On June 3, 2026, the Texas Animal Health Commission confirmed New World screwworm in a calf in Zavala County, the first detection of the pest on US soil since it was eradicated in 1966. The fly slipped past the sterile-insect barrier at the Darien Gap and marched north through Central America and Mexico. This article explains the biology, the breach, and the 500-million-sterile-flies-a-week response.

What the screwworm is

The New World screwworm is the larval stage of a parasitic fly, Cochliomyia hominivorax. The species name reflects its appetite for living flesh. The adult female fly lays her eggs in the open wounds or body orifices of living, warm-blooded animals. When the eggs hatch, the larvae burrow into the flesh with sharp mouth hooks and feed on living tissue. This is what separates the screwworm from ordinary wound flies, which feed on dead or decaying tissue. The screwworm actively consumes healthy flesh, and as it feeds, the wound enlarges and deepens.

The resulting condition is called myiasis. It is painful, progressive, and, left untreated, frequently fatal. The Texas Animal Health Commission states that an infested animal can die within one week. The New Mexico Livestock Board puts the window at seven to ten days. The fly does not discriminate much by species. Cattle, horses, sheep, goats, and swine are the primary livestock hosts, but the screwworm also infests other mammals, birds, and, in rare but serious cases, people.

The danger is compounded by how quietly an infestation can begin. A female fly needs only a small opening: a tick bite, a barbed-wire scratch, a branding wound, a newborn's navel. She deposits her eggs there, and the larvae that hatch begin feeding almost immediately. Because the wound is living tissue, the animal's own immune response cannot clear the maggots the way it might clear a surface infection. The infestation instead grows, and the animal's condition deteriorates as the wound deepens and secondary infection sets in.

The biology matters because it dictates the only control strategy that has ever worked. Because the larvae feed on living tissue, they are not reached by the ordinary wound treatments that clear dead-tissue maggots. And because the adult female mates only once in her lifetime, the entire population can be driven down by flooding the environment with sterile males.

The sterile insect technique

The sterile insect technique, or SIT, is the tool that eradicated the screwworm from the United States in 1966. The method is elegant in principle and industrial in practice. Facilities rear enormous numbers of screwworm flies, sterilize the males with radiation, and release them by air over infested regions. A wild female that mates with a sterile male produces eggs that never hatch. Because she mates only once, that single sterile mating removes her from the reproductive pool entirely. Over successive generations, the wild population collapses.

SIT is not a one-time fix. It requires continuous production and continuous release, because the moment the sterile flies stop arriving, any surviving wild population can rebuild. The scale is industrial: the program that eradicated the pest from the United States relied on rearing and releasing hundreds of millions of flies every week. That is why the United States did not simply declare victory in 1966 and walk away. It built a permanent barrier.