Screwworm Is Back in the U.S. for the First Time Since 1968. Here's Everything You Need to Know.

A three-week-old calf in La Pryor, Texas, a small town near San Antonio, just made history no one wanted to make. The USDA confirmed it as the first U.S. case of New World screwworm since the parasite was eradicated from American soil in 1968. One week earlier, a case had been confirmed in Mexico just 25 miles from the U.S. border.

For most veterinarians practicing today, this is a parasite they have read about but never seen. That is about to change. Here is the complete picture.

What New World Screwworm Actually Is

Cochliomyia hominivorax, literally "human flesh eater" in Latin, is a parasitic fly whose larvae are uniquely and exclusively adapted to feed on living tissue. This is not an opportunistic secondary invader of necrotic wounds. This is a primary parasite that targets healthy, living animals.

The female fly is attracted to the warmth and odor of open wounds, fresh blood, or natural body openings including the eyes, ears, nose, and mouth. She lays clusters of 200 to 400 eggs at the wound margins, and the eggs hatch within 12 to 24 hours. The larvae that emerge immediately begin burrowing into living tissue, feeding and expanding their tunnel as they grow. The wound enlarges, the odor intensifies, attracting more flies, and the cycle accelerates. Without treatment, a heavily infested animal can die within seven to ten days.

A single female screwworm fly lives approximately 20 days and can lay more than 3,000 eggs across multiple egg-laying events in that time. Her flight range exceeds 120 miles under favorable conditions.

The History: How We Lost It and How We Got It Back

New World screwworm once caused catastrophic losses across the American South, Southwest, and throughout Central and South America. Before eradication, U.S. livestock losses were estimated in the hundreds of millions of dollars annually. Animals that survived infestation often required intensive wound management. Wildlife populations had no treatment at all.

The eradication campaign that eliminated screwworm from the United States between 1958 and 1968 remains one of the most successful pest eradication programs in agricultural history. It used the sterile insect technique, or SIT, mass-rearing male screwworm flies, sterilizing them with gamma radiation, and releasing them in enormous numbers into the environment. Because female screwworms mate only once in their lifetime, mating with a sterile male means producing no viable offspring. Flood the environment with enough sterile males and the wild population collapses within a few generations.

The eradication was completed progressively southward across the continental U.S., then into Mexico and Central America. A biological barrier of sterile fly releases has been maintained near the Panama-Colombia border for decades, preventing reinvasion from South America.

That barrier is what began to fail. Starting around 2024, screwworm was detected moving northward out of Panama into Central America despite ongoing sterile fly releases. By 2025, the fly had reached Mexico. The USDA and multiple state veterinary agencies had been preparing for a U.S. detection for at least two years before the La Pryor calf made it official.

The Transmission Risk Everyone Needs to Understand

The fly itself does not travel long distances on its own. A 120-mile flight range sounds significant, but it is not the primary mechanism by which screwworm spreads across continents. The primary mechanism is, as Wyoming state veterinarian Hallie Hasel memorably described it, "help on wheels."

Cattle, horses, and other livestock move constantly through interstate and international commerce aboard transport vehicles. A single animal with a small wound containing undetected larvae, transported across state lines before anyone notices, can establish a new infestation hundreds of miles from the source. The same risk applies to rescue animals transported from affected regions — a dog or cat from Texas with an open wound or skin lesion arriving at a shelter in Colorado, Wyoming, or New York is a potential vector.

This is why veterinary surveillance in every state matters, not just Texas. The detection happened in Texas. The risk of secondary establishment is national.

Clinical Recognition: What to Look For

For practitioners who have never seen a screwworm case, recognition depends on knowing what distinguishes it from other wound conditions.

The defining features are the larvae themselves, cream-colored, spine-covered maggots that burrow actively into tissue rather than resting at the wound surface. They move in a characteristic screwing motion, which is the origin of the common name. A fresh wound infested with screwworm larvae will typically show a foul, distinctive odor, visible larval movement within the wound cavity, and a wound that appears to be enlarging or deepening despite standard wound management.

Animals with screwworm are often restless, off feed, and may be seen repeatedly turning to investigate a wound site. The behavioral change frequently precedes obvious wound deterioration.

Any wound in a patient with potential exposure to affected regions, particularly from Texas, but increasingly from any area with confirmed activity, that does not respond normally to standard treatment should prompt screwworm consideration.

What to Do If You Suspect Screwworm

The reporting protocol is specific and mandatory.

Do not move the animal. If the patient is livestock-sized, confine it immediately. Movement risks spreading larvae to new locations.

Do not discard larvae. Collect any larvae you remove from the wound into a sealed container. These specimens are critical for laboratory confirmation.

Photograph everything. Document the wound appearance and any visible larvae before intervention.

Contact your state veterinarian immediately. If you cannot reach the state veterinarian directly, the USDA APHIS Veterinary Services emergency line is available. Your state veterinarian will coordinate with federal authorities on specimen submission and any required containment measures.

For companion animal patients — dogs and cats — the same collection and reporting protocol applies. Take the animal in immediately for treatment and notify authorities. Companion animals arriving from Texas or other potentially affected areas with any wound, skin lesion, or unusual dermatological presentation should be examined with screwworm in mind.

Treatment

Once confirmed or strongly suspected, screwworm treatment involves manual removal of all larvae, thorough wound debridement, and application of approved larvicidal products. Organophosphate-based treatments have historically been the primary pharmacological intervention. Your state veterinarian will provide current product guidance as the response protocols develop.

The most important clinical point is that waiting is not a safe option. Every hour of untreated infestation allows larvae to burrow deeper, the wound to expand, and additional flies to be attracted to the site.

The Sterile Fly Response Already Underway

The same technique that worked in the 1960s is being deployed again. A facility in Panama is currently releasing 100 million sterile screwworm flies per week to suppress the population in Central America. A new, larger facility under construction in Mexico, partially funded by the United States, will release more than 300 million sterile flies per week when it becomes operational, currently projected for mid-2027.

The USDA has established a 12-mile infested zone around the La Pryor detection site and is implementing additional surveillance and containment measures in coordination with the Texas Animal Health Commission.

The veterinary and agricultural community's assessment is that the current case does not represent an imminent national crisis if containment holds. But containment depends on detection, and detection depends on every veterinarian in every state knowing what they are looking for.

The Bottom Line

New World screwworm has been absent from American veterinary practice for 58 years. The veterinarians who treated it before are largely retired. The clinical knowledge that once lived in the profession's working memory has faded to textbooks and case reports.

That changes now. Know the parasite. Know the presentation. Know the protocol. And if something looks wrong in a wound that should be healing, call it in.