Can the rare, tiny Key deer survive a flesh-eating worm?

As the rutting season for the planet’s lone surviving herd of Key deer got underway on Big Pine in late August, staff at the national refuge that manages the toy-sized animals started to notice something strange: the bucks, often wounded on the head and neck during the mating ritual, were turning up with gaping, festering wounds.

The numbers, and gory injuries, only worsened as the season deepened, leading staff to wonder what new peril faced a fragile herd threatened daily by speeding motorists, dogs and South Florida’s relentless development.

In late September, they got their answer: the flesh-eating New World screwworm, an aggressive pest that looks like a house fly. Reaction was swift and unprecedented. The state’s top vet issued a quarantine for all pets and livestock from the southern border of Key Largo to Mile Marker 0 in Key West, nearly the entire island chain. Highway checkpoints were set up and on Tuesday an army of sterile screwworms was released to stop the bugs from reproducing, with releases scheduled for twice a week for the next 25 weeks until the fly is wiped out.

READ MORE: The emotional toll of euthanizing ‘zombie deer’ infested with screwworm

How the fly wound up on Big Pine, a 10-square mile island about a two-hour drive from the mainland, may never be known. Though the worms pop up periodically — in 2010 a Florida vet identified one on a dog that had traveled from Venezuela — the U.S. hasn’t had an outbreak in 30 years. To have one now, attacking one of the globe’s most imperiled species, came as a shock.

“I never suspected in a million years that it would come through my lab,” said Heather Stockdale Walden, a University of Florida parasitologist who identified the larvae sent from refuge biologists on Sept. 30. The national veterinary lab in Ames, Iowa, verified the finding with a second sample.

There’s also concern that the standard treatment method, which uses sterile flies to halt reproduction, may not be enough to control the invasive pest. It’s never before been used on a wild herd that can’t be tagged or easily tracked.

“The [U.S. Department of Agriculture] doesn’t work on wildlife, it’s almost always an ag problem,” said Dan Clark, who heads the Florida Keys National Wildlife Refuges Complex that manages the herd. “To have a wildlife species be the first and then be one so endangered makes it even more complicated.”

As of Wednesday, wildlife managers had confirmed 70 dead deer in a herd estimated at just 800 to 1,000. It’s too soon to tell what toll the outbreak will take or whether it could wipe out the herd, Clark said, but the number of fatalities is also likely low because some deer may have perished unseen in the rugged scrub. Clark, who managed a Midway Atoll refuge where mosquitoes have decimated native forest birds, said endangered species have a long list of such predators managers need to protect them from.

“We really want to see what the fly does,” he said. “The bottom line is there’s certainly concerns. This is an event we haven’t experienced before in the refuge. There's reason to be concerned. But the caveat is we’re still at the height of the rut. What I’m hoping is when the rut tapers off in December, there’s fewer wounds out there.”

Getting the outbreak under control before spring fawning is critical, he said. For now, mostly males are turning up infested. The rutting season will likely continue into November, then taper off. If the flies linger until the spring, the flies could spread to does and newborn fawns with bloody umbilical cords and afterbirth.

For locals used to seeing a steady stream of havoc, from tourists riding manatees to an invasion of 9-pound rats, the gruesome infestation has been unnerving.

Last week, Kim Gabel, a UF horticulturist helping wildlife agents battle the outbreak, woke from a nap to the sound of scratching coming from under her Big Pine house. She followed the noise to a crawl space and found an infested deer, shaking its head wildly as flies circled, clearly frightened and in agony. She immediately called the Key deer hotline.

"It put my heart in the bottom of my stomach," she said. “The only thing you can do with a deer so far gone is put it down.”

The revival of the Key deer has been a success story. The deer, descendants of the mainland white tail deer, trekked to the Keys across a land bridge about 13,000 years ago that eventually sank into the ocean. As the land around them shrank, so did the deer. Today’s deer are about half the size.

Between early hunters and South Florida’s land boom, the deer barely survived into the 20th century. Only about two dozen survived by the mid 1950s. So in 1957, the deer were added to the endangered species list and the federal government created the 84,351-acre refuge. With an aggressive management plan focused on protecting the natural environment — the mangroves and thatch palm berries the deer feed on — managers set out to keep the gentle deer as wild as possible, Clark said.

Speed limits were set and fences erected to address the never-ending flow of traffic up and down the Overseas Highway that poses the biggest hazard. Penalties helped check the good intentions of well-meaning visitors stopping to feed the deer. The herd rebounded. A mascot was born: Big Pine has a Deer Run Bed and Breakfast. Internet tips abound on where to spot the deer. An annual Run With the Deer 5K is staged in the refuge.

What wasn’t anticipated was the screwworm.

The worm is like the Walking Dead of pests, feasting only on the exposed tissue of living warm-blooded animals. Human infections have occurred, but are rare. The flies, with iridescent blue armor and bulging orange eyes, lay their eggs in wounds so the larvae can feed off tissue until they become pupa, drop to the ground and emerge as adults. They generally only fly a couple of miles to find a host, but can fly much farther if conditions are right.

“If you don’t remove them, they’ll just keep eating until they develop,” Walden said. “They’re often called the first fly on the scene because they won’t go into a contaminated wound where there’s other larvae or dead, nasty wounds.”

The fly has been a hugely damaging pest to cattle and other livestock across the southern U.S., South America and the Caribbean, but has been controlled since the 1950s with the invention of mass-produced sterile male flies, said UF entomologist Jennifer Gillett-Kaufman. In Curacao, in an initial field test, 300 flies released weekly wiped out a livestock infestation in six months.

But the treatment has never been used in the wild, making it tougher to track its effectiveness.

“Wildlife are spread everywhere and this can go not only to deer but other wildlife,” Walden said.

If the fly were to spread north, consequences could be dire. The state’s cattle industry is ranked ninth in the nation. As Hurricane Matthew approached South Florida last week, wildlife managers worried a Keys’ landfall could have been disastrous.

“We were all very concerned that there would not be time to inspect all the animals leaving,” Gillett-Kaufman said. “It’s unknown how fast they can spread.”

This week, Florida agricultural agents set up tiny cardboard chambers containing sterile pupa purchased from a production lab in Panama. Flies take several days to grow and emerge to begin mating with wild flies. Because treatment depends on infestation rates and area involved, researchers can’t be sure how long it will take to tackle the outbreak.

The USDA is also using genetic material from Big Pine flies to compare to flies collected from other populations around the world in an attempt to determine the source, said Florida Department of Agriculture spokeswoman Jennifer Meale.

In the meantime, the agency is asking residents need to keep watch for open wounds on pets, avoid unnecessary surgery and email pictures of suspected infestations to AgVeterinarians@FreshFromFlorida.com. Some residents have asked why more can’t be done to treat injured deer rather than euthanize them. But the deer have proved too fragile in the past, Clark said.

“We have a nearly perfect track record of them not surviving,” he said.

Not reporting injured deer is also a concern because “everybody has their favorite pet deer,” Gabel said. But not reporting wounded animals not only prolongs suffering but helps spread the disease, she said.

And knowing whether the sterile flies work to eradicate wild ones will ultimately be “very hard,” Walden added. “I think it’s got a good chance because it’s very logical and very efficient. So it’s as good a shot as we’ve got.”

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