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Screwworm from canine

Photo by Dr. Benjamin Hada

After removing suspicious larvae from near the eye of a shih tzu patient, Dr. Benjamin Hada sent a photo to the Texas Animal Health Commission, which retrieved the maggots the next day to confirm their identity.

Saturdays are usually busy at Andrews Veterinary Clinic, a mixed animal practice in West Texas, and this one was no different — until it was.

Among the appointments on June 6 was a 10-year-old male shih tzu, a regular patient of Dr. Benjamin Hada, the clinic owner. The dog's owner brought in the pet to have a wound near his eye examined. There were two injuries, each about 1.5 to 2 centimeters in size.

During an initial look, a veterinary technician noticed a few maggots. But that wasn't atypical. Hada has, over the years, seen countless cases of flystrike, an infestation by blowflies, which lay eggs in dead tissue near a wound or in carrion.

Two things struck the doctor as odd, however: First, none of the tissue appeared to be necrotic. Second, even with a dulled sense of smell after a bout of Covid-19, Hada picked up a distinctly bad odor.

He sedated the dog and probed deeper. It was then that he saw that, beneath the skin, the wound was filled with larvae.

Hada ended up removing at least 40 maggots from the small dog's head, saving the larvae in a sterile container and sending a photo to a veterinarian at the Texas Animal Health Commission (TAHC). Early the next morning, a TAHC courier arrived to take the maggots to a research laboratory in Kerrville, 300 miles away.

Within 48 hours, Hada received confirmation: His patient was the first canine case of the New World screwworm fly in Texas in decades.

Although part of the same family, Calliphoridae, as blowflies that feed on dead or decaying flesh, the screwworm fly lays its eggs in living flesh. It targets wounds or bodily openings such as eyes, ears, genitals and the umbilicus of newborns. Once hatched, maggots burrow deeper into the skin.

Once common in parts of the United States, screwworm was declared eradicated in the 1960s (though there were outbreaks continuing into the 1970s), and eventually confined south of the Darién Gap, a narrow strip of land connecting Central and South America.

No longer. The pest breached the Gap and has been migrating north since 2023. In September 2025, screwworm was detected within 70 miles of the U.S. border, in the Mexican state of Nuevo León. The colder temperatures of autumn and winter brought a lull, as the insects like balmy weather. But with the return of warm temperatures, fly activity and migration picked up.

On June 3, the U.S. Department of Agriculture confirmed detection of screwworm in a newborn calf in Zavala County, Texas. The parasite struck the 3-week-old animal's umbilicus. The case was the state's first since 1982.

As of today, a USDA screwworm online dashboard shows 12 confirmed cases of screwworm in the U.S. Except for the dog, which was diagnosed in Texas but lives in New Mexico, all of the cases are in livestock — cattle, goats and one sheep — living in six Texas counties.

On June 5, the Texas governor declared a state of disaster for the entire state.

Recently authorized drug put to use

As Hada's patient was added to the USDA dashboard, and news of the detection made national headlines, the dog in question was being monitored at Andrews Veterinary Clinic. As advised by the TAHC, Hada kept the dog isolated from other animals, and his team watched their patient closely.

After the shih tzu woke up from sedation, Hada gave him NexGard, an oral parasiticide with the active ingredient afoxolaner that is given regularly to dogs to protect against fleas and ticks.

Because screwworm had been eradicated from the country until now, there are no fully approved drugs for treating infestations. However, since September, the U.S. Food and Drug Administration has authorized the use of 12 drugs for the prevention and/or treatment of screwworm in a variety of animal species. The drugs have received conditional approval – indicating that they have been demonstrated to be safe, with a reasonable expectation of effectiveness – or an emergency use authorization, a status indicating that the potential benefits of a product outweigh its potential risks during a public health emergency. Among the drugs authorized for emergency use against screwworm are NexGard chewable tablets for dogs and NexGard Combo, a topical solution for cats.

The treatment appears to have worked for Hada's patient. The doctor said that by the next day, three dead screwworm larvae were found at the bottom of the dog's cage.

In talking to the client, Hada's team wasn't able to determine where the dog might have encountered screwworm. But Hada thinks he knows what drew the parasite to the animal. Inside the wound, he found a few foxtail seeds. Foxtail grass awns are sharp and can become embedded in an animal's skin. It's a common injury, Hada said, and a physically small one.

A foxtail piercing or any number of other seemingly minor injuries could create an opening for the screwworm fly to exploit, Hada said: "Any little dermatitis or a little puncture, a cat bite – anything that may start off really small could attract that particular fly, and she'll lay her eggs, and there we go."

Foxtail

Photo by Dr. Benjamin Hada

Screwworm flies lay eggs in wounds, even small ones. Hada suspects that his patient's injury was originally caused by a foxtail grass seed.

Zones of surveillance

Three hundred miles east of Hada's clinic, screwworm larvae have been detected 11 times, on cattle, goats and one sheep. The TAHC has established six quarantine areas, or "infested zones," around those cases. Each zone extends 20 kilometers (about 12½ miles) from the site of detection, with an additional 20-km belt beyond that designated for surveillance. Animals may not be moved outside of the infested zones without permission from the commission, which involves an inspection and obtaining a permit from a TAHC representative.

The primary method of controlling screwworm spread historically has been by sterilizing males and releasing them to mate. Female flies, which mate only once in their lifetime, after mating with the sterilized male, will produce unfertilized eggs. Male flies can mate multiple times.

Today, sterile fly deployment is underway in the affected areas, according to a USDA announcement. The agency said on June 5 that it is releasing 2 million sterile flies twice per week through aircraft and 4 million flies per week from ground release chambers – containers filled with pupae, which hatch as adult flies – in or around the area of detection.

The sterile flies are imported while the U.S. develops its own production facility. The USDA announced in February that it had completed a sterile fly dispersal facility in Edinburg, Texas. In April, the agency broke ground on a sterile fly production facility, also in Edinburg, that is slated to be operational in November 2027.

As screwworm developments unfold rapidly in Texas, Dr. Will McCauley, who is on the board of directors of the Texas Veterinary Medical Association (TVMA), said it's important for veterinarians to stay up to date on accurate information. For that, he recommends the websites of the state animal health commission and Texas A&M AgriLife.

"This is not a reason for panic," McCauley added. "People should have a long-term mindset on this and just know that this is something that they're likely to see, particularly down here in Texas, and ... report it so that the proper regulatory authorities can respond to it."

Staying calm and keeping a sharp eye out

In the two weeks since finding screwworm on his patient, Hada has had many conversations about screwworm with clients. Some, ranchers with family members who remember when screwworm was last in Texas, are on high alert. One client asked if they needed to keep their dogs inside. Another told him a local boarding facility was denying admission to animals from his clinic. In response to the latter, Hada encourages people not to spread panic.  

"If you're doing a good job observing your animal for normal, everyday cuts and scrapes and changes like that, the likelihood of you getting a screwworm infestation in your pet is probably the least of any animal, honestly, just simply because they're looked at more often," he said.

Vigilance is key. Early detection helps curb spread and can save the life of an infested animal, as Hada's patient proved. The shih tzu got to go home to his family on Wednesday. Once the maggots were removed, the doctor reported, the wound healed quickly.

"He's barking, eating, drinking,” Hada said. “He's really made a spectacular recovery."

Correction: A previous version of this story reported that the June 3 confirmation of a screwworm infestation in a Zavala County calf was the first in Texas since the 1970s. The state's last known screwworm case before 2026 was in 1982.