California issues health advisory on a parasitic fly

A parasitic fly whose maggots can infest living livestock, birds, pets and humans could threaten California soon.

- BY RONG-GONG LIN II

THE maggots of the New World screwworm fly burrow into living flesh.

The New World screwworm has been rapidly spreading northward from Panama since 2023 and farther into Central America. As of early September, the parasitic fly was present in seven states in southern Mexico, where 720 humans have been infested and six of them have died. More than 111,000 animals also have been infested, health officials said.

In early August, a person traveling from El Salvador to Maryland was discovered to have been infested, federal officials said. But the parasitic fly has not been found in the wild within a 20-mile radius of the infested person, which includes Maryland, Virginia and the District of Columbia.

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 “man-eater.”

“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.”