A full-grown screwworm looks like any other fly. It is an insect small in size, dark, six-legged and compound-eyed. The peculiar horrors of its life cycle, however, could have been cooked up by the likes of David Cronenberg or H.R. Giger. A pregnant screwworm seeks out the bodies of much larger animals, and, upon finding an open wound or other fleshy crevasse, delivers her eggs. When they hatch, the screwworm maggots earn their name, carving corkscrew burrows into the skin to grow fat off their host.

Until the United States managed to eradicate the flies in the late 1960s, they were a devastating agricultural pest. Mentioning the screwworm “sends shivers down every rancher’s spine,” to hear Adam H. Putnam, Florida’s agriculture commissioner, tell it.

But they are back, at least in a corner of Florida. Putnam, speaking in a statement, was the bearer of grim news: A screwworm infection broke out in a population of Florida’s wild Key deer, a subspecies of the far more common white-tailed deer, the federal Agriculture Department’s National Veterinary Services Laboratories confirmed Monday.

It is the first time in three decades the screwworm has infested a group of animals in the United States, and the first time in 50 years the insect appeared in Florida.

“This foreign animal disease poses a grave threat to wildlife, livestock and domestic pets in Florida,” Putnam said. “Though rare, it can even infect humans.”

In the age of mosquito-borne West Nile, malaria and Zika, it should not come as a surprise that invertebrates can be lethal. The New World screwworm is different, however, in that it does not harbor a virus or other deadly microbe within its exoskeleton. The fly itself is the killer.

“Unlike most barnyard flies, its larvae feed on living tissue,” the New York Times wrote in 1977. “They can kill a fully grown steer in 10 days. Last year, they infested and killed an elderly woman in San Antonio who could not get help nor care for herself.”

The New World screwworms infesting the Florida deer were not supposed to be in the United States. In the 1950s, the USDA embarked on an ambitious project to rid the country of the agricultural pest. Its plan was a bit closer to “kill it with fire” than “shoo, fly”  with gamma radiation and X-rays supplying the fire.

The government raised young screwworms by the millions, and bombarded the larvae with gamma and X-rays. Thus rendered infertile, the adult flies were released en masse across the Southeast and West. By the end of the 1950s, a “fly factory” in Sebring, Fla., churned out 50 million sterile flies a week. Unable to find fecund mates, the U.S. screwworm population crashed, first in pockets and then across the country.

By the end of the 1960s, the fly had vanished from the United States. In each subsequent year, the lack of screwworms has saved the livestock industry $900 million, according to a U.S. Department of Agriculture estimate. 

The Agriculture Department also set its sights southward, helping eliminate the flies in Jamaica, Mexico and parts of Central America. Panama marks the “buffer zone” between the fly zones in South America and the screwworm-free north, Edward B. Knipling, the son of the entomologist who came up with the birth control plan, told NPR in June.

U.S. infections since the eradication effort have been isolated cases, typically the result of traveling abroad. In 2007, for instance, a 12-year-old returned to Connecticut after vacationing with her family in Colombia. She ended up in the emergency room, worried about the extreme pain in her scalp. Using “bacon therapy” — a combination of luring out and smothering the flies with pieces of meat placed over the wounds  plus petroleum jelly, doctors removed 142 screwworm larvae from her head.

Such instances aside, the flies were contained below the buffer zone. Until late 2016.

No livestock or human cases have been reported. But in the island refuge of Florida’s Big Pine Key and No Name Key, three samples taken from Key deer confirmed the screwworm infection. Other deer in the area, as well as a few pets, have shown signs of infection over the past two months, the Florida Department of Agriculture and Consumer Services said in the news release. To curb the further spread of the flies, about 40 of the deer were killed, a wildlife refuge manager told the Associated Press.

Florida announced it will once again release sterile flies, and Putnam was optimistic the flies could be beaten twice.

“We’ve eradicated this from Florida before, and we’ll do it again,” he said. “We will work with our partners on the federal, state and local level to protect our residents, animals and wildlife by eliminating the screwworm from Florida.”