Flesh-eating parasite may soon threaten CA. What is the New World screwworm?
A parasitic fly with larvae capable of infesting living humans and animals — including livestock, birds and pets — may soon threaten California, health officials said.
The New World screwworm (NWS) could show up in the Golden State from an infested traveler, animal, or simply from the natural travel of the flies, the California Department of Public Health warned in a new health advisory this month.
The flesh-eating parasite has increasingly been detected in southern Mexico and Central America — then a new case popped up in a cow in northern Mexico, just 70 miles south of the U.S. border with Mexico, the country’s ministry of agriculture confirmed on Sunday, Sept. 21.
What does that mean for California? Here’s what to know about the parasite and how it could affect the Golden State:
What are the symptoms of New World screwworm infestation?
New World screwworm infestations — called myiasis — occur when the fly’s larvae (maggots) “infest the tissue or flesh of warm-blooded animals and people,” according to the Centers for Disease Control and Prevention.
“The adult fly is attracted to wounds or mucous membrane openings, where it will lay eggs,” California’s Department of Health said in the Sept. 4 news release. “The larvae feed on healthy tissue, leading to pain, bloody discharge, and a foul-smelling odor from the site of infestation, with the potential for extensive tissue destruction and secondary bacterial infections.”
Infestations are painful, the CDC said. You may see maggots in or around an open wound — but “they could also be in your nose, eyes, or mouth.”
You also might feel maggots moving around in a “wound or sore, ears, nose, eyes, or mouth,” the agency said.
Infestations are usually accompanied by a “foul-smelling odor,” and you could notice unexplained sores that won’t heal and even get worse within days, the CDC said.
Gruesome images of infestations shared in a gallery by the Animal and Plant Health Inspection Service show open wounds across the bodies of livestock animals.
“People have to be aware of it,” Dr. Peter Chin-Hong, a University of California San Francisco infectious diseases specialist, told The Los Angeles Times. “As the New World Screwworm flies northward, they may start to see people at the borders — through the cattle industry — get them, too.”
People who travel in areas where the screwworms are present, including South America, Cuba, Haiti and the Dominican Republic; people who spend time around livestock in rural areas; and anyone with an open wound are at highest risk of infestation, the CDC said.
Anyone who has traveled to areas where New World screwworms are present in the past 10 days should visit their healthcare provider if they see any signs of infestation, California health officials said.
Has there been a human case of New World screwworm infestation?
The first human case of the flesh-eating parasite was confirmed in Maryland on Aug. 4 in a person who had returned from traveling to El Salvador, federal officials said.
The parasite “has not been found in livestock, pets, wildlife or in the environment” in the U.S.; “no locally acquired cases of NWS infestation in humans have been reported to date;” and “the risk to human health in the U.S. remains very low,” California health officials said in the advisory earlier this month.
Still, the “northward creep from South America has put the country’s cattle industry on high alert in recent months,” NPR reported.
The U.S. Department of Agriculture called it “a devastating pest” and a threat not only to the country’s ranching community — “but it is a threat to our food supply and our national security,” the department said in a news release in August.
This story was originally published September 26, 2025 at 5:00 AM.
