AABP Executive Director Dr. Fred Gingrich is joined by Dr. Chris Ashworth, Global Veterinarian for Zinpro Performance Minerals and Dr. Dave Sjeklocha, technical services veterinarian for Merck Animal Health to discuss New World Screwworm (NWS). Recently, the United States Animal Health Association and National Institute of Animal Agriculture hosted an NWS symposium in Kansas City which was sponsored by the American Veterinary Medical Foundation, Merck Animal Health and Elanco. Both of our guests attended and are providing an update from that symposium.

NWS was recently discovered in Nuevo Leon, Mexico which is 70 miles south of the U.S.-Mexico border. The infected animal was a feedlot steer that was detected with larvae upon arrival at the feedlot. Mexican authorities have restrictions on animal movements both at the point of origin and arrival.

The life cycle of the NWS fly is about 3 weeks. Fertilized eggs are laid in packets of 200-350 eggs that hatch in 12-21 hours with the emergence of larvae within 3 days. Two important aspects of the life cycle are that female flies mate and lay eggs once, then die and they lay eggs in fresh wounds, not decaying tissue like other fly strikes. The larvae burrow deep into wounds and cause significant destruction which can result in the animal dying of septicemia in 1-2 weeks. It is important to inspect for larvae and remove all of them, placing them in alcohol to kill them. 

The USDA continues to work with Mexican authorities to protect the U.S. from this fly. Releasing sterile male flies works based on history of control of this pest, but the current production rate of 110 million per week is not enough to control the areas that have the fly. It is also important to remember that half of the sterile flies are females, although scientists are working to try to produce only male flies to increase production. The U.S. is also constructing a facility in Texas, but the timeline for completion to release of flies is 2-3 years.

The most important thing veterinarians can do is to inspect animals aggressively. Pay special attention to any wounds – superficial wounds or scrapes, foot rot lesions, open umbilical cords, wounds in the vulva from calving, castration wounds or dehorning sites. Veterinarians and producers should remain hypervigilant and report any suspicious wounds or larvae and contact their state animal health official. Treatment involves the use of avermectin products and doramectin has received emergency use authorization for NWS by FDA. Fly control and wound care is also very important for control of this infestation.

AABP members can also view the presentation from Dr. Bud Dinges at the 2025 AABP Annual Conference in Omaha by going to the NWS page on the AABP website (https://aabp.org) as well as other NWS resources.