Flesh-eating screwworm larvae poised to invade the US have snuck into Maryland via the flesh of a person who had recently traveled to El Salvador, upping anxiety about the ghastly—and economically costly—parasite.

Reuters was first to report the case early Monday, quoting Andrew Nixon, spokesperson for the US Department of Health and Human Services, who said in an email that the Centers for Disease Control and Prevention had confirmed the case on August 4 in a person who had returned from a trip to El Salvador.

While other outlets have since reported that the screwworm case found in Maryland is the first human case in the US, or first travel-related case in the US, or the first case in years—none of those things are true. Screwworms are endemic in parts of South America and the Caribbean and travel-related cases have always been a threat and occasionally pop up in the US. While the CDC doesn’t keep a public tally of the cases, experts at the agency have noted several travel-related human cases in the US in recent years, including one as recent as last year.

The new case in Maryland doesn’t change anything in the US. “The risk to public health in the United States from this introduction is very low,” Nixon wrote to Reuters. But, what has changed is that the risk of an incursion at the US-Mexico border is no longer low—in fact it’s rather high currently.

Savage parasites

Screwworms were once endemic to the US before a massive eradication effort that began in the 1950s drove the population out of the US and Central America. The flies were held at bay with a biological barrier of constant releases of sterile male flies along the Darién Gap at the border of Panama and Colombia. The flies were declared eradicated from Panama in 2006. But, in 2022, the barrier was breached and the flies have worked their way back up through Central America, including El Salvador, since then. Now they are merely 370 miles or less from the Texas border, and state and federal agencies are preparing for an invasion, including with plans to build a sterile fly facility in the state.

(The latest human case may mark a “first” in the US in that it may be the first travel-related case from a country in Central America experiencing an outbreak since the previous eradication.)

Screwworms—or technically New World Screwworms (NWS)—are parasitic flies that spawn hundreds of ravenous larvae in the wounds and orifices of a wide range of warm-blooded animals. The parasites get their names from the larvae, which look and act like screws, boring and twisting into their victim’s flesh. As the larvae feast on living flesh, they create repulsive, excruciatingly painful festering wounds that can easily turn fatal in wild animals and livestock. The US Department of Agriculture has estimated that an outbreak of screwworms in Texas—a major cattle-producing state—could cause $1.8 billion in economic damage.

While the infection is typically not fatal in humans, it is still extremely painful and gruesome. Adult female flies can lay up to 400 eggs at a time, and each larva in a human patient has to be physically dug out from infected flesh. As CDC Medical Officer Rebecca Chancey remarked in a clinical presentation last October about their resurgence, the larvae are “pretty tenacious and hang on pretty tightly, so oftentimes, you know, a great deal of force is required to remove them.” After that, treatment can involve removing necrotic tissue, cleaning the wounds, and treating for secondary infections while trying to manage the pain.

Gruesome cases

While details of the case in Maryland are not known, the CDC in that presentation last year laid out details of three other travel-related cases. One case in 2024 was in a Florida man who had traveled to the Dominican Republic and unknowingly had a screwworm fly lay eggs in his nose. The man had previously had a cancerous tumor removed from his nose and was immunosuppressed. Back in Florida, his face abruptly began swelling, and he developed constant nose bleeds. When he went to the hospital, doctors were shocked to find his nose and sinus cavities erupting with 100 to 150 larvae.

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