and treat wounds quickly. Other practices, like manure management and fly traps, can help reduce fly reproduction.
She said communication is key to protecting other producers, as well.
“In short, New World screwworm preparedness depends on vigilance, rapid response and a strong biosecurity plan,” Da Silva said.
Da Silva and other experts say eradicating New World screwworm from the U.S. alone would require releasing 500 million sterile flies per week. The facility in Panama, which is jointly managed by the USDA and Panama’s Ministry of Agriculture Development, produces 100 million.
A renovated facility in Mexico is slated to open soon with capacity to produce 100 flies each week. Another being built in Texas by the USDA and U.S. Army Corps of Engineers is expected to open in 2027 with the ability to release 300 million flies per week.
