New World Screwworm and Community Cats

For comprehensive guidance, please view all resources on our Screwworm resource page, including:

Use this guide to help protect your community's cat population from New World Screwworm (Cochliomyia hominivorax) with a few straightforward adjustments to your existing protocols.

The good news: New World Screwworm (NWS) is preventable and highly treatable, and if your area does not have active confirmed NWS cases, your cats are not currently at risk. Keep this guide on hand so you're prepared and confident if cases are ever confirmed in your area.

When NWS is active in a region, warm-blooded outdoor animals with open wounds are the primary concern. With simple additions to your surgical protocol, wound monitoring and prompt care, your community cats can stay safe and healthy. This guide walks you through easy protocol adjustments to help keep them protected.

A few important things to know as you plan:

NWS is federally reportable, so your local veterinary and wildlife networks will have up-to-date information on active cases.
Treatment and drug authorizations are updated frequently — your veterinarian is your best resource for confirming current approved options before use.

Why This Matters

Unlike common maggots, which feed on dead tissue, New World Screwworm larvae feed on living tissue in warm-blooded animals — they burrow into a fresh wound, enlarge it, and attract more flies, so an infestation can become fatal within days if left untreated. However, it is highly treatable when caught early.

Wounds such as scrapes, bites, surgical incisions or ear-tips are at risk for infestation in areas where the NWS fly is active. Risk is highest in or near areas with confirmed cases during warm months; screwworm flies are largely inactive in cool weather.

Widely accessible flea and tick medications that you may already have on hand can help prevent NWS, so expensive protocol revisions are not warranted in most situations.

Give a systemic isoxazoline while the cat is anesthetized

For community cats, their surgical appointment is your opportunity to provide them with month-long protection from NWS. A systemic isoxazoline circulates in the cat’s tissue and kills larvae that try to feed there.

Choose a product authorized for cats

Credelio CAT (oral lotilaner) and NexGard COMBO (topical esafoxolaner) carry FDA emergency use authorizations for screwworm in cats. The month-long efficacy of these products protects cats while their incision heals and for weeks afterwards. The topical is also very practical for TNVR cats when applied while the cat is anesthetized. Topical Bravecto (fluralaner) lasts longer but is extra-label for screwworm and Nitenpyram (Capstar) works rapidly, but its duration of action is only about a day.

Product efficacy

As with fleas, Isoxazolines kill larvae that feed on the treated cat. They do not repel flies or stop eggs from being laid, but the drug limits how far an infestation progresses.

Considerations for kittens and nursing queens

Check label minimums for weight and age before dosing juveniles, and discuss pregnancy and lactation guidance with your veterinarian; these products are typically extra label for use in very young kittens and nursing moms.

Never put permethrin, pyrethroids, or DEET on a cat.

Cats cannot metabolize these compounds, and exposure can cause severe, potentially fatal neurologic toxicity. This includes most products marketed as fly repellents.

Avoid livestock and equine products.

Fly sprays and the screwworm wound sprays and ointments authorized for cattle, horses, and other species are not authorized for cats.

Systemic isoxazoline and careful wound management are the safe and effective methods to protect cats; cat-safe topical fly repellents are not available.

Surgical closure

A fully closed, non-oozing incision is important as blood and discharge attract egg-laying flies. Surgical technique should include a secure intradermal closure with good hemostasis. Thoroughly clean residual blood from the cat prior to discharge. Apply sterile tissue adhesive as a protective layer when appropriate for the specific surgical procedure.

Ear tip

Ensure complete hemostasis and clean edges before the cat is returned. Electrocautery is recommended over chemical cautery, and battery-operated units are inexpensive and available from veterinary distributors. Avoid the use of electrocautery when oxygen is being delivered via a face mask.

Post-operative cleaning

Wipe away any blood or discharge and confirm the incision and ear-tip are clotted and dry before release or discharge.

Considerations for areas with confirmed cases

In high-risk areas with confirmed cases, during fly season, weigh the potential benefit of holding cats for a brief period (24–48 hours) against the stress of confining unsocialized cats and your holding capacity. It may also be considered for cats with larger incisions (e.g., procedures other than routine spay/neuter). This is a risk-based decision and is likely unnecessary except in the highest risk areas.

Euthanizing the host animal does not kill larvae. A euthanized animal with suspected or confirmed NWS is still a biosecurity hazard until the carcass is frozen or incinerated.

Alert colony caretakers to watch community cats for enlarging, foul, or maggot-bearing wounds and to report them immediately. New World Screwworm is reportable: in Texas call the Texas Animal Health Commission at 1-800-550-8242; in New Mexico call the New Mexico Livestock Board at 505-841-6161 (after-hours AgroGuard 1-800-525-2782).

Every companion-animal authorization is for treatment of an existing infestation, not prevention, so using these products at the time of surgery as a means of prevention is an extrapolation. Texas A&M describes their preventive efficacy as “likely,” but these products have not been tested for efficacy when used for prevention.