The risk to humans from the fly, particularly in the U.S., is relatively low, but experts say infestations can happen.

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

The New World Screwworm has rapidly spread 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 "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.

There could be grave economic consequences should the New World Screwworm get out of hand among U.S. livestock, leading to animal deaths, decreased livestock production, and decreased availability of manure and draught animals, according to the U.S. Department of Agriculture.

"It is not only a threat to our ranching community -- but it is a threat to our food supply and our national security," the USDA said.

Already, in May, the USDA suspended imports of live cattle, horse and bison from the Mexican border because of the parasitic fly's spread through southern Mexico.

The New World Screwworm isn't new to the U.S.

But it was considered eradicated in the United States in 1966, and by 1996, the economic benefit of that eradication was estimated at nearly $800 million, "with an estimated $2.8 billion benefit to the wider economy," the USDA said.

Texas suffered an outbreak in 1976. A repeat could cost the state's livestock producers $732 million a year and the state economy $1.8 billion, the USDA said.

Historically, the New World Screwworm was a problem in the U.S. Southwest and expanded to the Southeast in the 1930s after a shipment of infested animals, the USDA said. Scientists in the 1950s discovered a technique that uses radiation to sterilize male parasitic flies.

Female flies that mate with the sterile male flies produce sterile eggs, "so they can't propagate anymore," Chin-Hong said. It was this technique that allowed the U.S., Mexico and Central America to eradicate the New World Screwworm by the 1960s.

But the parasitic fly has remained endemic in South America, Cuba, Haiti and the Dominican Republic.

In late August, the USDA said it would invest in new technology to try to accelerate the pace of sterile fly production. The agency also said it would build a sterile-fly production facility at Edinburg, Texas, which is close to the Mexico border, and would be able to produce up to 300 million sterile flies per week.

"This will be the only United States-based sterile fly facility and will work in tandem with facilities in Panama and Mexico to help eradicate the pest and protect American agriculture," the USDA said.

The USDA is already releasing sterile flies in southern Mexico and Central America.

The risk to humans from the fly, particularly in the U.S., is relatively low. "We have decent nutrition; people have access to medical care," Chin-Hong said.

But infestations can happen. Open wounds are a danger, and mucus membranes can also be infested, such as inside the nose, according to the CDC.

An infestation occurs when fly maggots infest the living flesh of warm-blooded animals, the CDC says. The flies "land on the eyes or the nose or the mouth," Chin-Hong said, or, according to the CDC, in an opening such as the genitals or a wound as small as an insect bite. A single female fly can lay 200 to 300 eggs at a time.

When they hatch, the maggots -- which are called screwworms -- "have these little sharp teeth or hooks in their mouths, and they chomp away at the flesh and burrow," Chin-Hong said. After feeding for about seven days, a maggot will fall to the ground, dig into the soil and then awaken as an adult fly.

Deaths among humans are uncommon but can happen, Chin-Hong said. Infestation should be treated as soon as possible. Symptoms can include painful skin sores or wounds that may not heal, the feeling of the larvae moving, or a foul-smelling odor, the CDC says.

Patients are treated by removal of the maggots, which need to be killed by putting them into a sealed container of concentrated ethyl or isopropyl alcohol then disposed of as biohazardous waste.

The parasitic fly has been found recently in seven Mexican states: Campeche, Chiapas, Oaxaca, Quintana Roo, Tabasco, Veracruz, and Yucatán. Officials urge travelers to keep open wounds clean and covered, avoid insect bites, and wear hats, loose-fitting long-sleeved shirts and pants, socks, and insect repellents registered by the Environmental Protection Agency as effective.

California issues advisory on a parasitic fly whose maggots can infest living humans