The New World Screwworm Has Reached Human Flesh In the US

A flesh-eating parasite that has devastated cattle herds south of the border has now infected a human in the United States. The New World screwworm was detected in August in a Maryland resident who had recently traveled to El Salvador. It’s one of the first times in decades that a human in the U.S. has been infected by the parasite, and comes months after the USDA shut down all cattle imports from Mexico in an attempt to prevent the screwworm from reaching the country.

Despite all of this, the risk to public health from the screwworm is relatively low, experts say. Although the parasite can potentially infect any mammal, it mostly seeks out non-human animals, cattle in particular. It is not contagious, and the Maryland resident who contracted it has since recovered, officials said.

But this doesn’t mean the U.S. is in the clear. Though it’s not a big risk to humans, New World screwworm is a very big risk to cattle — and by extension, the beef industry.

“While the immediate threat to public health is relatively low, the parasite’s capacity to devastate the cattle industry cannot be overstated,” Dr. Tyler Evans, epidemiologist and former Chief Medical Officer for New York City, told Sentient in an email statement. “Screwworm infestations in livestock lead to immense suffering, rapid herd losses, and billions in potential economic damage if not swiftly contained.”

What is the New World screwworm?

Despite its name, the New World screwworm is a fly. Female screwworms lay their eggs in the wounds and bodily orifices of mammals, and once those eggs hatch, the larvae inside burrow into the muscles of said animals. After a week or so, the fully-developed larvae drop out of the animal and into the soil, where they develop into adult screwworm flies.

When a living creature is infected by screwworm larvae, the resulting condition is called myiasis. When untreated, myiasis can kill the infected host in 1-2 weeks; thankfully, the condition is easily treatable with insecticides, larvicides and daily cleaning of the infection.

How big of a problem is screwworm for humans?

Screwworm doesn’t infect humans very often, and isn’t considered a significant public health risk, as it isn’t contagious and is easily treatable. It’s also easy to detect: Todd Thrift, associate professor of animal sciences at the University of Florida, told Sentient in May that when a human is infected with screwworm, they’ll realize something is wrong soon enough to get treated for it.

That doesn’t mean infections are unheard of, though. In Mexico, there have been 41 recorded cases of screwworm in humans over the last year. Mexican health officials say most of the patients have recovered or are being treated, but at least one of them, an 86-year-old woman in the Campeche state, died. However, she didn’t die from the screwworm, but from the skin cancer that she already had, which was exacerbated by the parasite.

The person in Maryland who was infected with screwworm has recovered and now has a clean bill of health, officials say. But a lot of animal hosts aren’t so lucky.

Why is the screwworm such a problem for cattle?

The primary concern regarding New World screwworm is its capacity to infest cattle. As of mid-August, there were over 5,000 recorded cases of screwworm in non-humans in Mexico, most of those cases in cattle — a 53 percent increase from the numbers for the entire month of July.

Unlike humans, cows can’t talk, and thus can’t tell anybody about the symptoms they’re experiencing. Treating a screwworm infestation in cattle first requires a human to detect it early on, but on many cattle ranches, farmers only see each cow once every two weeks, according to Thrift. At that point, infested animals are often past the point of no return.

What’s more, the fact that screwworm larvae lay dormant in the soil after falling out of an animal’s body means that when an infestation is detected, the next generation of screwworms may already be lying in wait, ready to strike again.

Finally, although female screwworms typically fly six to nine miles before laying their eggs, they can fly as far as 125 miles. From a policy standpoint, this means that preventing screwworm from reaching U.S. cattle effectively means eradicating it in Mexico as well.

When did the current outbreak start?

The screwworm was a serious problem on U.S. farms until the 1960s, at which point it was eradicated from North America. For decades afterwards, the screwworm was mostly contained to south of Panama, barring some isolated but quickly contained outbreaks in non-humans in the 21st century.

But in 2023, Panama went from experiencing an average of 25 cases per year, to over 6,500 screwworm detections in one year. Shortly afterwards, New World screwworm was found in Costa Rica, Guatemala, Belize and several other Central American countries. By the end of 2024, it had managed to reach a cow in southern Mexico.

What’s being done to fight the spread of the screwworm?

When the parasite was first detected in Mexican cows at the end of 2024, the USDA allocated $165 million in emergency funds to try to stop its spread to the U.S. But the fly continued to proliferate, and in May, the USDA halted all cattle imports to the U.S. across the southern border. This constriction of supply is one reason why beef has become so expensive in the U.S..

The primary strategy countries used to fight the screwworm in the past is called the sterile insect technique. Female screwworms only mate once in their lives, so scientists sterilize male screwworms en masse and release them into screwworm hotspots, effectively stopping the fly from reproducing within those areas.

This strategy successfully eradicated the screwworm from the U.S. in the 1960s, and since then, the U.S. and Panama have cooperated to regularly release sterilized flies in Panama and beyond, effectively creating a “buffer zone” that, until recently, has prevented the screwworm from moving north.

Currently, the only facility that can create sterile screwworms is in Panama. But that’s about to change: In mid-August, USDA Secretary Brooke Rollins announced that the U.S. will build a sterile fly production facility in Texas. It’s expected to take two to three years to complete the project; meanwhile, Mexico is building its own sterile fly facility, which is set to be finished by 2026.

In addition, the Department of Health and Human Services has allowed the FDA to authorize emergency use of animal drugs off-label to treat the screwworm, if and when it reaches U.S. soil.

The Bottom Line

While “flesh-eating parasite” certainly sounds scary, the screwworm isn’t a serious public health concern for humans. But it is for animals, and because of how deeply embedded the meat industry is in the U.S., it’s a significant economic risk as well. The fly hasn’t yet been detected in any U.S. animals, but if it is, today’s beef prices might soon seem quaint by comparison.