The New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae feed on the living tissue of livestock, pets, wildlife and, in rare cases, humans. Though eradicated from the United States in 1966, recent outbreaks in Mexico and Central America have raised concerns about its potential reintroduction into the U.S., prompting heightened vigilance from the U.S. Department of Agriculture and the Animal and Plant Health Inspection Service. APHIS is particularly alarmed by the increased volume of screwworm cases in southern Mexico. The risk of the parasite entering the U.S. increases through the movement of infested animals or wildlife. To mitigate this threat, APHIS has suspended imports of live cattle, horses and bison from Mexico and allocated emergency funding to increase surveillance and control efforts.

The New World screwworm (NWS) fly lays eggs in open wounds or mucous membranes of animals. Upon hatching, the larvae burrow into the flesh, causing severe tissue damage, secondary infections and, if untreated, death within seven to 10 days. Infestations can lead to significant economic losses in the livestock industry due to decreased meat and milk production, increased veterinary costs and animal deaths.

As of May 2025, the U.S. has suspended imports of live cattle, horses and bison from Mexico due to screwworm concerns. Before traveling, ensure that horses are free of open wounds, are inspected by a veterinarian, and have a valid health certificate and current Coggins test as interstate travel inspections are increasing, especially when traveling through Texas. Use extreme caution when purchasing horses from sale barns or from other states. Upon return, monitor horses for signs of infestation for at least two weeks.

NWS will burrow into the flesh of the animal, causing extensive damage. This damage is amplified when other larvae hatch and begin burrowing. This action will create nonhealing wounds with foul odor. Horse owners may notice the presence of maggots in wounds or body openings, behavioral changes such as depression, weight loss, reduced appetite, fever, irritability and head shaking.

The NWS fly can travel up to 12 miles to lay eggs. Eggs can also be transported by animals and people traveling from infested areas. To reduce the potential damage caused by the NWS, horse owners should increase fly control methods such as using insecticides and fly traps and reducing waste and standing water in the area. Conduct routine checks for wounds or signs of infestation. Clean and cover any wounds promptly. If you suspect your horse has a wound caused by the NWS, contact your veterinarian immediately. The Louisiana Department of Agriculture and Forestry and APHIS must be notified immediately if a New World screwworm infestation is confirmed. Your veterinarian will create a treatment protocol that includes quarantine, removal of the larvae, wound treatment and prevention of secondary infections. While livestock are the primary hosts, screwworm can also infest pets and humans. In humans, infestation occurs when flies lay eggs in open wounds, leading to painful lesions and potential complications if untreated. Pets, particularly those with outdoor exposure, are susceptible and should be monitored for signs of infestation.

The resurgence of the New World screwworm in regions close to the U.S. border underscores the importance of vigilance among horse owners, veterinarians and livestock producers. Adhering to preventive measures, recognizing early symptoms and ensuring prompt reporting and treatment are crucial steps in safeguarding animal and public health against this devastating parasite.

Adobe stock photo