Screwworms in US: Human risk is low—but they can burrow through your skull

Ravenous, flesh-eating flies have busted through containment barriers and have now reemerged in the US. On Monday and Tuesday, the US Department of Agriculture reported three new cases, bringing the tally to five.

One of the cases is in a dog, though it’s unclear where it became infected; the dog lives in New Mexico, had its infection reported in Texas, and may have recently traveled to Mexico, where the flies are also spreading. But the other four US cases were all in Texas—and all in calves—two in Zavala County and two in La Salle County.

Almost all the attention over screwworm’s resurgence has focused on the threat to livestock, like the calves and, in turn, the financial risk to the cattle industry. The fly’s voracious, screw-shaped larvae can fell cattle if given the chance, and preventing infestations requires intense vigilance. The USDA has estimated that if the flies stage a comeback rivaling isolated outbreaks of the past, they could cost Texas producers $732 million per year and the Texas economy $1.8 billion.

But while livestock are the easiest and costliest prey, humans are also at risk. Human cases are far less frequent than those in livestock, but when they do occur, they are just as severe. As researchers noted in a 2025 review, infestations in humans “cause rapidly enlarging, painful wounds that can progress to deeper tissues, with risks of secondary infection, sepsis, and mortality.” The fly’s larvae can destroy muscle, cartilage, and bone if they aren’t caught in time. They can even break through a human skull.

Human risk

Understanding the threat requires examining the parasite’s lifecycle. The screwworm—technically New World screwworm or Cochliomyia hominivorax (Coquerel)—is a parasitic blowfly. Females mate only once in their 10–30-day lifespan but can lay up to 3,000 eggs. The flies are attracted to the smell of wounds, mucous membranes, and orifices of warm-blooded animals, and females deposit hundreds of eggs when they find an opening. The eggs hatch within a day, and the resulting eponymous screw-shaped larvae quickly begin ruthlessly boring into and feasting on their victim’s living flesh. This savagery can last up to a week before the mature larvae fall to the ground. There, they pupate in the soil and emerge, 7–54 days later, as adult flies.