The parasitic screwworm fly has surfaced in the U.S. Southwest in June for the first time since the 1970s, raising alarms among veterinarians and livestock managers. The insect's larvae feed on living flesh and can kill an infested animal if the infestation is not discovered and treated promptly. Authorities say the renewed presence poses risks to livestock, wild animals and household pets, particularly in areas near the U.S.-Mexico border.
Veterinarians emphasize several basic facts about screwworms and what pet owners should do to protect animals under their care.
What are screwworms?
Female screwworm flies deposit eggs into wounds on any warm-blooded host. After hatching, the larvae - known for their sharp mouthparts - consume living tissue. Hundreds of larvae may be present at a wound site, using their feeding activity to tunnel into flesh. Infested animals may die if the infestation progresses without veterinary care.
Experts note that screwworms typically spread through movement of infested animals rather than via other means. The parasite does not present a food safety threat and only rarely affects humans. When discovered early, treatment that removes the maggots, cleans the wound and includes antibiotics generally results in recovery for the animal.
The recent northward advance of screwworm from areas where it is endemic in Central America and Mexico has already produced at least one documented case in the United States: a dog in New Mexico was found infested in June. That case underscores that household pets in the U.S. Southwest are at elevated risk following the pest’s movement across the border.
Recognizing symptoms
Pet owners should examine animals for wounds that are swollen, produce pus or simply will not heal. Behavioral signs of pain may include excessive licking or chewing of a particular area, lethargy and a decreased appetite. One of the most distinctive indicators of a screwworm infestation is a foul odor associated with rotting flesh.
Veterinarians advise owners to bring animals to a clinic if they notice these signs, even when maggots or eggs are not clearly visible. Stray cats and dogs may be particularly vulnerable to infection and could act as vectors, transferring the parasite to household animals. Residents who observe possible screwworm in strays are urged to contact local animal control.
"The consequences of missing one or two cases is so great that if the public sees anything suspicious, take it into your vet," said Dr. Pancho Hubert, a veterinarian in Corpus Christi, Texas, and the president of the Texas Veterinary Medical Association. "It might not be anything, but these wounds carry so much significance."
Prevention options for pet owners
Some monthly parasite-control medications commonly used to prevent ticks, fleas and other parasites may also offer protection against screwworm. These chewable products, including Credelio and Simparica, are absorbed into an animal’s bloodstream and can kill parasites that feed on the host. Pet owners should consult their veterinarians about whether existing parasite prevention regimens could help reduce screwworm risk.
Treatment protocol
When a veterinarian suspects screwworm, they will examine the wound, mechanically remove larvae and eggs, and submit samples to animal health authorities for confirmation. After removal and collection of diagnostic material, the wound is disinfected and the animal is typically given an oral insecticide. Vets will also likely prescribe pain relief and antibiotics to address secondary infections.
The U.S. Department of Agriculture has authorized roughly a dozen drugs for use against screwworm infestations. Experts caution that pet owners should not attempt to treat screwworm themselves, since missed larvae can continue to burrow into tissue and discarded maggots might mature into flies that perpetuate spread.
Prognosis and recovery
When detected early and treated appropriately, animals usually make a full recovery. Conversely, untreated screwworm infestations are fatal. The need for prompt veterinary attention is therefore central to preventing severe outcomes.
Federal and local response
The U.S. Department of Agriculture has taken several steps to confront the outbreak. Actions cited by officials include investments in the production and research of sterile flies designed to interrupt screwworm reproduction, ramped-up surveillance, and movement restrictions intended to limit spread.
Although two new sterile fly production plants are under construction, current output falls well short of what experts say is necessary to suppress the outbreak. In addition, a reported decline in staffing at the USDA’s animal health unit has reduced the agency’s specialist capacity to respond to animal disease events. Sources reported that hundreds of veterinarians, support staff and laboratory workers departed the agency after the previous administration pushed for resignations, leaving fewer specialists available to address emerging outbreaks and prompting concerns about preparedness.
Agriculture Secretary Brooke Rollins has said screwworm was forecast to cross into the United States by last year, and that the previous administration’s actions delayed that movement, allowing the USDA additional time to mount a rapid response.
For pet owners, the core message is vigilance: check animals regularly for wounds that will not heal, seek veterinary care promptly if signs or symptoms appear, keep an eye on stray animals that might spread the parasite, and follow veterinary advice on preventive parasite control. For livestock managers and public authorities, intensified surveillance, movement controls and increased sterile fly production are central elements of the containment effort.