On October 28, 2026, the USDA confirmed the first official detection of New World screwworm in Arizona. This is now the third state to report the parasite, after Texas and New Mexico. So far this year, more than 70 screwworm cases have been confirmed in the U.S. in dogs, cats, horses, cattle, sheep, goats, and elk.
Officials say the current risk to animals in the United States remains low. But with confirmed cases in several southern states, including our neighbor Arizona, it’s important to understand the situation and potential risk change. Horse owners, especially those who travel with their animals, should pay particular attention.
What Is New World Screwworm?
New World screwworm (Cochliomyia hominivorax) is a species of parasitic fly whose larvae feed on the living tissue of mamals and birds. Unlike maggots that feed on decaying material, screwworm larvae burrow (screw) into healthy flesh, which can cause serious and potentially fatal wounds. Any mammal, even humans, can become infested.
The adult flies are about the size of a common housefly. Female flies are attracted to open wounds and body openings, where they deposit their eggs. A small scratch, insect bite, or tick wound can provide an opportunity, but the larva can also burrow into healthy soft tissues around the eyes, mouth, and genitalia.
New World screwworm is endemic in Cuba, the Dominican Republic, and countries in South America. It was eradicated from the United States in the 1960s, thanks largely to a program that released sterile male flies to interrupt reproduction. However, the parasite began spreading northward again and, in 2026, it reached the United States.
The Screwworm Life Cycle
The New World screwworm has the potential to spread rapidly. Understanding its life cycle can help explain why early detection is so important.
- Egg laying: A female fly deposits a cluster of approximately 200–300 eggs along the edges of a wound or near a body opening.
- Hatching: Within approximately 12–24 hours, the eggs hatch into larvae.
- Feeding: The larvae burrow into living tissue and feed for roughly a week. As they grow, they enlarge and deepen the wound, creating an increasingly serious infestation.
- Pupation: Mature larvae drop from the animal and burrow into the soil, where they develop into pupae.
- Adult emergence: New adult flies emerge from the soil, mate, and continue the cycle.
A single female can produce thousands of eggs during her lifetime. Left untreated, infestations can rapidly cause severe tissue damage, secondary infections, and death.
Signs and Symptoms in Horses
Early infestations can be difficult to recognize, particularly if a horse has a small wound hidden beneath its hair or in a hard-to-see area. Watch for:
- Wounds that worsen quickly, becoming larger, deeper, or more inflamed instead of healing.
- Unusual discharge or bleeding, sometimes accompanied by a foul odor.
- Visible egg masses or maggots in or around a wound.
- Pain or behavioral changes, including irritability, sensitivity to touch, reduced appetite, or lethargy.
- Lesions around body openings, particularly the eyes, ears, nostrils, mouth, and genital area.
Because screwworm larvae feed on living tissue, an infestation can cause much more damage than an ordinary superficial wound.
It’s important to remember that not every wound containing maggots is caused by New World screwworm. Other fly species can cause similar-looking infestations, and laboratory testing may be necessary to confirm the species. If you suspect screwworm, contact your veterinarian immediately. Do not wait to see whether the wound improves.
How to Reduce Your Horse’s Risk
At this time, there is no reason for Nevada horse owners to panic. But good wound care and routine inspections are especially important for horses who have been to areas where screwworm has been detected.
Don’t overlook other animals! Remember, New World screwworm can infect any mammal or bird. The most recent case in Arizona is a dog. If you travel with dogs or keep multiple species on your property, be sure include them in your routine inspections.
What Happens if a Horse Becomes Infested?
New World screwworm infestations require prompt veterinary treatment. Treatment generally involves removing the larvae and thoroughly cleaning and managing the wound. Veterinarians will also administer medications. Additional treatment may be necessary for secondary infections or extensive tissue damage.
The USDA and state animal-health agencies also need to know about suspected cases so they can investigate and help prevent further spread. If you discover a suspicious infestation, contact your veterinarian immediately and avoid moving the animal until you receive guidance. Your veterinarian can coordinate testing and reporting with animal-health officials.
Where Has New World Screwworm Been Detected?
New World screwworm has been confirmed in three states, including the most recently reported case in Cochise County, Arizona, on October 28, 2026. The USDA and state agencies say they are responding with surveillance, animal movement controls in affected areas, and releases of sterile flies. Because this situation is evolving, we recommend checking official sources rather than relying on social media reports.
Stay Up to Date
USDA New World Screwworm Current Status: https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/current-status
- The best starting point for current detections in the United States, Mexico, and Central America, including links to the U.S. detection dashboard.
USDA Confirmed Detections Dashboard: https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/current-status/confirmed
- Track confirmed animal cases and wild fly detections by location and date.
Equine Disease Communication Center (EDCC): https://equinediseasecc.org/ALERTS
- Timely alerts about infectious and emerging equine diseases across the United States.
USDA Reporting Information: https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/report-suspected-cases-screwworm
- Learn how to report suspected infestations and find the appropriate animal-health contacts.
CDC New World Screwworm Information: https://www.cdc.gov/new-world-screwworm/index.html
- Information about the parasite, its life cycle, symptoms, and human health considerations.
Again, there is no reason for Nevada horse owners to panic. Continue practicing good wound care, check your animals regularly, and stay informed about developments in neighboring states. If you do travel with your horse to an impacted state, take a few minutes to review the latest detection information and travel restrictions before heading out.
This article is not a substitute for veterinary advice. Contact your veterinarian immediately if you suspect a New World screwworm infestation. Information is current as of October 29, 2026 and we will update this page as updates become available.
