New World screwworm cases have now been confirmed in five Texas animals across three counties, with the latest case, a calf with an umbilical lesion in La Salle County, confirmed yesterday by the Texas Animal Health Commission. The parasite's spread into the Hill Country, confirmed just two days ago in a Gillespie County goat, has prompted the establishment of a formal quarantine zone just outside Fredericksburg.
The five cases have all involved livestock: two calves in Zavala County detected June 3 and 5, two more calves in La Salle County on June 7 and June 9, and a goat near Harper in Gillespie County on June 8. A dog examined by a Texas veterinarian was also confirmed as infected, but the animal resides in Lea County, New Mexico, and is counted separately.
Federal officials are escalating their primary eradication tool: the release of laboratory-bred sterile flies. When wild screwworm flies mate with sterile ones, no offspring are produced, eventually collapsing the population.
Officials are deploying ground release chambers to supplement the four million sterile flies already being dispersed aerially in the region each week, with billions of additional sterile flies being expedited into the affected areas.
Ranchers in infested zones who report cases will have sterile flies released directly over their properties and from tubes on the ground. Those who don’t report lose access to that resource.
Sonja Swiger, a livestock entomologist with Texas A&M AgriLife Extension and an expert on screwworm, first spoke with TPR about the parasite as it marched through Mexico last summer. Now that it's back in the United States, she says concern is warranted, but panic is not.
“It is treatable, it is manageable, but it does require cooperation,” Swiger said. “Treating your animals, if they’re infested, is a necessity because you can’t let these larvae continue to feed. They will eventually kill that animal.”
A confirmed infestation triggers a 72-hour inspection period, not a ranch closure. Officials inspect all animals on the property and any adjacent herds, administer approved treatments, and release sterile flies. Once animals are confirmed larvae-free, they can move. Swiger urges ranchers not to let fear of a three-day restriction keep them from reporting.
“It’ll get found out eventually, and you’ll still be in the middle of it. So it always helps to just do it sooner,” she said.
WHAT ABOUT PETS — AND PEOPLE?
For pet owners anxious about the outbreak, Swiger offers measured reassurance. Screwworm flies are drawn to animals that are resting or left outdoors unattended, not to those out for a midday walk.
“Walking the dog is not going to be a problem,” Swiger said. “Ignoring your dog and leaving it outdoors for lengthy periods of time — those are the problems.”
For additional protection, several products — including Credelio and NexGard — have received federal authorization to guard against screwworm infestation while also covering fleas, ticks, and internal parasites.
As for humans, screwworm myiasis is rare in the United States, accounting for a handful of travel-related cases over the past decade. The flies are daytime insects; sleeping indoors at night carries no elevated risk. The danger arises when a person with an open wound falls asleep outdoors in an area where the fly is active. Humans represent roughly one percent or fewer of all screwworm cases.
FOOD SUPPLY: NO RISK
Misinformation circulating on social media has suggested the food supply could be compromised. Swiger says that is categorically false. Screwworm larvae infest only living animals and cannot survive in meat that enters the commercial food chain. “There will not be maggots in the food,” Swiger said.
The economic impact is another matter. Ground beef averaged $7.06 per pound in May — a record, and 13 percent above year-ago levels — according to the Bureau of Labor Statistics. A prolonged screwworm outbreak that further shrinks the already-depleted U.S. cattle herd could push those prices even higher.
THE GOAL: ERADICATION, AGAIN
The mission has shifted in the year since TPR first spoke with Swiger on the approaching threat. The goal then was to keep New World screwworm out of the United States. Now that it has arrived and spread as far north as the Hill Country, the objective is to push it back out.
“Our main goal, of course, is to minimize the spreading of them and then to get rid of them as fast as possible,” Swiger said.
Screwworm Resources:
New World Screwworm Fact Sheet
Texas Animal Health Commission NWS
Screwworm.gov
Texas AgriLife Extension NWS
Texas Parks and Wildlife NWS
