Watch continues for New World screwworm

Although it has not made its way to the Northeast, New World screwworm (NWS) has made inroads in the U.S. with 15 confirmed cases as of June 24. The Northeast Dairy Producers Association and New York Farm Bureau recently presented a webinar on the parasite to help keep producers vigilant. Panelists included Rob Lynch, DVM, dairy herd health and management specialist, PRO-DAIRY Animal Science; Joy Bennett, DVM, NYS Department of Ag & Markets; Michael Capel, DVM, Perry Veterinary Clinic; and Carrie Telgen, DVM, Diamond V. “New World screwworm is not in New York State and is not a food safety issue,” Lynch said. “It is in the U.S. but far away from us.” According to USDA, all of the active cases – those still involving ongoing disease mitigation efforts – and resolved, inactive cases for the past 30 days are in Texas and New Mexico, including 10 cattle, one dog, two goats and two sheep. The NWS fly is about the size of a standard housefly. It lays eggs in broken skin and the subsequent larvae burrow into the wound. Unchecked, the parasite infects any warm-blooded creatures including livestock, pets, wildlife, birds and people. NWS was eradicated in the U.S. in 1966. “The maggot infestation is usually at the surface but this burrows into living tissue,” Telgen explained. “We have one case with a dog. It can affect people and birds but it is more common in wildlife and livestock.” Once female flies lay eggs in a wound or orifice, they die. In addition to accidental wounds, the opening could be umbilical or from open work castration, ear tags or dehorning. Within 12 to 24 hours, the eggs hatch and in five to seven days, larvae appear. “That’s what does the damage as they burrow down into the skin,” Telgen said. “They can drop off into the soil to pupate.” In six to eight days, the pupae are adults and ready to mate. One of the reasons it’s unlikely to become a widespread issue in the north is that the parasite cannot overwinter. Ideal conditions for NWS are between 77º and 86º F with relative humidity of 30% to 70%. “This has been endemic in Cuba, Haiti, Dominican Republic and South American countries,” Telgen said. “We’ve been watching this march up Mexico for quite some time.” She advised producers to look for lethargy, draining or enlarging wounds, maggots or egg masses, foul odor near a wound or orifice and discomfort, irritability and head shaking. Foot rot or cuts on the feet can be unexpected places for NWS to strike. The sterile fly technique proved the most effective means for USDA to eradicate the pest in the past. This involves releasing irradiated, sterile males that mate with females – but their eggs will not hatch and the females die, as they can breed only once. This causes NWS numbers to plummet. Sterile fly production locations include Pacora, Panama; Metapa, Mexico; and Moore Air Base in Texas. Sterile fly dispersal sites are Tuxtla, Mexico; Tampico, Mexico; and Moore Air Base. “USDA is working to increase sterile fly production by building new facilities and developing techniques to produce only male flies, effectively doubling our production,” Capel said. “The current production is 100 million flies per week with a goal of 300 million per week in 2027, which is the target for control.” Capel advised producers to monitor their cattle for maggots. “This bug is not here,” he said. “This bug is miles, miles and miles away from us but we thought it prudent to have some conversation in New York. Be educated and put a plan in place but don’t overreact.” Any producers who observe maggots in living tissue should contact their local veterinarian who may choose to contact a state veterinarian and then the USDA area veterinarian in charge. Capel said, “The best thing we could do is be vigilant for it. Our biggest risk is species traveling from infected areas or areas that might be infected in the future. If you see a maggot infestation that doesn’t look superficial, contact your local veterinarian.” In the meantime, it’s important to clean wounds. Remove and destroy maggots. Dropping them onto the ground is ill-advised as they pupate in the soil. Topical larvicides or injectable larvicides and systemic antibiotics and pain management may be recommended. Veterinarians can provide guidance as to treatment and prevention protocols with FDA-approved medications, since “prior to two years ago, there were no drugs approved to treat screwworm. We had eradicated it,” Capel said. He warned that overreacting by stockpiling drugs or giving drugs unnecessarily can increase the risk of the spread of NWS because those who need the drugs won’t have them and it can risk generating resistance. Bennett noted that one case in 2025 in Maryland was confirmed and contained. She outlined the “New World Screwworm Order” protocols, such as not allowing domestic animals infested with NWS to enter the state, and those suspected should first be examined by an accredited veterinarian. Animals coming from an infested area or an area under investigation should not enter the state until declared free of NWS by the originating state’s animal health authority. “Domestic animals originating from an infested state – currently Texas and New Mexico – must have a Certificate of Veterinary Inspection within five days prior to entry,” Bennett said. “The CVI must include a statement from the accredited vet that the animals were inspected and found to be free of NWS.” The prohibition includes dogs and cats but hatching eggs and day-old chicks are exempt. The concern about importing animals from infected areas is that it promotes a faster spread of NWS and can create more hotspots requiring treatment with sterile males. Capel explained that flies can travel only two to five miles daily but with available hosts, they can go much farther. An infested zone is at least 12.4 miles beyond the perimeter of an infested premises. The Texas Department of Agriculture offers an interactive map on its website. “There are some things we can do from a prevention standpoint, from management, that can reduce risk factors associated with NWS,” Lynch said. “Look at those animals daily.” Reduce the risk of animal injuries with safe handling practices and animal facilities. Keep up the farm to reduce fly habitat. by Deborah Jeanne Sergeant