34 NEBRASKA CATTLEMAN April/May 2026 PRODUCTION New World Screwworm Implications for Nebraska Producers MATT HILLE, DVM, PH.D., DACVP | VETERINARY DIAGNOSTIC PATHOLOGIST AND EXTENSION VETERINARIAN, UNIVERSITY OF NEBRASKA-LINCOLN In recent years, confirmed cases of New World screwworm (NWS) in cattle have spread from Panama northward to within approximately 100 miles of the southern U.S. border. The threat of NWS is not new to the United States, although it may feel that way to some since it has been 60 years since the fly was officially eradicated. Thankfully, the successful eradication campaign carried out in the 1960s will serve as the blueprint for managing the current threat. NWS Biology and Disease Risk in Cattle Adult NWS flies lay eggs in open wounds or on mucous membranes of warm-blooded animals. Livestock and wildlife are most commonly affected, but pets, birds and occasionally humans can also serve as hosts. After hatching, larvae (i.e., maggots) feed on living tissue of a susceptible host. Once mature, the larvae drop to the ground to pupate and eventually emerge as adult flies. Under ideal conditions (around 80°F), the insect’s lifecycle can be completed in a few as 21 days. Female flies mate only once but may lay eggs two or three times in her life, depositing several hundred eggs per batch, allowing for rapid population growth. Reproduction can occur between 65°F and 91°F, though the lifecycle is prolonged outside the optimal temperature. Sustained soil temperatures of 46°F or cooler are lethal to larvae and pupae. For Nebraska, this suggests reproduction could occur from late spring through early fall, but successful overwintering of the fly population would be extremely unlikely. The larval stage causes disease and represents the greatest concern for livestock. Feeding larvae will burrow deep into tissue – hence the name “screwworm,” which can lead to extensive tissue destruction and allow for secondary bacterial infections, septicemia and even death. Mortalities from NWS are not uncommon, especially in untreated cases. Even animals that survive may suffer reduced performance leading to indirect economic losses. The umbilicus of newborn calves is a common site for NWS to lay eggs, but even much smaller wounds from syringe needles, implant guns or tick bites can suffice. Eradication: Then and Now In the 1960s, male flies were mass reared, sterilized and released into and adjacent to infested areas, a strategy termed sterile insect technique (SIT). Because female flies mate only once, those that mated with sterile males produce no viable offspring, although they will still lay eggs. Over time, the relentless release of hundreds of millions of sterile flies eliminated the wild population in the United States. Strict control of livestock movement from affected areas and visual animal inspection played a large role in limiting geographic spread, as the transport of infested animals to unaffected areas poses significant risk. Surveillance for adult flies using traps and/or bait was also a critical component to eradication. Early detection of adult flies in areas previously unaffected allowed for the targeted release of sterile flies before widespread infestation of livestock. In many ways, today’s response efforts mirror the 1960s campaign. However, the current sterile fly production capacity is approximately 100 million flies per week – less than 20 percent of what officials estimate to be the current need. The U.S. Department of Agriculture has unveiled plans to expand production and distribution, but a meaningful increase in the number of available sterile flies is not expected until 2027. To reduce the risk of importing NWS, U.S. Secretary of Agriculture Brooke Rollins closed the southern U.S. border to animal movement in May 2025, and it remains closed. Officials in Mexico and the southern U.S. states have expanded surveillance to guide strategic sterile fly placement, something particularly important given the current limited supply. To date, there have been no confirmed U.S. cases of NWS identified from the outbreak in Central America, although the sterile fly release perimeter was recently expanded to include parts of extreme southern Texas. Bottom Line for Nebraska Producers • Historically proven eradication methods are being pursued to combat the NWS threat, including sterile fly production and release as well as enhanced surveillance programs. • Nebraska’s climate could support NWS reproduction during the warmer months, but NWS overwintering is not a realistic threat. • For Nebraska, importing infested animals currently presents the greatest risk and strict adherence to animal movement regulations is essential. • Suspected cases of NWS in any species should be reported immediately to your veterinarian or state/ federal officials. A key sign to look for are larvae feeding on living tissue. Note that larvae feeding on a dead carcass is not suggestive of NWS. • Additional resources containing more detailed information on the history, current outbreak status, clinical signs, and prevention or treatment of NWS can be found by scanning the QR codes or visiting these organization’s websites: • U.S. Department of Agriculture • COPEG • Nebraska Department of Agriculture • Texas Animal Health Commission ~NC~
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