September 2025 NEBRASKA CATTLEMAN 17 could slow down new drug approvals or indications for screwworm. For guidance on how to prevent and treat a potential NWS infestation, refer to www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm for official resources. How to Prepare As pupae, NWS cannot survive soil temperatures that are consistently below 46 degrees. For perspective, last year, soil temperatures stayed above 46 degrees from mid-April to late-October in Pullman, Wash.; Grand Island, Neb.; Minot, N.D.; and Brookville, Penn. From spring to the early fall, there was almost no part of the United States that was completely inhospitable to this pest – that means all American producers need to be prepared and stay vigilant for screwworm. When screwworm was endemic in the United States, the response for farmers and ranchers was time and labor intensive. Start planning now, especially if you are in a border state, on how you will effectively monitor your herd for signs of screwworm. Familiarize yourself with the signs of a screwworm infection and talk to your vet about treatment options. Closely watch your state animal health offices and departments of agriculture for information on any emergency response plans they might be updating. If you suspect a screwworm case, report it to your state veterinarian for proper testing and confirmation. Sharing unverified information online can cause unnecessary alarm and negatively impact the livestock industry. Accurate reporting helps protect both animal health and our agricultural community. Additional information and resources are also available at ncba.org. ~NC~ NEBRASKA DEVELOPS NWS RESPONSE PLAN Kaylie Fritts, DVM | Deputy State Veterinarian, Nebraska Department of Agriculture I feel it’s necessary to qualify the following article with this statement: New World screwworm has not been identified in the United States as of July 25, 2025. New World screwworm (NWS) is a species of fly that develops from larvae (maggots) that feed on live flesh. This differs from the common maggot that feeds on dead flesh. Infestation with NWS larvae – often called myiasis – is a painful and sometimes debilitating condition leading to production losses in food animals. Notably, NWS infestation can also affect many other species, including dogs, cats, humans and birds. The life cycle of the NWS is about three weeks long and begins when a female fly and male fly mate; a female fly will only mate once in her life. She is then attracted to blood from an open wound as small as an injection site. The female fly will lay her eggs next to an open wound; the larvae hatched from those eggs will begin feeding on live flesh. After about a week, those larvae drop into the soil to pupate and will eventually emerge as adult flies. Fortunately, we have several tools that allow us to interrupt the NWS lifecycle. 1. The United States has the capability to produce sterile male NWS flies. Since the female only mates once in her life, if a female mates with a sterile male, her eggs will not be viable. This is known as sterile insect technique (SIT) and the United States has been employing this control tactic since the middle of the 20th century. 2. If the eggs are viable and reach the larval stage, ivermectin is highly effective in treating and preventing active infestations (off-label use). 3. The adult NWS fly prefers temperatures around 77 degrees and starts to slow down significantly below temperatures of 70 degrees. Similarly, the pupae cannot continue their life cycles in sustained soil temperatures below 46 degrees. The climate in Nebraska prevents the overwintering of this insect. The Nebraska Department of Agriculture (NDA) has been working diligently to develop practical response plans for the possible incursion of NWS into the United States. These plans involve brief movement restrictions from affected states, prophylactic treatment of incoming animals and different CVI requirements for animals moving into Nebraska. If there is an NWS detection in Nebraska, we will implement control zones, quarantine the affected site, prioritize treatment and isolation of animals on the affected premises and initiate robust outreach protocols to local producers, veterinarians, physicians and constituents. It’s important to note that depopulation is not a part of USDA’s response plans for NWS, and indemnity will not be offered for sick or dead animals. As the NWS continues to move northward, the likelihood of its incursion into the United States increases. Regardless of the outcome, NDA will continue to prioritize animal health and continuity of commerce. New World screwworm infestation in a horse’s leg. Photo by Dr. Feliciano Bravo, Commission for the Eradication and Prevention of Screwworm (COPEG). Eartag wound infested with New World screwworm. Photo by Dr. Feliciano Bravo, COPEG.
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