Screwworm Parasite Reappears in US Cattle, Triggering Emergency Response

The USDA confirmed the first case of New World screwworm in U.S. livestock since 1966 — a 3-week-old calf in Zavala County, Texas. The infection was found in the calf's umbilical area, a classic entry point for the parasitic fly larvae.

That 1966 eradication was a big deal. The sterile insect technique — releasing millions of sterilized male flies to collapse the wild population — wiped the pest out of the southern U.S. and Mexico. It worked so well that screwworm disappeared from American livestock for six decades.

How a Parasite That Cost $6.1 Billion Returned

Screwworm never left the Caribbean and South America. By 2005, annual economic losses in those regions hit an estimated $6.1 billion in today's dollars. The fly has been creeping northward through Central America for years, and experts warned it was only a matter of time before it crossed back into the U.S.

Now it's here. The calf's infection marks the first documented livestock case since the eradication campaign ended. The fly lays eggs in open wounds or body openings of warm-blooded animals — including humans — and the hatched maggots burrow into flesh, causing foul-smelling, painful lesions.

The Response: Sterile Flies and Quarantines

USDA undersecretary Dudley Hoskins called it a national security issue for the livestock industry. Officials have already formed an incident command team with the Texas Animal Health Commission, established a 20-kilometer quarantine perimeter around the infection site, and started releasing sterile New World screwworm flies from ground and air.

Trapping along the border and wildlife screening in the area are also underway. The goal is to contain this single case before the flies establish a breeding population. Sterile insect technique worked before — but that was a coordinated, multi-year program across two countries. A single hot spot requires a faster, more localized response.

What This Means for Livestock and People

For ranchers near Zavala County, the immediate task is checking cattle, pets, and livestock for draining wounds or visible maggots around eyes, ears, mouth, and genitals. The USDA is asking anyone who suspects an infection to call their state animal health official or a local USDA veterinarian immediately.

Human infections are rare but possible — especially for people who spend time outdoors with open wounds in endemic areas. The CDC advises keeping wounds clean and covered, sleeping indoors with closed windows, and using insect repellent. Removing maggots yourself is a bad idea; a healthcare provider may need to remove them surgically.

This case will test whether modern surveillance and containment can replicate the success of the 1960s eradication program. If it works, the sterile flies and quarantine zone will snuff out the threat. If not, the U.S. livestock industry faces a return to the days of devastating screwworm outbreaks that took decades to defeat.

Source: A flesh-eating New World screwworm was just found in a Texas cow-here's what to know
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