New World Screwworms

Current Background

• On June 3, 2026, New World screwworm (NWS) was confirmed in a calf in Zavala County, Texas. The calf has since recovered from the infestation and this individual animal case is now considered inactive.
• Federal and State partners work jointly in the infested zone to conduct animal and fly surveillance, epidemiological investigations, and implement existing response protocols.
• NWS was last eradicated from the United States in 1966 using the sterile insect technique. Currently, sterile fly dispersal efforts, including ground release and aerial dispersals, to eradicate NWS are underway in affected areas.
• In Central America, cases of NWS have quickly increased since 2023, expanding to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador.
• In November 2024, NWS was detected in a cow at an inspection checkpoint in Chiapas, Mexico. Progressive Northern spread of NWS has been confirmed in Mexico since the original detection.
• The USDA's Food Safety and Inspection Service (FSIS) is the regulatory agency responsible for ensuring the nation's commercial supply of meat, poultry, and egg products is safe and properly labeled. Any evidence of screwworm infestation would be identified during FSIS inspection processes, and adulterated product derived from the affected animal would not be allowed to go into commerce.
• Livestock and pet owners can help avoid infestations of NWS by staying vigilant, monitoring animals, and reporting any suspicions of NWS flies, maggots, larvae, and secondary infections.

New World screwworms (NWS) are larvae or maggots of the NWS fly (Cochliomyia hominivorax), that cause the painful condition NWS myiasis. NWS flies lay eggs in open wounds or orifices of live tissue. These eggs hatch into dangerous parasitic larvae, and the maggots burrow or screw into flesh with sharp mouth hooks. The wound can become larger, and an infestation can often cause serious, deadly damage. NWS primarily infest livestock, but can also affect mammals, including humans, and birds.

There are several flies associated with wounds, but only NWS feed on living tissues, compared to dead tissues and fluids. The identification of NWS is done by larvae collection and evaluation. NWS larvae have a series of backwardly protruding spines around a tapered body, giving a screw-like appearance, helping to identify the pest.

Eradication Method

Eradication of NWS infestations is accomplished through the sterile insect technique. Sterilized male flies are released to mate in an area with an established NWS population. Females, only mating once in their lifetime, then lay nonviable eggs. The population decreases without the addition of new larvae and dies off naturally over a few lifecycles.

In combination with producer management practices, effective and efficient treatment of wounds, and diligent response tactics, the sterile insect technique is a proven tool for fighting the spread of NWS. According to the USDA, the release of sterile flies is safe and poses no risk to wildlife, livestock, or people in infested areas.

Monitor

Look at your animals closely and often. Diligent monitoring of animals is key to preventing infestation and avoiding large impacts.

• Check livestock and companion animals for evidence of NWS infestations, including flies, maggots, larvae, or eggs.
• Monitor body openings such as the nose, ears, umbilicus, or genitalia for drainage, foul odors, or enlargement.
• Observe animals for clinical signs of NWS myiasis and secondary infections.

View the USDA NWS Best Practices for Evaluation of Animals guidance.

Prevent

Avoid introduction of NWS to animals to prevent infestations.

• Keep open wounds clean and covered.
• Work with a veterinarian to establish a veterinary-client relationship for effective preventative treatments.
• When traveling, especially in NWS-infested areas, ensure pets and vehicles are inspected for NWS flies and larvae.

Treat

Animals impacted with NWS myiasis should be treated immediately. A veterinary-client relationship is critical to administering effective treatments. Treatments may vary and should be evaluated using current recommendations for specific species and areas affected.

Treatment guidance should be discussed with a veterinary practitioner. Learn more about FDA and EPA approved preventions and treatments using the Animal Drugs for New World Screwworms page.

This is a highly preventable and treatable condition using approved products and animal care. However, left untreated, animals can die within one week of infestation.

Learn

Resources for NWS prevention, treatment, and general information is available for livestock producers, pet owners, and the general public.

Discover NWS video resources in the TAHC YouTube playlist.