Alex Cerda, veterinary assistant, inspects canine fecal matter to check for parasites at the Oklahoma City Animal Welfare Shelter on Tuesday, July 11, 2023.
California – The California Department of Public Health is urging travelers to take precautions after a rare case of a parasite known as the New World screwworm was detected in the United States this summer. Though the warning is precautionary, health officials say the risks—both to humans and to livestock—are serious enough to merit attention.
The parasite is small, but its impact can be severe. Less than an inch long, screwworm larvae burrow into wounds and feed on living tissue, creating painful sores that can spread if left untreated. The case that prompted the new warning was found in Maryland, in a traveler returning from abroad. Now California officials want physicians and residents alike to recognize the signs of infection should it appear closer to home.
Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco, did not mince words. “It’s so gross,” she said. The process begins when a female fly lays eggs inside even a minor cut—a nick from shaving, for example. Once the eggs hatch, the larvae can dig deeper into the skin. “It can really be very painful,” Gandhi explained.
For humans, the condition is rare but treatable if caught early. For livestock, however, the stakes are much higher. In the 1930s, the parasite spread across the American South, devastating cattle herds. Ranchers inundated the Department of Agriculture with pleas for help as animals suffered gruesome injuries. Eventually, a coordinated fly control program wiped out the screwworm in the United States by 1966, and eradication efforts expanded into parts of Central America and southern Mexico.
But the story didn’t end there. In recent years, outbreaks have reappeared in Central America, raising fears that the parasite could return to the U.S. In August, the Department of Agriculture announced new measures to help Mexico fight the fly and prevent its spread northward.
For Californians, the warning is as much about prevention as it is about awareness. Gandhi emphasized that bug spray won’t stop the parasite. “It’s more about covering your wounds,” she said. Travelers visiting regions where screwworm remains active should take care to keep cuts and scrapes clean and protected, especially when near livestock.
The Department of Public Health has also alerted doctors across the state to be vigilant. Because the symptoms can mimic other infections at first, early recognition is key.
For now, there are no reported cases in California. But the health department’s warning serves as a reminder that even a long-eradicated threat can find its way back. The screwworm may be small, but history has shown how destructive it can be when overlooked.
