The Return of the New World Screwworm: Livestock and Border Threats

A flesh-eating parasite that was supposedly wiped out from the United States decades ago is back. It’s found in American livestock now, kicking off serious alarms in one of the country’s biggest cattle regions.

The USDA confirmed a case. A three-week-old calf in La Pryor, Texas, got hit with New World screwworm. That’s about a hundred miles southwest of San Antonio, right near the US-Mexico border. This is the first time they've seen it in Texas since 1966.

It matters because Texas is huge for cattle. We’re talking about $17 billion worth just in cattle there. The whole US industry? That’s closer to $113 billion. Officials keep stressing that this fly doesn’t infect food, not a food safety issue. But if it spreads among livestock? That changes things fast.

So they started quarantines. Surveillance ramped up. They moved animals around and started releasing sterile flies in South Texas. Secretary Brooke Rollins basically said the immediate threat to human health is super low. But she warned something else. A very serious threat to our livestock.

What exactly is this thing?

The New World screwworm. It’s a parasitic fly hanging out in the Western Hemisphere. Its relatives are in Africa and Asia, but these flies are weird. Their larvae—the maggots—don't eat dead tissue. They feed on live flesh and fluids.

They lay eggs in open wounds or mucous membranes of warm-blooded animals. Once hatched? The larvae devour living tissue. It messes up vital organs. Triggers nasty bacterial infections. If you don’t treat it right, the animal just dies.

It isn't contagious like a disease spreading from one animal to another. The real danger is where the adult females lay their eggs in wounds. Even a small scrape can attract them. Stephen Diebel, a big Texas rancher and head of the cattle association, pointed this out. He said wounds "as small as a tick bite" are enough to put cattle at risk.

Why are the cattle so vulnerable?

Simple. Routine handling causes scrapes or breaks in the skin. Shearing, de-horning, moving them in and out of pens—all that stuff leaves openings. And birth itself makes both mother and calf targets. That’s key. The Texas case involved a very young calf.

The parasite can infest any warm-blooded animal. Period.

Human Health and International Spread

Can humans get it? Cases are rare, but they can be deadly if you catch it. CNN reported the last US human infection happened in Maryland, linked to travel through El Salvador. That person made it through.

But people need to watch out. The CDC notes that folks who work with livestock, spend a lot of time outside—especially sleeping outdoors—or have medical conditions causing open sores are at higher risk.

Outside the borders, things are way bigger. AP reported that by June 2nd, this parasite had sickened over 171,700 animals and about 2,000 people across Central America and Mexico. The CDC mentioned ten human deaths there too.

Why is this so alarming to US officials? Because they thought it was gone. They had a major program decades ago. Sterile flies, public awareness campaigns, international controls. For years, the pest was supposedly locked down near southern Panama.

That barrier? It’s getting shaky. AP said an outbreak in Panama triggered a state of emergency back in early 2023. Cases jumped to Costa Rica and Nicaragua. Then, a case popped up in southern Mexico in November 2024. That just pushed US officials and the whole cattle industry into panic as this thing started creeping closer to our border.

These flies reproduce fast. They can travel far by hitching rides on hosts, like deer. Edward Burgess, an entomologist studying them, admitted it’s hard to keep up with their speed and regeneration. “It’s hard to stay ahead of it,” he said.

Control Strategy and Response

How are they trying to stop the spread now? The USDA labs in Ames confirmed the Texas case by testing that calf sample. Then things moved fast: response teams deployed in Texas, an infested zone marked out at 20 kilometers, quarantines started, and movement controls hit.

They’re also pushing sterile fly releases in South Texas. They’ve been dropping them since February. Now they're doing it twice a week—four million flies released total this cycle. Plus, they are putting four million more pupae into the ground every week.

The whole strategy hinges on those sterile flies. That’s the core of how the US fought this decades ago. Female screwworm flies only mate once. Scientists raise these flies in factories, sterilize the males, and release them en masse. If a fertile female mates with a sterile male? No eggs hatch.

If you release enough sterile males over time, the population just crashes. It can die out completely. That’s how they managed to eradicate it back then.

Right now? They are trying to stop this single confirmed case from blowing up into a full-blown outbreak. It means sticking to the treatment protocols, enforcing movement rules, watching animals, and keeping those sterile fly releases going. Rollins said if everyone follows the guidance on animal treatment and restrictions, there’s no reason to think this incursion will establish itself in our country. A fragile hope, maybe.