New World Screwworm in Pets: What Pet Owners Should Know About the U.S. Return
- Dr. Zachary Glantz
- 7 hours ago
- 5 min read
New World screwworm (NWS) is back in the news after the USDA confirmed the first animal case in the United States in the current outbreak on June 3, 2026. The case was identified in Texas near the U.S.-Mexico border and marks the first detection in the United States since a small outbreak in the Florida Keys was eradicated in 2017.
For most pet owners in Philadelphia, the immediate risk remains extremely low. However, New World screwworm is one of the most economically and medically important parasites in the Western Hemisphere. Unlike most maggots, which feed on dead tissue, screwworm larvae feed on living tissue. Left untreated, they can cause severe wounds, secondary infections, and even death.
Because of the potential impact on livestock, wildlife, pets, and occasionally people, the United States has spent decades and hundreds of millions of dollars preventing this parasite from becoming re-established. Based off a Texas outbreak in 1976, USDA APHIS estimated the potential economic loss of a similar scale outbreak in 2024 dollars to be $1.8 billion.
What Is New World Screwworm?
New World screwworm, or Cochliomyia hominivorax, is a parasitic fly found primarily in parts of South America and the Caribbean. Since 2023, however, the parasite has spread northward through Central America and Mexico, prompting increased surveillance and control efforts throughout North America.
The adult fly itself does not cause disease. Instead, female flies are attracted to wounds and body openings such as the ears, eyes, nose, mouth, or genitals. They lay eggs on these sites, and the eggs hatch into larvae (maggots).
Unlike the maggots commonly seen in neglected wounds, which generally consume dead tissue, screwworm larvae actively burrow into healthy, living tissue and feed as they grow. The wound often becomes progressively larger, deeper, more painful, and foul-smelling over several days.
The parasite gets its name from the way the larvae burrow into tissue, resembling a screw being driven into wood.
Why Is Screwworm Such a Big Deal?
New World screwworm was once widespread throughout the southern United States. Through a remarkable eradication effort using sterile insect technique, the parasite was eliminated from the United States in 1966.
Sterile insect technique works by releasing sterile male flies into affected areas. Female screwworm flies generally mate only once during their lifetime. When they mate with a sterile male, they produce no offspring. Over time, the population collapses and can ultimately be eliminated.
This strategy was so successful that New World screwworm remained absent from the United States for decades, with only occasional incursions such as the Florida Keys outbreak in 2016–2017.
The current concern is that screwworm has steadily moved northward since 2023. According to the CDC's outbreak updates, cases have now been reported throughout Central America and Mexico. As of June 2026, more than 185,000 animal cases and more than 2,100 human cases have been reported in Mexico and Central America.
Can Dogs and Cats Get Screwworm?
New World screwworm can infest virtually any warm-blooded animal, including dogs, cats, livestock, wildlife, birds, and people.
Dogs and cats become infested when a female fly lays eggs on a wound or body opening. One of the more concerning aspects of screwworm is that the wound does not need to be large. According to USDA and CDC guidance, infestations can occur in wounds as small as a tick bite, insect bite, scratch, surgical incision, or other minor skin injury.
Pets may be at higher risk if they:
Spend significant time outdoors in affected areas
Have allergies or chronic skin disease
Have recent surgical sites
Have wounds or skin infections
Have flea, tick, or mite infestations
Have skin folds or areas of chronic irritation
For pets living exclusively in Philadelphia that have not traveled to affected regions, screwworm is currently an extremely unlikely cause of skin disease. However, the disease becomes much more relevant for imported animals, rescue dogs, and pets that travel internationally.
What Clinical Signs Should Pet Owners Watch For?
The most common sign is a wound that appears to worsen rapidly instead of healing.
Owners may notice:
A painful wound or sore
A foul odor from the wound
Bleeding or discharge
Visible maggots in a wound or body opening
Excessive licking, scratching, or chewing at a specific area
Head shaking or irritation around the ears
Lethargy or decreased appetite
Fever or signs of secondary bacterial infection
The presence of visible maggots should always prompt veterinary attention, especially in any pet with recent travel to an affected area.
Can People Get Screwworm?
Yes, although human infestations are uncommon.
Most human cases occur in areas where screwworm flies are circulating and involve people who have open wounds, significant outdoor exposure, or close contact with livestock and other animals.
Importantly, no locally acquired human infestations have been reported in the United States during the current outbreak.
The CDC has documented one travel-associated case in a person who returned to the United States from El Salvador in 2025.
How Is Screwworm Treated?
Treatment requires prompt medical or veterinary intervention. The primary goal is complete removal of all larvae from the affected tissue. Depending on the severity and location of the infestation, this may require sedation, anesthesia, wound management, antibiotics, pain control, and antiparasitic medications.
Recently, the FDA issued guidance regarding products authorized for prevention and treatment of New World screwworm myiasis. Oral flea/tick preventatives in the isoxazoline class have been used to treat and prevent screwworm in South America, and most have conditional FDA approval, including Credelio and Bravecto..
If you suspect screwworm infestation in your pet, do not attempt to treat it yourself. Veterinary evaluation is important both for appropriate treatment and for disease reporting.
What About International Travel With Pets?
International travel is currently one of the most important risk factors for exposure.
Veterinarians are being encouraged to discuss screwworm risk with owners traveling to affected regions, and USDA APHIS launched Screwworm.gov.
Dogs entering the United States from affected countries must meet specific USDA import requirements, including certification that they were examined and found free of screwworm infestation shortly before travel.
If your pet will be traveling internationally, particularly to Mexico, Central America, South America, or the Caribbean, discuss travel plans with your veterinarian well before departure.
How Can Pet Owners Reduce Risk?
For most families in Philadelphia, the most important preventive measures are simple:
Maintain routine flea and tick prevention like Credelio or Bravecto
Seek veterinary care for wounds and skin infections
Monitor surgical sites carefully during healing
Avoid unnecessary travel to affected areas with pets that have wounds or skin disease
Have pets examined before and after international travel
Owners traveling to areas where screwworm is present should inspect their pets daily for wounds, swelling, discharge, unusual odor, or signs of irritation.
What Does This Mean for Philadelphia Pet Owners?
At this time, New World screwworm is not a common risk for pets in Philadelphia.
For a local dog or cat with a skin lesion, common problems such as allergies, abscesses, bite wounds, skin infections, ear disease, or anal gland disease remain far more likely explanations.
However, the return of screwworm to the United States serves as an important reminder that animal diseases do not respect borders. Increased international travel, animal movement, and changing environmental conditions can allow diseases once considered eliminated to reappear.
The good news is that the United States has successfully eliminated New World screwworm before, and extensive surveillance and response efforts are already underway. Unfortunately, cuts to USDA and USAID have significantly slowed the response.
For most pet owners, there is no reason for alarm.
