California Issues Advisory for New World Screwworm

California Issues Advisory for New World Screwworm

We have been covering the battle to control the New World Screwworm (NWS, Cochliomyia hominivorax) , a flesh-eating parasitic fly whose larvae infest open wounds of warm-blooded animals, including livestock, wildlife, pets, and occasionally humans, causing severe tissue damage and often death if left untreated.

In our last report, the first U.S. case of screw worm infection in a human was reported in Maryland. Apparently, the patient had recently visited El Salvador.

Now the state of California is warning people that more infestations of this particular parasite are a possibility.

After the Maryland incident, the California Department of Public Health decided to issue a health advisory this month warning that the New World Screwworm could arrive in California from an infested traveler or animal, or from the natural travel of the flies.

Graphic images of New World Screwworm infestations show open wounds in cows, deer, pigs, chickens, horses and goats, infesting a wide swath of the body from the neck, head and mouth to the belly and legs.

The Latin species name of the fly — hominivorax — loosely translates to “maneater.”

“People have to be aware of it,” said Dr. Peter Chin-Hong, a UC San Francisco infectious diseases specialist. “As the New World Screwworm flies northward, they may start to see people at the borders — through the cattle industry — get them, too.”

Other people at higher risk include those living in rural areas where there’s an outbreak, anyone with open sores or wounds, those who are immunocompromised, the very young and very old, and people who are malnourished, the U.S. Centers for Disease Control and Prevention says.

But, hey…this is California! How is one more blood-sucking parasite going to matter?

California issues advisory on a parasitic fly whose maggots can infest living humans https://t.co/zeb5wPSsvS

— Los Angeles Times (@latimes) September 17, 2025

Strangely, much of the California Department of Public Health’s warning is focused on people who have recently come from areas where infestations are prevalent, such as Central America and Mexico.

Meanwhile, the Trump administration’s Department of Agriculture (USDA) already has a response team in Mexico fighting to contain the spread with sterilized flies. It appears they are not relying entirely on the resources or data from Mexico.

Ag Secretary Brooke Rollins says the USDA now has staff on the ground in Mexico to help in the fight against New World screwworm. “We are not just able to accept the data from our friends in Mexico,” he says. “We hope to always be able to trust their data, but this is too important. We’ve got to verify the data.”

She tells Brownfield that building a strong working relationship with Mexico has been critical. “When I first started, we couldn’t even land our planes several days of the week, which had the flies, on the Mexican side of the border,” she says.

Rollins says work has already begun to retrofit the facility in Mexico. “Hopefully that comes online very soon,” he says. “That is our best guess, 50 to 60 million more fruit flies a week.”

And she says there is the larger facility that is located at the former Moore Air Force Base in South Texas. “That’s probably 12 months away,” she says. “Which again, for the short term, is not helpful, but we’ve got to get moving and get that online for the future, and we’re already moving in that direction.”

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