Companion Animal Care in the Face of the New World Screwworm (NWS) Threat

Introduction: Understanding the NWS Threat to Companion Animals 

The New World screwworm (NWS), Cochliomyia hominivorax, is a myiasis-causing parasitic fly native to the Americas. In 1966, the NWS was successfully eradicated from the United States. This was done through the mass release of sterile male flies (sterile insect technique). 

While livestock and wildlife represent most NWS cases in a given area, dogs and cats are also at risk, as well as other companion animal species (e.g., rabbits, ferrets, guinea pigs, etc.). The ensuing myiasis from a larval infestation may be fatal for dogs and cats, depending on anatomic location and severity of the infestation, care, and timing of the diagnosis. 

Monitoring and Surveillance 

• Check pets daily for the presence of potential wounds, including sites of previous tick bites/attachment. 
• Monitor for a foul odor, tissue damage, or signs of fly larvae in wounds (myiasis). 
• Body secretions and wound discharges may attract NWS females for oviposition. 
• Assess the pet’s environment for possible fomites that may cause injury (e.g., metal collars or chains, wire fencing, sharp vegetation, etc.). 
• Avoid risky behavior for dogs and cats, especially those with outdoor access (e.g., animal aggression/fighting and accidents). 
• Provide adequate care and dressing of any wounds, including recent surgical incisions (e.g., spay/neuter). 
• Observe pets for unusual behavior, such as wound-licking, limb-shaking or irritation. Animals may lose interest in food, and may become aggressive or lethargic due to irritability and associated infestations. 
• Visit your local veterinarian immediately if myiasis is suspected.

Treatment

Consult your veterinarian immediately if you suspect myiasis from New World screwworm to ensure that the correct diagnosis, treatment, and wound care are provided.

A veterinary professional should inspect and must promptly remove (and kill) ALL fly larvae. They must also clean suspect lesions and wounds. Sedation may be necessary. 

Mechanical removal of maggots should occur immediately, and removed larvae should be stored in 70 percent ethanol and submitted for diagnostics. Veterinarians may submit up to 10 maggots to the National Veterinary Services Laboratories for identification using the specimen submission form. All maggots that are not submitted must be killed, either by freezing or in alcohol. 

Isoxazoline-based oral products (tablets and chewable tablets) have a proven treatment efficacy and are likely to have preventive efficacy*. In October 2025, the U.S. Food and Drug Administration (FDA) issued an emergency use authorization for lotilaner to be administered to dogs for treatment of NWS. In November 2025, this authorization was extended to cats. In February 2026, products containing afoxolaner received emergency use authorization for use in dogs and cats. In these cases, administration requires a valid veterinary-client-patient relationship (VCPR) with a veterinarian.

More recently, nitenpyram-based oral products for dogs and cats received Emergency Use Authorization. These products are available over the counter, and while effective for treatment of existing infestations, do not have preventive efficacy.*

Nevertheless, consultation with a veterinarian is warranted to ensure the animal receives appropriate care, beyond antiparasitic treatment.

* Consult the FDA website for information on products with emergency use authorization or extra label use as new products are being approved on an irregular basis: New World Screwworm: Information for Veterinarians.

Conclusion

Pets near the U.S. southern border and those traveling to and/or from Latin America should be monitored regularly. 

Contact your veterinarian to discuss whether pretreatment for dogs and cats traveling to and from Latin America is warranted. 

Early detection of NWS myiasis is essential for effective treatment and clinical recovery of pets. 

Daily inspection for wounds and proper care of existing wounds/lesions are necessary to avoid NWS infestation and reinfestation.