A flesh-eating maggot is spreading in America – it could threaten the livestock industry in the USA

Clyde Sommerlatte still remembers the maggots vividly, even though he was just a boy at the time, eleven or twelve years old. "When I think back, I can still smell it today," says the 75-year-old. "A horrible, acrid odor. It smells of decay, like a carcass that's been lying next to the highway for a few days." Except the calves and cattle that smelled like that back then were actually alive. Sommerlatte, now a rancher himself in South Texas, helped a neighbor in the 1960s scrape the New World screwworm fly larvae from the navels of newborn calves with a pocketknife.

Only the older ranchers in the southern United States can still remember the plague; the younger ones know it only from stories. Cows, goats, and deer with gaping, festering wounds teeming with maggots. Many animals died. But then the New World screwworm fly disappeared from the United States in the 1960s, and later from Mexico and Central America.

It was a success story of scientific ingenuity and international cooperation. Until now. The New World screwworm fly is spreading again. And in Texas, preparations are underway to eradicate it once again.

The New World screwworm fly belongs to the blowfly family. It is about twice the size of a housefly, and its body has a metallic sheen. Blowflies typically lay their eggs in rotting meat and other organic matter, explains Phillip Kaufman, professor of entomology at Texas A&M University. "But the New World screwworm fly only lays its eggs in living animals."

A female fly lays around 200 eggs, usually in wounds: small scratches, cuts, or the navels of newborns. After 12 to 24 hours, the larvae hatch. "They make the wound larger and larger, penetrating deeper and deeper, eating their way through the flesh." With sharp hooks on their mouths, the larvae screw themselves into the flesh, hence their name.

When the larva is fully developed, it crawls out of the wound, falls to the ground, and pupates into a fly. Soon after, it mates – only once. Scientists have taken advantage of this.

In the 1950s, entomologists Raymond Bushland and Edward Knipling invented a revolutionary method for eradicating a species: the sterile insect technique. In the laboratory, they irradiated screwworms with gamma radiation while they were pupating—strong enough to damage their reproductive organs, but not so strong that they could no longer mate. Starting in 1958, the sterile male flies were loaded onto airplanes and dropped over the southern states. Barely a decade later, the screwworm was eradicated from the United States.

The sterile males serve as a weapon against their own species. When they mate in the wild, the female lays unfertilized eggs. This causes the population to dwindle over time. It's a method that, unlike pesticides, causes no harm to the environment. Only small cardboard boxes used to trap the flies remained. Clyde Sommerlatte remembers finding them in the fields as a boy.

The sterile insect technique became an export hit and has since been used internationally to combat a variety of insects. Sterile tiger mosquitoes are currently being released in Germany, Austria, and Switzerland. The method is also used in Europe to combat agricultural pests, such as fruit flies.

In the Americas, the New World screwworm fly disappeared from an increasing number of countries in Central America over the decades. Their governments cooperated with the US, even though political relations were strained at the time. Since 2006, an invisible barrier has been maintained in southern Panama—to South America, where the New World screwworm fly has never been eradicated.

Every week, millions of sterile larvae are produced in a production facility in Panama. Instead of ground meat, as in the early days, they now feed on a mixture of dried beef blood, meat protein, poultry eggs, and honey. For decades, the flies have been dropped almost daily from airplanes over the Darién Gap, the rainforest area that served as a barrier—until it broke.

Since 2023, the New World screwworm fly has been spreading north again, slowly at first, then increasingly rapidly. "No one knows for sure what happened. There are a number of possible causes," says Kaufman. Inspections may have been neglected during the coronavirus pandemic. In addition, more people and animals are moving through the Darién Gap these days.

The rapid spread of the plague recently is likely due to illegal and thus uncontrolled animal trade. The screwworm fly has traveled thousands of kilometers; it has been detected just 600 kilometers from the Texas border. Over 5,000 cases of infected animals have already been reported in Mexico.

In the US, people are now trembling in fear of the plague. "It could devastate the livestock industry," said Texas Governor Greg Abbott at a press conference in mid-August. According to the U.S. Department of Agriculture, the cost of an outbreak in Texas alone could reach $1.8 billion. U.S. Secretary of Agriculture Brooke Rollins even called the screwworm fly a national security threat. "If our livestock farmers are overrun by foreign pests, we won't be able to feed ourselves."

The US now wants to eradicate the New World screwworm fly once again. The problem: The production facility in Panama can produce a maximum of 100 million sterile flies per week. According to experts, however, at least 500 million per week are needed to suppress the parasite. Therefore, construction is now underway. A new facility is scheduled to open in Mexico in 2026. Maggots will also be sterilized for the first time in Texas. However, it will take another two to three years until then.

Scientists are therefore also working on new methods to combat the screwworm fly: genome editing, for example, to breed exclusively male screwworms. Or traps that attract the flies with specific smells. To reduce the risk of introduction, the US also stopped importing cattle from Mexico months ago. Normally, more than a million cattle are sold across the border annually.

Cochliomyia hominivorax, the Latin name for the New World screwworm fly, translates as "man-eater." Human infestation is very rare but potentially fatal. Last week, the first case was reported in the United States. The person had previously visited El Salvador and is now doing well.

In Mexico, over 40 cases have already been reported this year, and an 86-year-old woman with underlying health conditions has died. Dozens of people have also been affected in Costa Rica, Guatemala, Honduras, Panama, and Nicaragua. The greatest risk comes from open wounds, but the fly also lays its eggs in mucous membranes, such as the nose.

The parasite can also nest in cats and dogs, says entomologist Phillip Kaufman. "The fly lays its eggs in all mammals." But he's particularly concerned about wildlife. "In the late 1960s, when we last eradicated the screwworm fly, there were a good 50,000 white-tailed deer in all of Texas. Today, there are five and a half million." Unlike cattle, no one would likely treat the deer. Their movements are also difficult to control. The US government plans to deploy more mounted patrols along the border to look for infected wildlife.

Could the parasite also appear in Central Europe? Experts doubt this. "It's a tropical species of fly. It spreads to temperate zones like here in Texas and retreats again when temperatures drop," says Kaufman. The New World screwworm fly doesn't like low temperatures; it dies in frost. Therefore, it wouldn't find the right climatic conditions in Central Europe. Rancher Clyde Sommerlatte also hopes that winter will halt its advance northward for the time being.