New World screwworm
Learn about New World screwworm, where it is found, how it spreads, and how to protect you and your pet
New World screwworm, or NWS, is a parasitic fly with metallic blue-green coloring and orange eyes, roughly the size of a common housefly. The New World screwworm feeds on healthy, living tissue and lays its eggs in open cuts, wounds or body openings, such as the nose, eyes and ears.
Once the eggs hatch, the larvae burrow into the wound and feed on living tissue, which is what causes the damage. This is different from common maggots, which usually feed on dead tissue. The larvae can cause extreme pain, severe infection, and serious illness if left untreated.
While the scientific name for NWS is Cochliomyia hominivorax — pronounced “KOCK-lee-oh-MY-uh HOM-in-ih-VOR-ax” — it’s commonly known as a screwworm. “Screwworm” is actually a general term for fly species whose larvae burrow into living tissue. NWS is one of two main types found around the world, alongside the Old World screwworm found in parts of Africa, Asia and the Middle East.
New World screwworm most commonly affects livestock, such as cattle, goats and sheep, but can also affect dogs, cats or wildlife. In cattle, infestations typically start at an existing wound and can worsen quickly without treatment, which is why early detection matters for ranchers and veterinarians. The larvae are not directly spread between animals or from animals to people. People are rarely affected by New World screwworm.
This illustration was designed to avoid graphic imagery. For a more detailed explanation of the infestation process, explore the CDC’s clinical overview of New World screwworm.
NWS is native to South America, Central America and the Caribbean, hence its “New World” name. The U.S. eradicated it by the mid-1960s using the sterile insect technique, and a biological barrier near the Panama-Colombia border kept it contained to South America for decades until the 2023 breach that’s driving the current outbreak in the southern U.S.
Disclaimer: The Arizona Department of Agriculture, Arizona Department of Health Services and Arizona’s local county and tribal public health departments are the regulatory and advisory authorities on all matters of animal and public health in the state. The Health Observatory supports their mission to protect Arizonans from emerging disease threats.
Current situation
Recent detections of New World screwworm have occurred in animals in Texas and New Mexico. As of July 2026, 33 animal cases have been documented by the U.S. Department of Agriculture. Most of these cases have involved cattle, goats and sheep, with two cases identified in dogs. The outbreak is ongoing.
State and federal agencies are actively working to contain the outbreak and prevent the spread of NWS. Most dogs and cats in the United States remain at low risk. To date, New World screwworm detections have not been reported in Arizona.
For additional detailed and up-to-date information, visit the USDA New World screwworm dashboard for official case data, or GenomicEpi.com, an independent tracker following the situation.
How can you protect yourself?
While the risk for New World screwworm is low for humans in the United States, there are many things you can do to protect yourself. First, keep open wounds clean and covered. If you live, work, travel to or spend time near livestock in areas where NWS flies are present, be sure to sleep indoors with closed windows. Sleep under a bed net or inside a tent if you have to sleep outside.
Be sure to protect your skin and prevent insect bites by using an EPA-registered insect repellent, treating clothing and gear with products containing 0.5% permethrin, and wearing loose-fitting, long-sleeved shirts and pants, hats and socks. These steps will not only prevent NWS flies from biting, but also other insects like mosquitoes that can carry diseases.
If you see or feel maggots on a wound or other area of your body, call your health care provider or local public health department immediately. Make sure to tell them if you have recently traveled to Central or South America, the Caribbean or Mexico.
Do not try to remove any maggots or egg masses yourself. If maggots or eggs fall out of a wound, place them in a leak-proof container with rubbing alcohol to kill them. Bring them with you to your health care provider. Do not throw any live maggots in the trash or outside as this could result in NWS spreading in your area.
Are pets at risk?
Most dogs and cats in the United States are currently at low risk. But, early detection and quick veterinary care are important to prevent severe infection.
If your pet has spent a lot of time outdoors, travelled to or been rescued from areas where the New World screwworm flies are, especially if they have an open wound, you can watch for changes in wounds or behavior.
Changes in a wound
Wounds that do not heal
Increasing redness or swelling
Foul odor or discharge coming from a wound
Tissue damage around an injury
Drainage from the ears, nose, or other body openings
Visible larvae (maggots)
Changes in behavior
Frequently licking or chewing at one area
Seems uncomfortable or irritated
Loses interest in food
Becomes unusually tired or lethargic
Suddenly become aggressive or sensitive to touch
Visit your local veterinarian immediately if you suspect a parasitic infestation.
All veterinarians and animal owners should immediately report potential NWS infestations to the Office of the State Veterinarian at [email protected] so specimens can be collected for prompt identification and investigation.
How can you protect your pets?
Prevention is key to ensure your pets do not become infested with New World screwworm flies and larvae. Pay close attention to pets that spend time outdoors. Check pets daily for the presence of potential wounds, including cuts and scrapes, sites of previous tick bites, or skin irritation.
Preventing injuries helps reduce opportunities for flies to lay eggs. Check your pet’s environment for items that could cause an injury that attracts female flies wounds, such as wire fencing, sharp vegetation, metal collars or chains.
Female flies are attracted to any open wound, including recent surgical incisions due to spay or neuter operations or tail docking. So, it is important to keep wounds clean and protected by applying antibiotic cream and clean dressings.
Maintaining flea and tick prevention year-round can help reduce bites and skin lesions that may attract flies. Talk with your veterinarian about the best parasite prevention products for your pet.
New World Screwworm: A One Health Issue
New World Screwworm can affect pets, livestock, wildlife, and people. Protecting animal health through surveillance, early detection, and veterinary care helps safeguard public health, agriculture, and ecosystems. Collaboration across veterinary, medical, wildlife, and public health sectors is essential to preventing the spread of this parasite.
Ask, answer, act
Got a question about New World screwworm? We’ve got a quick answer and the right place to go.
Answer: No. Most pets in the United States are currently considered at low risk, although animals with open wounds or recent exposure to affected areas may be more vulnerable.
Act: Watch for signs of infestation, especially if your pet spends time outdoors or has open wounds, recent surgical incisions, or skin injuries. Prompt veterinary care is important because early detection and treatment can prevent severe complications.
Answer: Yes, but human cases are rare and there have been no reports of NWS cases in humans acquired in the U.S. The parasite primarily affects animals.
Act: Visit the CDC website to learn more about risk factors for exposure to the flies and acquiring NWS.
Answer: No. It is spread by adult flies laying eggs in wounds rather than through direct
contact between animals.
Act: Learn how to prevent NWS flies from laying eggs in wounds on your pets and livestock.
Answer: The risk is lower, but any pet with an open wound could potentially be exposed if flies are present.
Act: For the latest official information, visit screwworm.gov and follow guidance from your veterinarian, state animal health officials, USDA and FDA.
Want to know more about New World screwworm?
Want to learn more about New World screwworm and its importance to human and animal health? These resources might offer more detail depending on what you’re looking for:
USDA Animal and Plant Health Inspection Service (APHIS)
the federal agency leading the U.S. response to NWS, including case reporting and prevention guidance:
American Veterinary Medical Association (AVMA)
veterinary guidance on recognizing and responding to NWS in animals.
American Kennel Club Canine Health Foundation (AKC CHF)
resources focused specifically on dogs and NWS risk.
University of Arizona: Cooperative Extension
guides for pet owners, veterinarians and animal shelter personnel to protect animals
Arizona Department of Agriculture
Arizona-specific updates and guidance.
CIDRAP: How to Protect Outdoor Pets from New World Screwworm
guidance on protecting outdoor pets, including wound checks and prevention tips.
USDA NWS Factsheet for Local Veterinarians
Tufts University: NWS Q&A with Dr. Isabelle Louge
an accessible expert Q&A on the outbreak