Bottom line: New World screwworm is not a reason to panic, but it is a reason to pay attention. The best owner actions are daily wound checks in higher-risk areas, prompt veterinary care for suspicious wounds, careful fly and tick control, and awareness that suspected cases are reportable.
What is Screwworm
New World screwworm, often abbreviated NWS, is caused by the larvae of the blowfly Cochliomyia hominivorax. Although owners may see maggots in a wound, this is not a harmless surface problem. New World screwworm larvae feed on living tissue, which distinguishes them from many common fly larvae that primarily consume dead or decaying tissue. As the larvae burrow, a small wound can enlarge and deepen, leading to pain, tissue death, secondary bacterial infection, systemic illness, and, in severe untreated cases, death.
How Transmission Happens
Transmission requires exposure to the New World screwworm fly. It does not spread by routine direct contact from animal to animal. A female fly is attracted to the odor and secretions of wounds and can lay hundreds of eggs near the wound edge. Eggs can also be laid on normal tissue areas, including the eyes, nose, mouth, ears, genitals, and the umbilicus of newborn animals, even when an obvious wound is not present.
Eggs hatch quickly, and larvae may become visible within about three days. After feeding and burrowing for approximately one week, larvae can drop from the animal, enter the soil, pupate, and later emerge as adult flies. Because the fly itself drives spread, animal-health officials focus on surveillance, movement controls, rapid case identification, and sterile-fly releases.
Where Risk is Highest
New World screwworm is considered a foreign animal disease in the United States and is reportable in every state. It is known to exist in parts of the Caribbean, Central America, and South America, and recent activity in northern Mexico and Texas has raised concern for U.S. animal health. Owners in states bordering Mexico, owners traveling with pets or horses, and farms or barns receiving new animals from higher-risk regions should be especially vigilant. Risk can change quickly, so owners and veterinarians should follow current USDA, FDA, CDC, state animal-health, and AAEP guidance.
Climate and season matter. New World screwworm is a tropical fly that is most active in warm, humid conditions. Cooler weather can suppress fly activity and larval development, while freezing temperatures limit its range. During warm months, especially in southern regions, careful wound monitoring is the most important practical step for owners.
Animals Most Prone to Infestation
Any warm-blooded animal with a wound can be affected. Higher-risk animals include outdoor dogs and cats, barn cats, working or hunting dogs, horses on pasture, newborn animals with healing umbilical stumps, and animals with chronic skin disease, ear infections, fly-bite lesions, abscesses, surgical incisions, tick bites, saddle rubs, wire cuts, fresh dental extraction sites, or traumatic wounds. In horses, risk can also follow procedures or management practices that create wounds, such as castration, branding, clipping, or injuries from fencing and sharp objects.
Signs Owners Should Watch For
Monitor for wounds that enlarge, deepen, drain, bleed, smell foul, or become increasingly painful. Other warning signs include visible larvae or white egg packets near the wound edge, head shaking, licking or chewing at a wound, decreased appetite, depression, self-isolation, fever, lameness, or reluctance to move. In horses, owners should carefully inspect the face, ears, mouth, sheath, vulva, tailhead, legs, withers, girth area, skin folds, umbilicus of newborn foals, post-surgical sites, dental extraction sites, and any place tack, blankets, or fly gear may rub. In dogs and cats, dense hair can hide wounds, so any moist, painful, swollen, or malodorous area should be examined closely by a veterinarian.
Important: Not all wound maggots are New World screwworm. Many wound infestations involve common fly larvae. Laboratory identification is needed to determine the species involved, so owners should contact a veterinarian before treating or moving the animal when NWS is suspected.
What Owners Can Do to Help Prevent it
Prevention starts with daily inspection and prompt wound care. Check animals carefully if they spend time outdoors, travel, live in a higher-risk region, or have any wound. Keep wounds clean and protected according to your veterinarian’s instructions. Use species-appropriate fly, flea, and tick control, while reducing fly pressure by removing manure, wet bedding, spilled feed, and decaying organic material. Use screens, fans, fly masks, fly sheets, approved repellents, and environmental fly control where appropriate.
In regions where NWS is present, owners and veterinarians may need to delay elective procedures or husbandry practices that create wounds until fly risk is reduced or control measures are in place. Examples include castration, branding, clipping, and other procedures that leave fresh wounds. For horses traveling to or through affected regions, owners should consult their veterinarian before travel and confirm current state entry, certificate of veterinary inspection, and movement-permit requirements before loading the trailer.
How Veterinarians Generally Manage Cases
Veterinary care typically includes killing or immobilizing larvae with an approved product, cleaning the wound, removing eggs and larvae, collecting samples for identification, preventing larvae from falling into the environment, covering the wound when possible, treating the animal for pain or secondary infection, and isolating affected animals as directed by regulatory officials. Prognosis depends on the wound location, severity, speed of diagnosis, and the animal’s overall condition.
Treatment and Product Cautions for Horse Owners
Equine-specific prevention and treatment information is still evolving. AAEP notes that there is limited published horse-specific research on New World screwworm treatment and prevention, and there are no published studies establishing long-term duration of efficacy for any product for preventing screwworm infestation in horses. Current guidance emphasizes vigilance, wound monitoring, reporting suspicious cases, and veterinarian-directed product selection rather than broad preventive treatment for horses outside affected areas.
Horse owners should not assume routine dewormers or fly products are proven protection. Fenbendazole and pyrantel do not treat screwworm infestation. Oral ivermectin has been used historically in some settings, but AAEP notes that there are no published studies documenting its efficacy in horses for NWS, and extra-label or indiscriminate avermectin use raises parasite-resistance concerns. Injectable ivermectin is not labeled for horses and should not be used as a do-it-yourself prevention or treatment approach.
FDA-authorized or emergency-use options for horses continue to evolve. AAEP materials identify Dectomax injectable as authorized for prevention of NWS myiasis in horses at least one year of age, not treatment of active infestation, and F10 topical products as wound-level prevention and treatment options under emergency-use authorization. Product selection, dosing, and whether treatment is appropriate should be directed by a veterinarian and aligned with current FDA, USDA, state regulatory, and AAEP guidance.
Treatment and Product Cautions for Dog & Cat Owners
Along with appropriate wound care in an affected dog or cat, it’s important to give a medication that kills any remaining larvae quickly because tiny larvae can be missed and continue to move under the skin, In the United States, the FDA has given special approval for using lotilaner (Credelio®) in dogs and cats, and full conditional approval for a combination product for dogs that contains lotilaner (Credelio®), moxidectin (Advantage Multi®/ProHeart®), praziquantel (Droncit®), and pyrantel (Strongid®). In a small field study of 11 dogs, a single dose of lotilaner killed all larvae within 24 hours.
Other commonly used parasite preventives may also work well. In small studies, afoxolaner (NexGard®) and sarolaner (Simparica®) both showed 100% effectiveness in naturally infested dogs, while nitenpyram (Capstar®) removed 94% of larvae within 18 hours and reached 100% effectiveness by 24 hours.
All of these medications were used at their normal label doses in the studies.
What To Do if You Suspect New World Screwworm
Call your veterinarian immediately, keep the animal contained, protect the wound from additional flies, and do not move the animal unless directed by veterinary or regulatory officials. Do not attempt to manage a suspected screwworm case with home remedies or unapproved insecticides. If larvae are present, your veterinarian may need to collect samples before treatment so the species can be identified and the case can be reported correctly.
Because NWS is reportable, suspected cases should involve a veterinarian and regulatory animal-health officials. For affected animals, guidance states that a regulatory veterinarian, such as the State Animal Health Official or federal Area Veterinarian in Charge, should be notified before sample submission. More information can be found on the USDA APHIS website at screwworm.gov. Veterinarians may be instructed to collect larvae in 70 percent alcohol, not formalin, and submit samples to the National Veterinary Services Laboratories or another approved laboratory as directed.
Our Veterinary Advisory Board Message
New World screwworm is a significant concern, but calm, consistent prevention is effective. Inspect animals daily in higher-risk areas, treat wounds promptly, control flies and ticks, isolate and examine new arrivals, check official guidance before animal movement, and contact a veterinarian for any wound that is worsening, painful, foul-smelling, draining, or contains larvae. Early recognition protects the individual animal and helps veterinary and public-health officials prevent further spread.
Sources and Additional Resources
- AAEP Infectious Disease Guidelines: New World Screwworm Infestation (Disease) Guidelines, 2026
- AAEP New World Screwworm: Equine Veterinary Prevention and Treatment Options, 2026.
- USDA APHIS – Stop Screwworm and New World Screwworm resources. https://www.aphis.usda.gov/livestock-poultry-disease/stop-screwworm
- CDC – New World Screwworm information and situation summaries. https://www.cdc.gov/new-world-screwworm
- FDA – New World Screwworm information for veterinarians and animal drug updates. https://www.fda.gov/animal-veterinary/safety-health/new-world-screwworm-information-veterinarians
- Brazos Valley Equine Hospital client education guide
- https://www.cliniciansbrief.com/article/new-world-screwworm-veterinary-guide
