New World Screwworm Is 62 Miles From the U.S. Border. Every Veterinarian Needs to Be Paying Attention.

This is not a livestock story. This is not a Texas story. This is a story about a flesh-eating parasite that is closer to the United States than it has been in decades, and about why every veterinarian in this country, regardless of species or geography, needs to understand what is coming.

In April 2026, the USDA confirmed a New World screwworm infestation in a four-day-old calf in the Mexican state of Nuevo León, approximately 62 miles south of the U.S. border. It is the northernmost active case recorded in Mexico. As of early May, Mexico had 1,717 active NWS cases across species, including 913 in cattle, 455 in dogs, 136 in swine, and 88 in horses. In the border states of Nuevo León and Tamaulipas alone, there were at least 206 active cases as of May 9.

Texas Agriculture Commissioner Sid Miller said it plainly in April: Texas is squarely in the crosshairs.

What New World screwworm actually does

The New World screwworm fly, Cochliomyia hominivorax, does not scavenge dead tissue. It feeds exclusively on the living flesh of warm-blooded animals, and it does so aggressively. After a female lays eggs in or near an open wound, body opening, or area of moist skin, the larvae hatch and use sharp mouth hooks to burrow deeper into living tissue. Untreated infestations are painful, rapidly progressive, and fatal. The fly can affect cattle, horses, dogs, cats, wildlife, and humans.

The United States eradicated NWS from within its borders in 1966 using the sterile insect technique, a method that involves releasing millions of radiation-sterilized male flies into affected areas. Because female NWS flies mate only once before dying, mating with a sterile male means producing no viable offspring. The technique was so effective that it pushed NWS eradication all the way south to the Darien Gap at the southern end of Panama, where a maintained barrier of sterile flies has held for decades. That barrier began showing cracks in 2023, when NWS incursions were detected north of the sterile fly distribution zone. Since then the parasite has moved steadily northward through Mexico, primarily carried by infested animals in transport.

The companion animal angle that is not getting enough attention

Former Arizona and Nevada state veterinarian Dr. Peter Mundschenk has raised a concern that deserves to be heard loudly in small animal practice: pets crossing from Mexico into the United States are still permitted entry with very limited oversight, and NWS cases have already been confirmed in dogs and at least one cat in Tamaulipas. Since May 1 alone, USDA data show four detected NWS infections in dogs from that state.

"There is concern that some companion animal veterinarians in the U.S. may not consider screwworm in their diagnoses because it is often viewed as a livestock-only issue," Dr. Mundschenk said. "In reality, New World screwworm can affect dogs, cats, and even humans, if eggs are laid in an open wound."

If you are in a border state and you see a wound that is enlarging, draining, or showing signs of larval activity near a body opening, screwworm belongs on your differential. Report suspected cases immediately to your state veterinarian, APHIS wildlife services, or public health officials depending on the context. One missed case, as Dr. Mundschenk put it, could lead to major economic losses and an expensive response effort.

This is a national supply chain problem, not a regional one

Jamie Jonker, chief science officer for the National Milk Producers Federation, made the argument that every large animal practitioner and food system stakeholder needs to internalize: if NWS breaches the border, routine livestock transportation becomes one of its most efficient transmission mechanisms. Dairy heifers move through Texas, New Mexico, Arizona, and California before being distributed across the country. Wildlife move without borders. A single undetected infestation in an animal entering that pipeline could establish NWS in a geography where it could thrive nine or ten months out of the year.

The USDA closed U.S.-Mexico border ports of entry to livestock movement including bison, cattle, and horses in July 2025. A sterile fly dispersal facility has been completed at Moore Air Force Base in Edinburg, Texas, and a production facility is under construction with a target of 100 million sterile flies per week by November 2027, scaling to 300 million per week in 2028. A white paper produced by the U.S. Animal Health Association's Committee on Parasitic and Vector Borne Diseases is direct about the current gap: sterile fly production capacity is insufficient to respond to a widespread outbreak or sustain extensive buffer zones. Complementary tools, better surveillance, clear animal movement policies, and public-private partnerships are all identified as critical needs.

What to watch for and what to do

Government authorities are urging animal owners and veterinarians in Texas, New Mexico, and Arizona to check animals daily for wounds that are draining, enlarging, or showing signs of larval activity near body openings. Screwworm eggs are creamy white and may be difficult to detect early. Any wound that looks unusual warrants a second look and a low threshold for reporting.

Suspected cases should be reported to APHIS, your state veterinarian, or your state wildlife agency immediately. Do not wait for confirmation before making the call.

The U.S. has beaten this parasite before. The infrastructure to do it again is being built right now. But the window between where NWS is today and where it could be tomorrow is narrow, and the profession's awareness is part of what keeps it from closing.

Stay vigilant. Know the signs. Make the call.

Report suspected NWS cases: USDA APHIS: 1-866-536-7593 Your state veterinarian's office State wildlife agency or public health officials depending on the species involved