The New World screwworm (NWS), a parasitic fly that had been eliminated from the United States for 60 years before reappearing in Texas earlier this month, has traditionally been considered a threat to livestock and wild animals.
But a report of screwworm in a New Mexico dog last week highlights the insect’s danger to pets.
A screwworm infestation can be fatal for dogs and cats, depending on the severity of the infestation, which part of the body is affected, the timing of the diagnosis, and the healthcare the animal receives, according to the Texas A&M AgriLife Extension Service.
Infestations occur when Cochliomyia hominivorax flies lay eggs in a wound or body opening. The larvae (maggots) will burrow into the living tissue of any warm-blooded animal, devouring flesh and causing severe, painful wounds and infections.
A total of 12 domestic animals in Texas and New Mexico have been infested in recent weeks, with most of the cases in cattle or goats.
While most dogs and cats in the United States are at low risk, pets are at elevated risk if they live in counties with documented NWS cases, including the Texas counties of Edwards, Gillespie, LaSalle, Sutton, Tom Green, and Zavala, as well as Lea County, New Mexico.
Monitor outdoor pets closely
There’s no need to keep cats and dogs inside the house, said Jenny Green, a spokeswoman for the New Mexico Department of Agriculture.
“New World screwworm is not infectious and cannot be transmitted from animal to animal, from animal to human, or vice versa,” Green said. “The larvae themselves are what cause the damage.”
Because NWS infestations can cause extensive tissue damage, early treatment is essential, according to the Centers for Disease Control and Prevention (CDC). So pets should be monitored for signs of infestation.
“The most important thing us pet owners can do is to put our eyes on our animals every day,” said Guy Loneragan, BVSc, PhD, dean of and professor at Texas Tech University’s School of Veterinary Medicine.
“At this point, we don’t need to dramatically change how our animals behave on a day to day basis,” Loneragan said. “If you notice anything concerning, contact your veterinarian. They have tools to effectively treat animals.”
But monitoring animals for signs of infestations isn’t as easy as it sounds.
“Sometimes, the infestation could be in the nose or another body opening that you cannot see,” Loneragan said.
The Texas A&M AgriLife Extension Service recommends that pet owners take these steps to protect their animals:
- Check pets daily for the presence of potential wounds, including sites of previous tick bites or attachment.
- Watch out for a foul odor, tissue damage, or signs of fly larvae in wounds. Female flies are attracted to body secretions and discharge from wounds.
- Check your pet’s environment for items that could cause wounds, such as wire fencing, sharp vegetation, metal collars or chains, which could cause an injury that attracts female flies.
- Take good care of any open wounds on your animal, applying antibiotic cream and sterile dressings. Flies are attracted to any open wound, including recent surgical incisions due to spay or neuter operations or tail docking.
- Monitor your pets for unusual behavior that can be a sign of an infestation: Are they licking a wound? Does an area of their body appear irritated? Have they lost interest in food? Have they become suddenly aggressive? Are they unusually sleepy?
- Visit your local veterinarian immediately if you suspect a parasitic infestation.
- Keep pets on a year-round flea and tick removal medication, which can help prevent them developing bites and sores that attract flies.
Treating New World screwworm
The Food and Drug Administration (FDA) last week issued an emergency use authorization for a generic drug, nitenpyram, to treat NWS in dogs and cats. Nitenpyram, the first generic animal drug authorized for use against the parasite, can be used in animals who weigh at least two pounds and are at least 4 weeks old.
The elimination of NWS was considered a triumph of animal health. The insect was eliminated in the United States in 1966, largely by releasing sterile male flies, along with active surveillance and livestock inspections. The elimination effort, waged over decades, cost more than $800 million, according to the Wildlife Conservation Society.
Illegal cattle trafficking has allowed the insect to reemerge and spread.
The parasitic flies reappeared in Panama and Costa Rica in 2023. Since then, at least 185,000 NWS cases have been reported in animals and more than 2,100 cases in people in Mexico and Central America.
