It’s not clear exactly how that stray dog got infested or where it had been before it was found in Homestead. The USDA heard about the animal from a vet in the area. It has since be treated. “It’s a very treatable condition if caught early,” says USDA veterinary medical officer Robert Dickens. “The dog is doing really well.”

Individual animals can get anti-parasite drugs. But the best large-scale weapon against screwworms are sterile screwworms, deliberately sterilized using X-rays in a factory. Release enough of them and they will prevent any of the non-sterile ones from finding a mate.

This was the strategy that eradicated screwworms from the U.S. 30 years ago, and this is the strategy USDA has been using to get screwworms out of the Keys again. The USDA has released 80 million sterile screwworms across 25 sites in the Keys, and now will add Homestead to the list of release sites. On Friday, state and federal teams released sterile screwworms at Homestead, and they will continue doing so twice a week for the next six to nine weeks.

It’s still unclear where the screwworms came from. After the U.S. eradicated the pests, it partnered with countries in South and Central America, releasing sterile screwworms further and further south until they reached one of the narrowest parts of the continent, the Darien Gap in Panama. Here, millions of sterile screwworms are still dumped by the airplane-load to form an invisible but permanent sterile insect barrier.

Perhaps someone or something brought it to the Keys from further south. Perhaps it came from Cuba, Haiti, or the Dominican Republican, which have not eradicated screwworms and are just short expanse of ocean away. Wherever it came from, it eventually reached a stray dog in Florida.