Cooperative Extension helping ranchers prepare for New World screwworm fly threat

A fly with flesh-eating larvae is raising the alarm in the U.S. cattle industry as the insect spreads through Mexico toward the border.

The New World screwworm fly, Cochliomyia hominivorax, came into the news most recently in May, when the U.S. Department of Agriculture halted live animal imports from Mexico for the second time in six months.

U.S. officials learned in November that the screwworm was in southern Mexico and halted imports then, resuming in April after Mexico agree to collaborate on eradication efforts including weekly release of millions of sterile male flies. But the insect continued to spread, leading to the new import ban, the USDA said in a news release.

Unlike most maggots, screwworm larvae eat living flesh, leaving behind open wounds that can kill cattle, their preferred host, within a week. They can also infest dogs, cats, birds and even people. The striped flies – slightly larger than house flies – are native to the Western hemisphere, including the U.S. Southwest.

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They prefer warm, tropical environments, said Dawn Gouge, an entomologist and Arizona Cooperative Extension’s interim assistant director for Agriculture and Natural Resources.

Screwworm flies have recently been found in Mexico about 700 miles from Texas, where in June the USDA began construction on a facility to produce sterile flies to head off the advance, USDA said.

Re-emerging threat

The fly was considered eradicated in the U.S. by the 1960s, but a few outbreaks plagued ranchers in the 1970s. International collaboration, including decades of surveillance and the release of more than 96 billion sterile male flies, eradicated the insects from Guatemala, Belize, El Salvador, Honduras, Nicaragua and Costa Rica by 2000.

A sterile fly production facility in Panama helped maintain a barrier along the relatively narrow land mass between South and Central America, Gouge said.

Now the fly is moving north again. Because flies fly and animals walk, the fly will always be difficult to contain, and it’s a question of when the flies get to the U.S. and Arizona, not if, Gouge said.

“There is this natural progression always for this fly to move. Until it is literally eradicated, it will periodically resurge,” she said.

Ranchers in the U.S. haven’t dealt with the threat in decades, and younger ranchers never have.

“I have zero experience with it, but as a little boy I can remember my grandpa and my dad talking about what a wreck it was,” said Nate Brawley, an Arizona rancher and Extension assistant area livestock agent in Graham and Greenlee counties. 

“All they did was doctor for screwworm, and that meant they were having to ride a lot, rope a lot, doctor in the country a lot.”

Extension working with AZDA on surveillance

Extension is working with the state Department of Agriculture on a monitoring and detection pilot program.

Extension’s Arizona Livestock Incident Response Team (ALIRT) is putting together test kits to be distributed among ALIRT veterinarians and Extension agents and offices, said Betsy Greene, a U of A equine professor and Extension specialist who leads the team.

ALIRT is a collaboration among Extension, the Arizona Department of Agriculture and the Arizona Cattle Growers Association. Extension produced a “BOLO” report to get concerned parties on the lookout for signs of screwworm flies and give some instruction on what to do. The report was reviewed by Arizona State veterinarian Ryan Wolker, Greene said.

“It takes the science and translates it for ranchers, horse owners, readers - John Q. Public – so we can make people aware of things in an understandable way, not vet talk or science talk,” she said.

Each kit contains two test tubes for larvae samples – one for the Arizona Veterinary Diagnostic Lab at U of A, and one for the USDA. The USDA sample is for official testing; the university one for local testing, she said.

“So if there is a positive (identification) we can react sooner,” Greene said, adding that no screwworm flies or larvae have been found in Arizona and that the main goals at this point are proactive surveillance, awareness and education. 

Those kits and Extension’s statewide network of experts could help make a difference, Gouge said.

“They're the right people, they're in the right places and they're already fairly well informed. As new information comes to light, we get those BOLOs out, straight away to our team, and then they are the people who disseminate it through the network within their counties.”

Not all ranchers are prepared

How ranchers react could impact how their herds fare when the flies get to the U.S., Brawley said.

“It's just doing the very basic things - making sure your herd is bio secure, making sure you have a good vaccination program and a pest control management program,” he said.

Calves can be more vulnerable than adult cattle, because the flies lay eggs in open wounds, such as umbilical cord sites or wounds from de-horning or ear tagging, Brawley said.

Arizona has a buffer zone to the south in the Mexican state of Sonora, which has stringent health protocols for cattle that could serve as an early warning system, Brawley said.

“But again, they struggle with the same issues we do. You have guys that are concerned about it and are going to do everything in their power to keep from getting it. But then there's also guys that that just take it as it comes, and are not real concerned,” he said.

Ranchers who are less prepared, including some who underestimate the threat, will likely be hit the hardest. Some ranchers see the threat as distant, Brawley said.

“Ranchers were, you know, ‘It's thousands of miles south in Mexico still. We're okay. We're safe.’ But I think it's surprising people how fast it is expanding north. Producers who are not being progressive, who aren't vaccinating, who aren't taking it seriously. They're going to be hurting.” he said.

To learn more about New world screwworm flies, see this Extension publication. To learn more about the ALIRT, see the team’s web page.