Florida fights flesh-eating screwworms

Adult screwworm is actually a fly

By Stephen Anthony Sobek
By Peggy Greb [Public domain], via Wikimedia Commons


In recent months, Florida has faced both Zika virus and hurricanes. Now, the Sunshine State must defend itself against one more natural scourge: flesh-eating worms of unknown origin.

After the US Department of Agriculture confirmed a local infestation of New World screwworm in Key deer on September 30, state Commissioner of Agriculture Adam Putnam declared an agricultural state of emergency in Monroe County, the home of Key West.

Despite its name, the adult screwworm is actually a fly, and it typically lays its eggs in an open wound on an animal. Infected animals usually separate from their herd and, if left untreated, die in seven to 14 days from toxicity or secondary infections.

No human or livestock cases have been reported, according to the Department of Agriculture and Consumer Services. The infestation is geographically limited to the Keys, specifically Big Pine Keys and No Name Keys.

Along with the confirmed cases in the National Key Deer Refuge of Big Pine Keys, a few local pets and additional deer have showed signs of possible infestations over the past two months. Key deer, a subspecies of white-tailed deer, once ranged throughout the lower Keys. They are protected under the Endangered Species Act.

This is the first local infestation in the nation in more than three decades and the first time in 50 years Florida has seen the screwworm, Cochliomyia hominivorax.

"While it sounds like a Halloween joke, it poses a grave threat to the last population of the subspecies of Key Deer," Putnam said. "And if it gets beyond the Keys, it represents an enormous threat to the US livestock industry, because of potential quarantines and trade barriers that could occur if it gets into the livestock population."

The New World screwworm can harm humans and their pets, but it is relatively easy to detect and simple to treat in both, explained Putnam. Though just confirmed within the past month, he said, officials believe this local infestation possibly began as early as July. Though the origin is unknown, previous infestations have been set off by animals entering from other countries.

'It is a horrible death'

The adult screwworm is about the size of or just slightly larger than the common housefly. The screwworm is different in color and appearance, a Technicolor vision with orange eyes and a metallic dark blue to blue-green or gray body, according to the USDA. Three dark stripes run down its back.

Typically, the female screwworm fly mates just once in her lifetime. She lands on or near an open wound or the mucous membranes of an animal's nose, mouth or ears and there lays hundreds of eggs.

The eggs emerge as maggots and begin to feed on the live tissue of the animal, the unwitting host to this parasite, explained Putnam. After the maggots feed till they reach a mature life stage -- five to seven days -- they drop off, burrow into the ground and emerge days later as flies. Then, the cycle begins once again.

In her lifespan, the screwworm fly can produce thousands of offspring.

"The way we interrupt that life cycle is to release high numbers of male flies that have been sterilized," Putnam said. The male flies will mate with multiple females. Tricked into believing that their biological imperative to reproduce has been fulfilled, the females stop breeding and die without laying any new eggs. Eventually, the sterile males die as well.

"That's how you break the life cycle," Putnam said.

The sterile fly technique was developed in the '50s, the last time Florida saw the New World screwworm, which probably originated in South America and moved into North America in the early 1800s. According to the USDA, by the 1930s, screwworms were present in the southeastern United States, causing livestock producers to lose millions of dollars annually.

"I grew up hearing stories about this," Putnam said. "It is a horrible death. The animals suffer terribly."

He noted that the flies are especially attracted to newborn fawns. But adult deer are also vulnerable.

"We're seeing the bucks, the male deer, suffer because this is the rutting season, so they're fighting with each other, which creates the open wounds that attract the screwworm fly to them, and once they have that, it's a very high mortality rate," Putnam said.

Humans and pets

With routine hygiene, it is highly unlikely that a person would develop an infestation, according to Jennifer Meale, communications director for the Florida Department of Agriculture and Consumer Services. That said, anyone who had screwworm "would know," said Meale -- it causes an itch -- and so be able to receive treatment before the worm caused any real health problems. Still, homeless people might be susceptible, and the department has begun outreach to them.

Using fly repellents and keeping skin wounds clean and protected from flies can help prevent screwworm infection in people and animals, according to the USDA's Animal and Plant Health Inspection Service.

An animal health checkpoint has been set up to assess pets and other animals before they are transported out of the Keys. Putnam said officials set up the interdiction station within 48 hours of the positive lab test, though they had to break it down for two days because of Hurricane Matthew.

"Any early detection of screwworm is easily treatable, and the animal can make a full recovery," Meale said. "Pet owners should look at their pets to see whether there are any obvious lesions that are oozing blood or pus or have a foul odor." If you think your animal might be infected, visit a veterinarian.

Meanwhile, animal health and wildlife officials at the state and federal levels are working jointly to eradicate the screwworm.

Among Putnam's greatest worries is the deer.

Unlike with pets, there's no opportunity to observe the Key deer until a screwworm infestation has reached an advanced stage. At that point, euthanizing the animal to eliminate its suffering is the only option.

"They have confirmed somewhere around 60 Key deer that have died as a result of the screwworm," Putnam said. "Now, keep in mind, they believe there are only approximately 1,000 of these animals left on this planet."

Taking into account undiscovered deer may be dying in the wild, "you are looking at a substantial impact to the population just in the last 90 days," Putnam said. "We will respond with overwhelming force to save the deer population."

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