We Beat a Vampire Fly in 1966. It’s Back.

Early this month, the U.S. Department of Agriculture confirmed what agriculture and public health officials had long been warning about: New World screwworm fly has reached livestock in the continental United States for the first time in nearly 60 years. The first case was in a three-week-old calf in Zavala County, Texas. Within days, more cases followed in cattle, and then a dog in New Mexico.

Most flies that lay their eggs in flesh are scavengers: their maggots feed on dead tissue, and they can even clean a wound. The screwworm is different. Its larvae feed only on living flesh. A female lays her eggs at the edge of a wound, and the larvae burrow in, eating healthy tissue and widening the wound as they go. Left untreated, an infection can kill in days to weeks. Because it feeds only on the living, you can think of it as a vampire fly. It strikes any warm-blooded animal—livestock, pets, wildlife, even people.

This didn’t have to happen.

Breeding and then deploying sterilized male flies to mate with females, called the sterile insect technique, is an approach that’s been effective for decades to reduce the screwworm population. For 60 years, a U.S.-funded sterile fly program kept screwworm at bay. Through the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG), USDA covered roughly 90 percent of all operations. When a confirmed outbreak hit the Florida Keys in 2016, the sterile insect technique eliminated it quickly. USDA’s Animal and Plant Health Inspection Services (APHIS) estimates the investment in COPEG saved the U.S. cattle industry $2.3 billion annually—protecting a $113 billion sector for a small fraction of what fighting an outbreak would cost.

So, how was screwworm able to resurge?

Screwworm has moved north through Central America into Mexico, driven partly by organized crime groups trafficking cattle without health screenings and accelerated by climate change expanding the territory where the parasite can thrive. Detections in Panama surged from roughly 25 cases annually to more than 6,500 in 2023 alone. It crossed into Mexico in 2024. The invasion of the screwworms is the result of failure to respond rapidly and appropriately to clear signals that the biological boundary to this pest had been breached. The Panama rearing facility, where sterilized males are bred, had been flagged as needing upgrades for years. In early 2025, U.S. funding for animal disease control—including screwworm programs—was cut and delayed, and USDA lost more than 2,000 APHIS employees. Veterinary expertise drained out. No single action caused screwworm to come back; the risks accumulated quietly.

The U.S. is now ramping up efforts to stop screwworm, at a steep cost. The Army Corps of Engineers has broken ground on a domestic sterile fly facility in Texas—projected to cost $750 million, with production not expected until next year. The Panama facility can produce only 100 million sterile flies per week; experts say 500 million per week are needed. The technique itself keeps improving—new genetic methods produce only male flies, which could make future programs cheaper and faster. But better tools mean nothing if no one funds the factory or heeds the warning signs.

In Resolve to Save Lives’ latest Epidemics That Didn’t Happen report, we document outbreaks stopped in El Salvador, Uganda, South Sudan, and elsewhere through early detection and rapid response, both of which require sustained investment. The throughline is that preparation cost money but pays off in savings of money and lives. The wins are invisible, which is why we shine a light on them.

Beyond investing in preparedness, there are three other important lessons. First, animal health is more closely linked to human health than many might think. The same systemic vulnerabilities—underfunded surveillance, climate change, inadequate biosecurity, and weak veterinary public health programs—that allowed screwworm to breach its barrier also increase vulnerability to a wide range of emerging zoonotic threats. Veterinary public health surveillance isn’t a niche agricultural concern, it’s part of the foundation that keeps all of us safer.

Second, the decades-long success in preventing screwworm was a multilateral achievement. Rebuilding it requires commitment. The U.S. didn’t protect us against screwworm alone. It did so in partnership with Panama, Mexico, and other Central American nations. Re-establishing those partnerships, sharing surveillance data, and expanding sterile fly capacity across the region is the operational path to getting this pest back under control in our backyards.

Third, this is a case for rules. The anti-regulation story is seductive: cut the red tape and let people decide for themselves. But there are some things no individual can do. No individual can inspect all the food and drugs they use, stop the factory upwind from fouling the air, or keep a flesh-eating parasite off a continent. Regulation and the agencies that enforce it do work no individual can. The screwworm barrier was that work, made routine.

The screwworm barrier worked for nearly six decades because people funded it, staffed it, and understood that public health actions were making a difference. The moment that logic erodes, in any country, in any administration, the risk starts climbing back.

Preparedness yields compounding returns. When the defenses hold, no one hears about it. When they fall, we pay in livelihoods, in billions of lost dollars, and, all-too-often, in lives. That’s the preparedness paradox. This is how defenses fall against preventable threats, in animals and people alike—quietly, when we stop watching. Let down our defenses and we will pay far more to drive them out again.

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