Screwworm in Pets and Livestock

New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae feed on living tissue of warm-blooded animals, including pets, livestock, wildlife, and occasionally people. Unlike most fly larvae, which feed on dead tissue, screwworm larvae invade and consume living tissue, causing severe tissue damage.

New World screwworm is attracted to wounds and open bodily orifices such as the nose, eyes, ears, mouth, and genitals where they lay eggs. The eggs hatch into larvae that eat the live tissue causing painful, enlarging wounds that often have a foul odor.  

The risk of New World screwworm infestation is low for most pets. However, pets with open wounds or healing surgical sites who spend time outdoors in areas where New World screwworm has been detected are at greater risk.

Awareness and early detection are important, and prompt veterinary attention for wounds can help prevent serious complications.

What Is Screwworm?

The New World screwworm is a parasitic fly whose larvae have rows of spines to burrow deep into wounds or bodily openings. It uses a screw-like motion as it feeds on living tissue, which can cause extensive tissue damage, severe illness, and even death in severe untreated cases.  

This parasite primarily attacks: 

• Livestock, including cattle, sheep, goats, pigs 

• Horses 

• Wildlife, such as deer 

• Dogs and other household pets 

• Humans, particularly those with open wounds or untreated skin injuries 

New World screwworm is typically found in South America and the Caribbean. But recently, this fly has been expanding north through Central America and Mexico. In June 2026, screwworm was detected in a cow in Texas and in a dog in New Mexico.

Although the detection has raised concerns and prompted efforts to contain the parasite, the risk to most pets remains low, especially for pets without open wounds or recent surgical sites.

Causes of Screwworm in Animals

Screwworm infestation is caused by the larvae of the New World screwworm fly (Cochliomyia hominivorax). While the adult stage is a fly, the term “screwworm” refers to the larval stage (maggot), which has screw-like barbs used to burrow into and feeds on living tissue.

The female adult New World screwworm fly lays hundreds of eggs that hatch within a day. The emerging larvae burrow into living tissue and feed for several days. The mature maggots then drop to the ground and pupate. Adult flies then emerge and repeat this life cycle, quickly causing an infestation.  

New World screwworm was eradicated from the U.S. by the mid-1960s after causing major losses to the livestock industry. Over the following decades, eradication programs pushed the parasite farther south through Mexico and Central America, helping prevent its reestablishment in the U.S.

Since its eradication, New World screwworm has only occasionally been detected in the U.S., including a localized outbreak in the Florida Keys in 2016 that was successfully eliminated. The recent detection of New World screwworm in Texas has renewed concerns about protecting livestock, pets, and wildlife from this destructive parasite.

Is My Pet at Risk?

For most animals in the U.S., the risk of screwworm is low. But the risk level depends on your location and lifestyle. 

Pets with open wounds, healing surgical sites, or skin injuries are at the greatest risk for screwworm infestation in areas where the parasite has been detected. Spending time outdoors can increase exposure to egg-laying flies, particularly during the daytime when adult flies are most active.

Symptoms of Screwworm in Animals

In infected animals, early signs of screwworm include:

• A wound that fails to heal, enlarges, or suddenly worsens 

• Excessive chewing, licking, scratching, or rubbing a wound 

• Swelling or redness around a wound 

• Bloody or blood-tinged discharge from a wound 

• Foul-smelling odor 

• Head shaking or pawing at the face (if the ears, eyes, or nose are affected)

• Pain and sensitivity when area is touched 

• Restlessness or discomfort

Advanced signs include:

• Visible maggots (larvae) in wound or natural body openings  

• Enlarging wounds despite appropriate treatment 

• Deep pockets of infection or abscess formation 

• Loss of appetite

• Lethargy

• Weight loss

• Fever

Common infection sites include:

• Surgical incisions

• Bite wounds

• Cuts or abrasions

• Umbilical cords of newborn animals

• Eyes, ears, nose, mouth, or genitals

Diagnosing Screwworm in Pets and Livestock

Veterinarians may suspect screwworm based on a physical exam and wound assessment. A definitive diagnosis for NWS is made by collecting the larvae and submitting them to a diagnostic laboratory. The larvae are examined under a microscope for proper identification. 

Molecular testing, or PCR, can be used to determine the identification of the larvae as well.

Many other fly species can cause similar-looking larvae, so proper identification is necessary to ensure proper treatment and precautions are taken.

Because New World screwworm is a reportable foreign animal disease in the U.S., suspected cases must be reported to the USDA Animal and Plant Health Inspection Service (APHIS) and state animal health officials. Diagnostic samples may be submitted to specialized laboratories, including the National Veterinary Services Laboratories (NVSL), for confirmation.

Treating New World Screwworm in Pets

New World screwworm is a serious condition that requires prompt veterinary attention.

If screwworm is suspected, veterinarians will remove and collect larvae for identification and begin treatment as quickly as possible. Because New World screwworm is a reportable foreign animal disease in the U.S., animal health authorities may become involved to help prevent the parasite from spreading.

Treatment focuses on eliminating all larvae, managing the wound, and preventing reinfestation. Wounds often require thorough cleaning, removal of damaged tissue, and ongoing wound care. Depending on the severity of the infestation, frequent wound monitoring, bandage changes, and topical treatments may be necessary.

Most animals will also require oral medications to kill remaining larvae and prevent further infestation.

Here's a list of treatments approved by the U.S. Food and Drug Administration:

Veterinarians may prescribe additional treatments, including antibiotics, if secondary bacterial infections are present or suspected.

Environmental management is also an important part of treatment. Animal housing areas and other environments where infested animals have spent time may need to be cleaned and managed according to recommendations from animal health authorities to reduce the risk of additional exposure.

The prognosis is generally best when screwworm is recognized and treated early. Pet parents who notice a wound that is worsening instead of healing, has a foul odor, or contains visible larvae should seek veterinary care immediately.

How To Protect Your Pet from Screwworm

Protecting your pet against New World screwworm requires meticulous attention to wounds and wound healing. When in doubt, it’s always best to have wounds evaluated by your vet to ensure proper treatment and healing occurs.

If you handle livestock or animals with wounds in areas where screwworm is present, wear disposable gloves when examining or treating wounds and wash your hands thoroughly afterward.

You can further help protect yourself and your pets by:

• Checking your pet regularly for wounds, skin irritation, or areas of drainage

• Keeping wounds clean and following your veterinarian's instructions for wound care

• Preventing your pet from licking, chewing, or scratching healing wounds

• Monitoring surgical incisions closely until they are fully healed

• Seeking veterinary care promptly if a wound becomes larger, develops a foul odor, or fails to heal

• Keeping your pet up to date on veterinarian-recommended parasite prevention products

• Reporting any suspicious wounds containing maggots to your veterinarian immediately