Even the name is grisly — but the behavior is even worse.
The parasite known as the New World screwworm, a type of fly larva that feeds on living flesh and can be fatal to its victims, has been confirmed as infecting a human in the United States, according to Reuters.
While human infections are vanishingly rare, the discovery has the nation’s beef industry on edge.
First human screwworm case linked to travel confirmed in U.S., HHS says https://t.co/CcMYRftQor
— CBS Evening News (@CBSEveningNews) August 25, 2025
As the Associated Press reported, screwworms are a species of fly with females that seek out orifices of living mammals to lay their eggs — such as the eyes, nose, mouth, or bleeding open wounds.
When the eggs hatch, the larvae then bury into the flesh of the host organism with sharp teeth that burrow in like screws — hence the name.
Screwworms have been recorded in the United States since the mid-19th century, according to the U.S. Department of Agriculture.
However, the flies were largely eradicated by the mid-1960s, with only periodic outbreaks occurring among animals. The most recent one was in 2016 when it hit the Key West deer population. That outbreak was controlled by 2017, according to the USDA.
Should US citizens returning from countries with parasites like this also be tested when they return home?
It’s a particular danger to the cattle industry, where animals are at risk of infection. In November, the USDA banned Mexican cattle imports because of the presence of the screwworm in northern Mexico, but the ban was lifted in February, as KSHB-TV in Kansas City, Missouri, reported.
In May, the USDA banned the import of cattle and other livestock from Mexico due to what a news release called the “unacceptable northward advancement” of New World screwworm.
That ban was eased at the end of June, with the USDA explaining in a news release that “progress has been made in several critical areas” in controlling the spread.
Only last week, the Department of Health and Human Services issued a declaration that represented the first steps toward getting emergency use authorization for drugs to treat animals infected with New World screwworm.
There are currently no Food and Drug Administration-approved drugs for treatment, the HHS declaration said.
The current method of treating infection in humans is physically removing “hundreds of larvae” and disinfecting the wounds, Reuters reported, describing the method as “onerous.” (“Painful” sounds like it would do just as well.)
The chief method of fighting the fly’s spread, according to NPR, is to raise millions of male flies, sterilize them with radiation, then release them to the targeted screwworm population.
The male flies then mate with the female flies, which lay eggs. But the eggs don’t hatch.
“If the females on the ground mate with a sterile male, at least with a screwworm, that’s all they’ll mate with… so that female won’t produce any offspring,” Max Scott, a professor in the Department of Entomology and Plant Pathology at North Carolina State University, told NPR.
According to Reuters, beef industry sources said the individual had traveled to Maryland from Guatemala.
However, the Department of Health and Human Services said the individual was returning to Maryland from a trip to El Salvador, Reuters reported.
The condition of the individual was not made public.
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