Screwworm threat found in U.S.

By HPN Staff

Federal officials have confirmed the first human case of the New World screwworm (NWS), a parasitic fly whose larvae feed on living tissue, in the U.S. The patient, a Maryland resident who recently traveled to El Salvador, has recovered, and officials say there is no evidence the infection spread to anyone else.

The case comes days after the Department of Health and Human Services issued a declaration allowing the Food and Drug Administration to grant emergency use authorizations (EUA) for animal drugs to prevent or treat NWS in livestock. There are currently no approved drugs for the parasite in the U.S.

It’s estimated that an outbreak could cost as much as $10 billion.

Why it matters

NWS primarily targets livestock, and outbreaks can devastate herds, disrupt the food supply and raise costs. HHS said the risk to humans is “very low” but framed NWS as a national security concern because of its potential economic impact.

The EUA allows veterinarians to use certain drugs — approved for other purposes or in other countries — to prevent or treat screwworm in U.S. livestock. Maryland health officials said the human case is a “timely reminder for health care providers, livestock owners and others to maintain vigilance through routine monitoring.”

Cattle imports from Mexico were halted in late 2024, briefly resumed in February, and suspended again in May. Officials say the restrictions will continue on a rolling basis until containment improves. 

The big picture 

NWS is native to South and Central America and the Caribbean. Adult flies lay eggs in wounds or natural openings. Larvae tunnel into flesh, causing infections that can be fatal if untreated. Livestock, pets and wildlife are most at risk, though humans can occasionally be affected.

This is not the parasite’s first brush with the U.S. In the 1960s and 1970s, American and Mexican officials eradicated NWS through a large sterile-fly program. Since 2022, NWS has been spreading north from Mexico, with more than 3,000 animal cases and 26 human cases reported by July 2025.

To fight the threat, U.S. Department of Agriculture and Texas officials are building the nation’s first sterile-fly production facility at Moore Air Force Base in Edinburg. The $750 million site will produce up to 300 million sterile flies per week and join facilities in Panama and Mexico to curb NWS populations.

Federal action follows pressure from the agriculture sector. Earlier this month, a coalition of 175 industry groups warned a modern outbreak could cost producers $4.3 billion annually and more than $10.6 billion in total economic losses.

Other measures are already underway: enhanced border surveillance, detector dogs, aerial releases of sterile flies and rapid reporting and treatment protocols for veterinarians. Together, officials say, these steps aim to stop NWS before it reaches U.S. herds.

The first U.S. human case highlights the parasite’s return, but does not change the assessment that public health risk is low