“We had no idea what this thing was,” said Kristie Killam, a ranger at the National Key Deer Refuge, the 9,200-acre center run by the United States Fish and Wildlife Service. “There was a few here and there in July and August, but it was September, when the rut started, that we started scratching our heads and saying, ‘This is a little bit out of the ordinary,’” she said, adding, “It looks like a horribly infected wound, an infected wound that never got treated, infected with mushy maggots.”

Lab test results in late September confirmed the presence of the screwworm. Native to South America, it first appeared in the United States in the 1800s. Screwworms are larvae laid by flies in the open wounds of an animal, where the larvae feed on flesh. The fly is a little bigger than a common housefly, with orange eyes and a metallic dark blue or gray body with three dark stripes down its back.

An animal quarantine was imposed by officials in Florida in early October, and an agricultural emergency was declared to avoid the spread of the parasite.

An infestation in the southeastern United States from the 1930s to the 1950s caused $20 million a year in livestock losses and took 20 years to eradicate, according to the United States Department of Agriculture.

Since that infestation, the federal government has spent millions of dollars in the last century to keep the screwworm out of Mexico and Central America, lauding it as “one of the greatest success stories in the history of agriculture in the Americas” — only to see it reappear in South Florida.

Some critics said the authorities should have reacted faster to the threat. They said the delay could cause the infestation to spread, imperiling the one million head of cattle in Florida.