Flesh-Eating Screwworm Fly Returns to Texas After Decades
USDA confirms first US case of New World screwworm in Texas since 1966, with larvae found on a three-week-old calf near the Mexican border
- On June 3, 2026, the USDA confirmed the first case of New World screwworm in Texas since 1966.
- Larvae were found in the navel region of a three-week-old calf in La Pryor, about 80 kilometres from the Mexican border.
- The parasite Cochliomyia hominivorax lays eggs in open wounds, and its larvae feed on the living tissue of warm-blooded animals.
- The response relies on the sterile insect technique, releasing millions of sterile flies each week to halt the spread.
The United States Department of Agriculture (USDA) confirmed on June 3, 2026, the first domestic case of New World screwworm in La Pryor, Zavala County, in southern Texas, roughly 80 kilometres from the Mexican border. The infested animal is a three-week-old calf, and larvae were found in its navel region. It is the first recorded case of the parasite in Texas since 1966.
The culprit is the larva of the fly Cochliomyia hominivorax, known as the New World screwworm. Unlike most related species that feed on dead tissue, this fly lays its eggs at the edges of open wounds on warm-blooded animals, and the hatched larvae then feed on the host’s living flesh. The name describes the way the larvae burrow deeper into the wound, like a screw.
How did the parasite reach Texas again?
The fly was eradicated from the United States back in the 1970s, using the sterile insect technique, one of the first biological pest-control methods applied on a large scale. Until the end of 2024, the parasite was confined to Panama, where it had been held in check for decades by a barrier of sterile flies. During 2025 the population pushed northward and spread through Mexico, gradually bringing the problem closer to the US border.
The larvae attack cattle, but also other mammals and even humans, through any open wound, an insect bite or a fresh vaccination site. Untreated cases in livestock can be fatal. The USDA stated that there have been no further detections beyond this single calf.
“The infested animal is a three-week-old calf, and there have been no further detections to date,” said Brooke Rollins, US Secretary of Agriculture.
Why is the response so urgent?
The stakes are high. Texas livestock is valued at around 15 billion dollars, so an uncontrolled spread of the fly would hit a key sector of the state’s economy. For that reason the USDA has set up a joint command and established a quarantine zone around the site of the find.
At the heart of the response is a proven method from the last century. The principle works as follows:
- Large quantities of flies are reared in a laboratory, and the males are sterilised through irradiation.
- The sterile males are then released from the air, millions of them each week, over the affected area.
- Because wild females mate only once, mating with a sterile male produces no offspring.
- By repeating the cycle, the local population gradually declines toward zero.
The method is slow by nature and depends on patient, continuous application. The value of this first detection is limited: a single infested animal does not reveal how far the parasite has already spread or whether undetected pockets exist nearby. The coming weeks of surveillance will show whether this is an isolated case or the start of a wider incursion. How aware are you of how much our agriculture relies on biological methods of pest control like this one?
