MSU Extension Animal Agriculture

What is NWS and how do you identify it?

New World Screwworm (NWS) is a pest that infests warm-blooded animals, including livestock, pets, wildlife and, less commonly, people and birds.

NWS flies may travel 6 to 15 miles to find a host and tend to travel along water. The ideal environment for survival and activity of NWS is between 77 and 86 degrees Fahrenheit, with a relative humidity of 30 to 70%. Temperatures consistently below 46 F typically kill the pupae (intermediate stage between larvae and adult fly).

Adult female NWS flies are attracted to and lay their eggs around or in open wounds (as small as a mosquito or tick bite) or mucous membranes (i.e., ears, nose, eyes, mouth or genitals). As these NWS eggs hatch 12 to 24 hours later, they can lead to a deadly condition if not treated, called myiasis (also known as flystrike). Myiasis is the parasitic infestation of fly larvae (maggots) that hatch from these NWS eggs and burrow into and feed on the living tissue of its host.

NWS larvae are difficult to see until three days after hatching and are typically seen by slight motion within the wound. After the NWS larvae feed through two molts (5 to 7 days), the larvae leave the wound and fall to the ground to pupate. Adult NWS flies emerge and feed on vegetation such as flowers or wounds of decomposing tissue.

Adult NWS flies have orange eyes, a metallic blue/green body and three dark strips along their back.  Male NWS flies may begin mating within 24 hours, while females mate within 3 to 5 days after emerging. Female NWS flies typically mate once and lay one to two batches of eggs. Adult NWS flies live up to 14 days for males and 10 days most commonly for females but may live longer up to 30 days.

Current Status of New World Screwworm (April 2026)

As of April 2026, the New World Screwworm (NWS) is not present in the U.S., however it remains a critical threat to North American agriculture. This pest has seen a significant resurgence in Central America and Mexico over the last three years, prompting mitigation practices being put in place to help control the spread and hopefully keep NWS from entering the U.S. Currently, active cases have been confirmed in the Mexican state of Nuevo León, approximately 60 to 90 miles from the Texas border.

The USDA Animal and Plant Health Inspection Service (APHIS) has launched an aggressive, multi-pronged mitigation program to prevent NWS from crossing the border. The primary offensive measure utilizes the Sterile Insect Technique (SIT). The USDA is releasing millions of sterile male flies to disrupt the pest’s breeding cycle. In January 2026, the USDA began dispersing sterile flies up to 50 miles into South Texas as a preemptive strike against the possible occurrence of NWS. The USDA has approved a sterile fly production facility at Moore Air Base in Edinburg, Texas. This facility is designed to produce up to 300 million flies per week. Continuous surveillance and trapping of NWS is currently being conducted with over 8,000 traps along the southern U.S. border.

Focused on preparing for the introduction of this pest to the U.S., the USDA released an updated NWS Response Playbook in April 2026. This guide standardizes how federal and state agencies will react if a single fly is detected, focusing on rapid quarantine and continuity of business for farmers.

To prevent accidental introduction of the parasite, the USDA has implemented strict trade and animal movement protocols from countries with identified infections. This includes border closures at livestock ports in Texas near active Mexican locations to prevent the entry of infested animals. Any animals coming from NWS-affected regions must have pre-movement inspections, sometimes requiring preventative treatment with antiparasitic sprays. If NWS is detected in the U.S., the USDA will implement immediate quarantine zones. Animals within these zones will require animal health certificates and official inspections of all open wounds before they can be moved. Additionally, to prepare for an NWS infection in the U.S., the FDA has issued Emergency Use Authorizations (EUAs) for new treatments, including a topical spray for dairy cows with a short 10-day milk withdrawal time, specifically to keep trade and production moving during an outbreak.

Farmers should remain vigilant. If you notice unusual maggots in a living animal's wound, do not move the animal. Contact your state veterinarian or the USDA immediately. Detection and containment are the only ways to prevent a billion-dollar hit to the livestock industry.

Can New World Screwworm infestations be prevented and treated?

Yes, an NWS infestation can be prevented and treated. If NWS is detected in the U.S., animal owners of affected areas are recommended to implement strategies to prevent infestation, such as:

• Frequently inspect animals for signs of NWS infestation and wounds.
• Treat wounds with approved insecticides immediately upon discovery.
• Check for sharp objects or pieces in pens and equipment used around animals to avoid skin injuries.
• Apply approved insecticides to the umbilical cord of newborns immediately after birth.
• Implement measures to control external parasites. For more information, check out this article on controlling external parasites in cattle.

Additionally, the FDA has a list of approved products for both prevention and treatment.

• Dectomax-CA1 (doramectin): injectable solution for prevention and treatment in cattle.
• Exzolt Cattle-CA1 (fluralaner): topical solution for prevention and treatment in cattle.
• Ivomec (ivermectin): injectable solution for prevention in cattle.
• F10 Antiseptic Wound Spray with Insecticide (benzalkonium chloride, polyhexanide, and cypermethrin): topical solution for prevention and treatment in cattle, horses, sheep, goats, deer, raptors and other wild birds, pet bird and captive wild, exotic and zoo mammals.

Before using any of these products, review their information on the FDA website or consult with your veterinarian. Always follow your veterinarian’s recommendations when treating an animal infested with NWS.

How to report:

If you identify any suspicious wounds, maggots, or infestations, contact the State Veterinarian at the Michigan Department of Agriculture and Rural Development at 1-800-292-3939, or for after-hours emergencies, call 517-373-0440 or 412-847-2255.

If you believe that you may have screwworm, contact your primary health care provider.

Where can I get more information:

More information for producers, veterinarians, and health care professionals can be found by visiting the United States Department of Agriculture Stop Screwworm: Unified Government Response to Protect the United States website.