Since the U.S. Department of Agriculture confirmed the first case of New World screwworm (NWS) in the United States on June 3, pet owners and shelters alike have expressed concern about what this might mean for their animals.
The larvae of the parasitic fly infest wounds and can create significant tissue damage if left untreated. Veterinarians are encouraging shelters and pet owners, including those who work with colonies of stray or other unowned animals, to remain vigilant and stay informed, especially because NWS infestations can be easily prevented and treated.
Dr. Guilherme Verocai, an associate professor and director of the Parasitology Diagnostic Laboratory at the Texas A&M University College of Veterinary Medicine & Biomedical Sciences, offers guidance for how owners can identify, prevent and manage the parasite.
How New World Screwworm Attacks Animals
NWS, or Cochliomyia hominivorax, is a species of parasitic blow fly with larvae known for their screw-like shape. While the adult fly is relatively harmless, the danger comes when female flies lay eggs in open wounds.
“The female fly is attracted to wounds or lesions on an animal. It can be as small as a tiny scratch or a post-surgical wound,” Verocai said. “They will lay eggs around the edge of the wound, and within 12-hours, the larvae will hatch from those eggs and begin feeding on live tissue.
“That’s really key with C. hominivorax,” he said. “Unlike other blow fly species, C. hominivorax requires live animals to survive.”
The adult fly thrives in warm climates, with activity typically occurring between 65 and 91 degrees Fahrenheit.
These flies, which Verocai describes as “strikingly metallic in color” — with metallic greens and blues and red eyes — should not be confused with their “first cousin,” Cochliomyia macellaria, another species of blow fly that is more common in the U.S., but which is typically harmless.
“Adult C. hominivorax are not going to be seen resting on the animals like horn flies or stable flies; they’re typically resting in forested areas,” Verocai said.
How Fast It Spreads and Why It’s So Dangerous
Female C. hominivorax will lay a couple of hundred eggs around an existing wound, and within 12 hours, depending on the temperature, the eggs will hatch. This can result in an infestation that can escalate quickly and become severe as the small larvae eat and grow larger.
“You’re not going to have just one or two larvae — it can be dozens or even hundreds feeding on live tissue,” Verocai said. “And it’s not a picky parasite; although it rarely infests poultry or birds, any warm-blooded mammal is quite suitable as a host, if given the opportunity.
“Screwworms can infest all warm-blooded animals, including dogs; cats; livestock; wildlife, including, rarely, birds; and even people,” he said. “In endemic areas cattle are most commonly affected, but dogs are usually the second most affected among domestic species.”
Stray or unowned dogs and cats are likely at higher risk of NWS infestation for several reasons, including being more prone to injuries and not having access to veterinary care and preventative medicines that may treat or prevent an infestation.
Because the parasite targets living flesh, it can cause extensive tissue damage and, if left untreated, may quickly become life threatening, depending on the location and severity of the infestation.
Early detection is key for managing NWS infestations; pet owners should watch for:
● Visible larvae in a wound
● Wounds that worsen quickly
● A foul odor, often described as decaying tissue
● Swelling or irritation around a wound
How To Prevent and Treat New World Screwworm
Preventing NWS infestation starts with routine care and awareness, which can include:
● Checking pets daily for wounds or injuries
● Cleaning and treating wounds promptly
● Monitoring surgical wounds daily until completely healed
● Keeping pets up to date on veterinary care
● Seeking veterinary attention for unusual wounds or odors
“If you notice anything concerning, don’t wait — reach out to your local veterinarian,” Verocai said. “Early diagnosis, confirming, and treating as early as possible makes a big difference.”
Because this is not a disease — it is an insect infestation — when the insects are removed, the animal typically recovers, especially if attention is given early.
It’s also important that owners keep their pets on routine parasite preventative medicines, some of which can be effective against screwworm larvae.
Since April, the U.S. Food and Drug Administration has issued conditional approval or emergency use authorization for a number of antiparasitic products that prevent and treat NWS in multiple species.
New World Screwworm Resources
There are many educational resources available through the Texas A&M AgriLife Extension Service, Texas Animal Health Commission, and U.S. Department of Agriculture, all of which provide practical guidance on identifying potential signs of screwworm infestation, monitoring wounds, and knowing when to seek veterinary care.
“If you suspect New World Screwworm in your dogs or cats, do not wait,” Verocai said. “Immediately isolate suspected animals and contact either the Texas Animal Health Commission (800-550-8242) for livestock cases; Texas Parks and Wildlife (512-389-4505) for wildlife cases; or your local veterinarian for your pets or other small animals.”
More information on reporting cases and submitting samples to the appropriate authorities also is available through Texas A&M AgriLife.
Verocai emphasizes that while NWS is in the U.S., and may be here for a while, there’s no need for owners to panic.
Though these parasites cause serious issues, by continuing the habits that help protect your companion animals — including the regular use of parasite preventatives — and by staying vigilant and addressing wounds early, owners can rest assured that their pets will be safe from NWS.
Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the web at vetmed.tamu.edu/news/pet-talk. Suggestions for future topics may be directed to vmbs-editor@tamu.edu.