Female screwworm flies lay eggs in any open wound, even a scratch or a newborn calf’s navel, and the hatched maggots feed on living tissue until the animal dies if untreated. The larvae have backward-pointing spines that make them nearly impossible to pull out, and they bore screw-shaped tunnels through living flesh, which is where the name comes from. The adult is a striking metallic blue-green fly with a reddish head.
The US eradicated the pest in 1966 by releasing sterile males. Wild females mate only once, so a sterile pairing ends her line, and saturating an area with sterile males crashes the population. A containment line held for decades.
That line broke. The fly spread north through Mexico to the Texas border, and the first US cases in decades turned up in June, starting with a calf in Zavala County. The count has climbed to 15 confirmed animal cases across Texas, from La Salle and Edwards to Gillespie, Tom Green, Sutton, and Crockett, plus a dog in New Mexico. No locally acquired human cases have been reported in this outbreak, though a travel-associated human case was recorded in 2025.
The response now runs to quarantines, insecticide treatment, and the release of tens of millions of sterile flies. Analysts warn of up to $1.8 billion in damage to a cattle industry worth billions if it spreads widely. A new US sterile-fly facility is under construction, but it won’t be fully running until late 2027.
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