A September 10, 2026, alert from the Equine Disease Communication Center identified screwworm in a working ranch mare in Presidio County, Texas. She was the first horse reported in the current U.S. incursion. Larvae had collected deep inside a long wound on her hindquarter.

This was a new horse case, not the beginning of the wider outbreak. The first U.S. animal case of this incursion was confirmed on June 3 in a calf in Zavala County, more than three months earlier. That calf recovered. The mare’s public notice doesn’t give her medical outcome. The attention around horn-like growths in wild rabbits shows why a report’s date needs to be distinguished from the history of the condition.

Screwworm can be treated, but a suspicious wound needs prompt veterinary attention. Recovery depends on the individual case, and having a dewormer or fly spray on hand isn’t a substitute for getting an infested animal assessed.

The current horse alert also includes an important tracing detail: investigators aren’t aware of the mare entering or leaving the property during the period they examined. That finding doesn’t identify the origin of the fly responsible or establish how many other infestations existed nearby.

An infested animal is evidence of exposure. Determining the extent of local transmission takes more investigation. USDA records animal cases separately from detections of wild screwworm flies in traps; those findings answer related but different questions. With infestations already documented across multiple Texas counties, the uncertainty around one property shouldn’t be confused with an absence of a wider outbreak. The virus identified in a Florida shrew offers a separate example of why detecting a biological threat and establishing its wider spread are different findings.

Texas’s September 9 order created Infested Zone 21 in portions of Presidio and Jeff Davis counties. It placed restrictions on animals leaving that zone, not a blanket prohibition covering every animal throughout both counties. A quarantine boundary defines where controls apply; it doesn’t mean every animal inside it is infested.

Start with the wound, not the medicine

Despite its name, New World screwworm is a fly, Cochliomyia hominivorax. Its larvae cause the injury. Females deposit eggs around wounds or vulnerable body openings, and the larvae that hatch feed on living tissue. The damage can deepen as they feed, creating an increasingly serious wound.

Owners don’t need to identify the fly species before asking for help. A wound can contain larvae from several fly species, so a specialist has to establish whether screwworm is responsible. Finding maggots is a reason to call, not a reason to wait until a photograph looks conclusive.

A useful inspection routine has four parts:

• Check vulnerable places. Examine cuts, recent procedure sites, newborn animals’ navels, and body openings. Don’t dismiss a wound simply because it looks small.
• Look for change. Worsening wounds, unusual drainage, a strong odor, visible larvae, and signs of pain deserve attention. A quick look at the surface may miss a deeper problem.
• Call promptly. Contact a veterinarian and the state animal health authority. Ask for instructions on collecting and submitting a sample rather than improvising removal or disposal.
• Check transport rules first. Animals leaving a Texas infested zone need authorization and must satisfy the applicable inspection, treatment and documentation requirements.

Good hygiene helps, but it doesn’t make an animal immune to infestation. Keep wounds protected as your veterinarian advises, maintain clean surroundings, and use an appropriate fly-control plan. Clean stalls don’t replace looking closely at the animal.

What does treatment involve?

Veterinary care generally includes cleaning the affected area, removing larvae and selecting suitable medication. A secondary bacterial infection may also require antibiotics. Larvae removed during care must be handled as animal health officials direct so they can’t continue developing in the environment.

Does ivermectin treat screwworm in horses?

With Ivermectin Liquid for Horses, the authorized aim is to protect a vulnerable animal before larvae take hold. FDA limits this use to newborn foals and horses receiving initial wound care. It’s a short window: estimated protection lasts up to 24 hours after dosing. The authorization doesn’t cover an existing infestation.

Doramectin, sold as Dectomax/Dectomax-CA1, also has an emergency authorization for prevention in horses at least one year old, not for treating an existing infestation in horses. Protection may extend past the first day, but its duration beyond that point in horses isn’t known. For both drugs, FDA notes that the evidence lacks horse trials demonstrating prevention of this particular infestation. The agency advises against indiscriminate preventive use in healthy horses without wounds or an expectation of developing them.

Those limits don’t mean veterinarians have no options. They mean the formulation, the animal and the purpose of treatment matter. The American Association of Equine Practitioners notes that oral ivermectin pastes have been used against screwworm in other countries, but published studies establishing their effectiveness in horses are lacking. A familiar paste isn’t interchangeable with the specifically authorized liquid product.

Certain topical medicines do have emergency authorizations for treating screwworm in horses. The FDA’s current veterinary guidance separates those uses from prevention and identifies which species each product covers. An emergency authorization isn’t the same as full approval for that use.

What about sprays and dip tanks?

Follow the specific product’s directions and your veterinarian’s advice. A pesticide intended for building surfaces isn’t a wound medicine, even if both products are described as fly control.

Texas also operates a cattle fever tick program with dipping facilities. Those vats target a different pest. Their existence shouldn’t be taken as a ready-made treatment plan for larvae feeding inside a horse’s wound.

Are dogs, cats and people at risk?

Screwworm can affect pets as well as livestock. FDA changed the options available to pet owners on September 3. Capstar and its generic counterpart, Nitenpyram Tablets, received emergency authorizations covering both prevention and treatment for dogs and cats. Eligible animals must weigh two pounds or more and be at least four weeks old. The generic’s earlier authorization was limited to treatment.

At the time of that announcement, the agency considered exposure risk low for most dogs and cats in the country because infestations were concentrated in a limited region. It urged closer monitoring for animals exposed to affected areas. Owners shouldn’t assume that every flea product provides protection.

People can develop infestations, too. CDC’s August 11 assessment put the general U.S. public’s risk at very low and reported no locally acquired human cases. Anyone who sees or feels larvae in a wound or another body area should seek medical care immediately rather than attempt removal.

Why would officials release more flies?

The established eradication method interrupts breeding. Screwworm pupae are exposed to radiation in a production facility, producing sterile adults. When wild females mate with sterile males, their eggs don’t hatch. Repeated releases reduce the opportunity for successful reproduction.

So deliberate fly releases are a documented control measure. Releasing sterile flies is different from introducing flies capable of producing another generation of feeding larvae. USDA describes the technique as a central part of its response, alongside monitoring that helps determine where releases are needed.

This approach has a much longer history than the current emergency. USDA’s archive records eradication in the Southeast in 1959 and the start of the Southwest campaign in 1962. The often-cited 1966 milestone needs qualification: the same archive records continued Texas infestations until 1982. Mexico wasn’t declared screwworm-free until 1991.

The history wasn’t finished then, either. Sterile flies helped eliminate a Florida Keys outbreak in 2017. Eradication in a place doesn’t guarantee that the pest can never be reintroduced.

That history also puts claims about newly manufactured worms in perspective. Screwworm was recognized as a distinct fly species in 1933. Modern laboratory research doesn’t establish that the species itself was recently created.

USDA is developing a newer male-only strain called NovoFly, but its published guidance says that strain isn’t currently being used in sterile-insect operations. Research into a future tool and deployment of that tool are different developments.

The medicines discussed here also shouldn’t be confused with vaccines: ivermectin and doramectin are antiparasitic drugs, not mRNA vaccines. The horse’s case notice makes no vaccine-related finding.

Questions about funding, staffing and border policy need their own documented timelines. Screwworm’s regional resurgence was underway in Central America in 2023, and Mexico detected it in November 2024. Those dates establish chronology, not the effect of a particular administration’s decisions. The mare’s case notice doesn’t attribute her infestation to a funding change or import policy.

A case marked inactive still isn’t a survival update. USDA closes mitigation for animals that recover, and also after required measures are completed following an animal’s death. Neither pathway automatically ends a zone’s restrictions.

In Texas, suspected infestations in livestock or pets can be reported at 1-800-550-8242. The Texas Animal Health Commission’s screwworm guidance provides current reporting and movement information. You don’t need a confirmed diagnosis to make that call.