The United States has reported its first-ever human case of New World screwworm, a dangerous flesh-eating parasite that normally infects cattle and wildlife. Health officials confirmed that a patient returning from Central America brought the parasite into Maryland, raising alarm about public health and livestock safety.

The screwworm is the larval stage of the screwworm fly (Cochliomyia hominivorax). Female screwworm flies lay their eggs in the open wounds of warm-blooded animals—including humans. When the eggs hatch, hundreds of larvae burrow into the living flesh of their host.

Unlike most parasites that feed on dead tissue, screwworm maggots consume healthy, living flesh. Over the course of a week, they enlarge wounds, cause severe tissue destruction, and eventually drop to the ground to pupate before becoming adult flies. If untreated, screwworm infestations can be fatal.

On August 4, 2025, the U.S. Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC) confirmed the first known human case of New World screwworm in the United States.

The patient, who had recently traveled from El Salvador, developed painful sores and required emergency medical treatment in Maryland. While no screwworm cases have been reported in U.S. livestock this year, the discovery has raised concerns about the parasite spreading northward from Central America and Mexico.

Screwworm infestations are not only excruciating but also easy to identify once larvae are present. Common screwworm parasite symptoms in humans include:

If untreated, screwworm infestations can lead to widespread tissue destruction, sepsis, and death.

Treatment is painful but necessary. All larvae must be manually removed from the wound, followed by disinfection and wound care. In humans, doctors also prescribe antibiotics to prevent bacterial infections.

In livestock, insecticides and topical treatments are used to kill remaining larvae and prevent reinfestation. With prompt care, both animals and humans can recover. But untreated cases are often fatal.

This is the first time screwworm has been officially confirmed in a human inside the United States. While eradicated from U.S. soil in the 1960s through sterile fly release programs, the parasite has continued to plague Central America, South America, and parts of the Caribbean.

Adult screwworm flies are capable of flying several miles to find new hosts, making containment difficult. For ranchers, the parasite represents a massive economic risk. Newborn calves, in particular, are highly vulnerable since their unhealed navels are an easy entry point for infestation.

The U.S. Department of Agriculture (USDA) estimates that a new screwworm outbreak in Texas alone could cost cattle producers nearly $1.8 billion in losses, treatments, and trade restrictions.

To prevent outbreaks, U.S. officials are reviving one of the most successful pest-control strategies ever used—the sterile insect technique.

The USDA has announced plans to build a new sterile fly facility in Texas. The program breeds large numbers of male screwworm flies, sterilizes them, and then releases them into the wild. When these sterile males mate with females, no viable eggs are produced, reducing the parasite population over time.

This method successfully eradicated screwworm from the continental U.S. decades ago and remains the most effective long-term solution.

Mexico has also launched its own $51 million sterile fly production plant to fight the northward spread. But with reports of screwworm just 370 miles south of the U.S. border in July, American ranchers are on high alert.

Each year, the U.S. legally imports over one million cattle from Mexico. But illegal cattle smuggling, often linked to organized crime, poses one of the biggest threats for screwworm’s reintroduction. Smuggled animals bypass veterinary inspections, increasing the risk of parasites crossing the border undetected.

While the confirmed Maryland case was linked to international travel, the discovery is a reminder that screwworm parasites can infect humans—not just animals.

Health experts stress that the risk of widespread outbreaks in humans remains low thanks to strong surveillance programs. However, cattle ranchers and veterinarians in Texas and other southern states are bracing for potential infestations.

The U.S. government is moving quickly to contain the threat. But as screwworm spreads north through Mexico, both public health officials and cattle producers will need to stay vigilant.

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