New World Screwworm: What Pet Owners Need to Know

By: Teleah Grand DVM, CVA, CVFT, CVCH, CVTP
For decades, New World Screwworm (NWS) was considered a disease of the past in the United States. Thanks to an extensive eradication program, the parasite was eliminated from the U.S. in the 1960s. Unfortunately, recent cases have been identified in Texas and New Mexico, reminding us that this parasite remains a threat to both livestock, wildlife, and companion animals.

While there is no reason to panic, there is good reason to be informed.

How Do Pets Get New World Screwworm?

New World Screwworm is not a worm at all. It is the larval stage of a fly.

Adult female screwworm flies are attracted to wounds, irritated skin, surgical incisions, hot spots, tick bites, ear infections, and moist tissues around the eyes, nose, mouth, and genital area. The fly lays eggs in these locations, and when the eggs hatch, the larvae begin feeding on living tissue.

Unlike most maggots, which feed on dead or dying tissue, New World Screwworm larvae feed on healthy, living tissue. As they grow, they cause wounds to become larger, deeper, and increasingly painful.

What Does a Screwworm Infection Look Like?

Early signs can be easy to miss.

You may notice:

As the infestation progresses, pets may become lethargic, lose their appetite, or develop extensive tissue damage.

The most important thing to remember is that screwworm infestations are medical emergencies. Early treatment dramatically improves outcomes.

How Is New World Screwworm Treated?

Treatment involves much more than simply removing visible larvae.

Most patients require:

Capstar® (nitenpyram) has recently received FDA Emergency Use Authorization for treatment of New World Screwworm infestations in dogs and cats. Capstar helps kill larvae quickly and has become an important tool in treatment protocols.

However, Capstar is not a substitute for veterinary care. Patients still require wound management, pain control, and treatment of secondary infections.

Prevention Is the Best Medicine

The best way to protect pets is to prevent infestations before they occur.

We recommend:

Prevention is especially important because the flies responsible for New World Screwworm are now being identified farther north than they have been in decades.

As of June 22, 2026, confirmed cases have been identified in Texas and New Mexico. A pet dog in New Mexico was diagnosed despite having no history of travel outside the United States, demonstrating that companion animals can be affected. Recent reports have also documented cases near San Angelo, suggesting the flies are continuing to move northward.

A Word About Isoxazolines and Natural Prevention

As an integrative veterinary practice, we understand that many of our clients are interested in reducing unnecessary medications and using natural therapies whenever appropriate.

We share that philosophy.

Nutrition, herbal medicine, acupuncture, physical rehabilitation, and other integrative therapies can play an important role in maintaining health and reducing disease risk.

However, when it comes to parasite prevention, we must balance potential risks against very real threats.

Many clients have questions about isoxazoline medications such as Credelio®, NexGard®, Simparica®, and Bravecto®. The FDA has acknowledged that rare neurologic side effects can occur in susceptible pets. These reactions are uncommon, but they are real and deserve consideration when making treatment decisions.

At the same time, we must consider the risks associated with the parasites these medications help prevent.

Today, those risks include:

Natural flea and tick products may provide some degree of repellency, but they generally do not provide the level of protection that has been demonstrated with prescription parasite preventives. While we support minimizing unnecessary pharmaceuticals whenever possible, we also believe it is important to use effective tools when the consequences of infection can be severe.

For most pets, the benefits of parasite prevention significantly outweigh the risks associated with these medications.

Why We Are Recommending Credelio Quattro

Because of the increasing parasite concerns in our region, Animal Care Center at Stonebridge Ranch is recommending Credelio Quattro for most canine patients.

Credelio Quattro protects against:

In addition, Credelio was the first isoxazoline to receive FDA Emergency Use Authorization for treatment of New World Screwworm in dogs, and Credelio Quattro became the first canine parasite preventive to receive FDA conditional approval for treatment of New World Screwworm infestations.

No medication is completely without risk. However, after reviewing the available evidence and considering the current parasite threats facing pets in Texas, we believe the benefits of year-round parasite prevention strongly outweigh the potential risks for the vast majority of dogs.

What About Cats?

Cats can also be affected by New World Screwworm and should not be overlooked when discussing parasite prevention.

While many people think of cats as “low-risk” pets, cats can develop wounds, abscesses, skin infections, and other conditions that may attract flies. Outdoor cats are at the highest risk, but indoor cats are not completely exempt.

Cats are not small dogs, and parasite prevention recommendations for cats differ from those for canine patients. The products we recommend depend on your cat’s lifestyle, health status, and risk factors.

If you have questions about the best parasite prevention program, please contact our team. We can discuss your pet’s individual risk factors and recommend a prevention program tailored to your dog or cat.