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Photo courtesy of Dr. Bill Folger

Members of a joint U.S.-Honduran veterinary team remove screwworms from a dog's forelimb in Honduras. Over six days last June, the group encountered infestations in scores of animals.

Dr. Bill Folger has made an annual trip to Honduras every year since 1998, traveling to remote villages to provide preventive care to small and large animals. Never in more than a quarter century of visits had the Texas veterinarian ever seen a case of New World screwworm — until last June.

An orange tabby cat was brought to him, with hair missing around its head. The skin there was paper-thin and fell away at a touch. Right away, he could see about 50 maggots.

"It's almost like a Frankenstein movie," Folger said. "It was really creepy seeing the maggots crawling out of the muscle tissue underneath the skin, [and] on top of the skull of that poor orange tabby — it freaked me out."

The case was the first Folger saw, and his team encountered many more. Over their six-day trip, the group saw screwworm infestations in as many as 60 animals, a mix of dogs, cats and cattle.

At first, they didn't understand what they were seeing.

Fortunately, the dean of a local veterinary school was working with them, as well as some of his students. "He knew exactly what it was," Folger said. That's because screwworm infestations had become their reality during the previous nine months. In September 2024, screwworm was documented in Honduras after years of absence, and after additional cases were confirmed, the United States Department of Agriculture added Honduras to its list of countries impacted by screwworm.

New World screwworm is a parasitic blowfly that targets live flesh, making its spread catastrophic for warm-blooded animals, including pets, livestock and wildlife — usually mammals but occasionally birds, too. Adult flies lay eggs in open wounds, where hatched maggots burrow deeper into the host. The pest was eradicated from the U.S. in the 1960s and eventually contained below the Darién Gap, a narrow strip of land between Colombia and Panama that creates a natural barrier between Central and South America. Since then, the U.S. has had only sporadic outbreaks.

But in 2023, screwworm began moving northward again. Mexican officials have documented several screwworm cases in the country's state of Tamaulipas, which borders Texas. U.S. officials have spent the last several months preparing for an incursion. During the past four months, the U.S. Food and Drug Administration has made six drugs available for the treatment and/or prevention of screwworm infestations in cattle, canines and felines.

In the latest surveillance development, officials at a Florida import facility last month detected an infestation of larvae on a horse from Argentina. The National Veterinary Services Laboratories in Iowa confirmed that the larvae were, in fact, New World screwworm. While the finding is not the same as documenting a northward expansion of the insect's range across the Mexico border, the USDA said in a press release that the event "underscores the need for vigilance in all of USDA's coordinated efforts to fight NWS."

As the parasite has been mostly absent from the U.S. for over 50 years, few in the country have had direct experience with screwworm. In addition to Folger, the VIN News Service spoke to a wildlife biologist and a preveterinary student who have seen it with their own eyes.

Incursion in the Florida Keys

Roel Lopez first heard of screwworm from his father, a former Texas rancher who remembers the havoc caused by the pest decades ago. Lopez saw it himself during an outbreak in 2016 in the Florida Keys — one of the few times screwworm has reappeared in the U.S. since its eradication.

Lopez, director for the Texas A&M Natural Resources Institute, has a doctorate in wildlife and fisheries sciences and did his dissertation on the endangered Florida Key deer, which was the main species affected by screwworm in the Keys. The Key deer is a subspecies of the white-tailed deer that lives only in the Keys. Lopez was part of a team addressing the outbreak.

To this day, it's uncertain how screwworm made it to the Florida Keys. Once the first suspected case was spotted, it took a while to formally identify. The first case was detected that June, and it wasn't confirmed until September.

That lag meant critical time was lost. "It wasn't identified early, and it had the opportunity to establish," Lopez explained.

Lopez arrived that November and stayed for several weeks, during which he placed monitoring collars on a segment of the deer population to help Florida officials get an accurate sense of the herd size and track how many individuals were infested with screwworm. Less than a year after it began, this outbreak was eliminated using what's known as the sterile insect technique — a practice of releasing sterile male flies to mate with females, which then produce eggs that won't hatch. Despite the short duration of the outbreak, it caused the deaths of 135 Key deer, about 15% of the area's herd of 900.

In the nine years since the outbreak, the Key deer population has bounced back. But managing a screwworm outbreak in wildlife in Texas would be more difficult than in the Florida Keys. Lopez points out that the landmass of the Keys is equal to only one-third that of a single county in Texas.

"One of the challenges with the potential infestation here in Texas is, when you think about free-ranging wildlife, it's a hard aspect of the population to monitor," he said. "They move around, they're carriers potentially for this parasitic screwworm, and so that's going to make things just a bit more challenging for us to try to contain."

Though screwworm has not been in Texas for decades, state officials have been preparing to see the blowfly again. An advantage that Texas has that Florida didn't is time to plan a coordinated agency response.   

"They'll be able to respond rather quickly," Lopez said. "That's a very different situation compared to what we saw in Florida."

Screwworm sighting in Honduras

For six days each June, Folger leads a team of veterinarians and other volunteers to Honduras to provide veterinary services in about two dozen villages, treating 2,500 to 3,500 animals.

Much of the treatment they provide is routine, such as administering vaccines and performing spays and neuters. As for parasites, it's not uncommon to see animals affected by Cuterebra larvae, also known as botflies. Cuterebra larvae are less aggressive than screwworm, settling just beneath the skin, as opposed to burrowing deep into the tissue. "Cuterebra larvae are easy to take care of," Folger said. "You just ... cut them out, get rid of them. No big deal."

Screwworms, he said, are much more vicious to their hosts. In his words, they "just eat them alive."

Folger and team did not have a protocol for treatment. In the cases they saw, the local veterinary students picked out as many of the maggots as possible, and Folger and his colleagues injected the patients with ivermectin, a drug commonly used to treat parasites. Whether the treatment was effective, Folger doesn't know, since the team had no way to follow up.

In preparation for this year's trip, Folger and team are trying to create a more formal protocol for handling screwworm cases. The effort is led by Anaïs Jayr, a sophomore preveterinary student at the University of California, Davis. They are being assisted by parasitologists, epidemiologists and pharmacologists at the Veterinary Information Network, an online community for the profession and parent of VIN News.

Last June, Jayr joined Folger and his team on their annual trip to Honduras, where she saw the screwworm for herself on the leg of a dog.

The experience stuck in Jayr's mind, motivating her to work on solutions. She began reading up on screwworm treatments.

"I think that part of my attachment to the screwworm is that it's very complex, and it's interesting because it's complex, and I just like thinking about all these different facets of the problem," she said.

Folger's team is raising money to employ members of the Honduran veterinary college to conduct follow-up visits to determine how well the protocol they design works. They're hoping this information can become widely applicable in the treatment of screwworm infestations.