How to protect pets from the New World screwworm

Pet owners may be diligent about routine care, but even a minor wound can put animals at risk for dangerous parasites. One parasite that poses a potential threat is the New World screwworm (NWS), a parasitic fly whose larvae infest wounds and can result in significant tissue damage if left untreated.

Although there have been no confirmed locally transmitted cases in animals within the United States since 2016, the NWS has been moving north through Central America since 2022, reaching Mexico in 2024. This movement has prompted veterinarians to encourage pet owners, and especially those who work with colonies of stray or other unowned animals, to stay informed and vigilant.

Dr. Guilherme Verocai, an associate professor and director of the Parasitology Diagnostic Laboratory at the Texas A&M University College of Veterinary Medicine and Biomedical Sciences, offers guidance for how owners can identify and manage the parasite, should it cross the U.S.-Mexico border. 

More than just a scratch

New World screwworm, or Cochliomyia hominivorax, is a species of parasitic blow fly known for its screw-like shape. While the adult fly is relatively harmless, the danger comes when they 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 conditions are common across much of the southern U.S., raising concerns about potential spread. 

These flies, which Verocai describes as “strikingly metallic in color” — with metallic greens and blues and red eyes — may 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.

The resulting infestation 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. “Cattle will be the No. 1 species affected, but what’s the second? Dogs.”  

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, including 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.

Staying one step ahead

Because 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

 Preventing infestation starts with routine care and awareness, which can include:

• Checking pets regularly for wounds or injuries

• Cleaning and treating wounds promptly

• Monitoring surgical wounds that may be healing

• 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. Some medications that are not currently labeled for use as treatment against NWS in the U.S. may also be effective, and researchers are continuing to evaluate treatment options.

Verocai emphasizes that although these parasites can cause serious issues, there’s no need for owners to panic about NWS. He points to resources such as the Texas A&M AgriLife Extension Service guide to companion animal care in the face of the NWS threat for owners who want more information.

For now, while the New World screwworm is not currently established in the United States, owners can continue habits that help protect their companion animals from this emerging threat, including staying vigilant and addressing wounds early.

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.