Screwworm infestation kills endangered deer

Eradication effort aimed at flesh-eating larvae in Florida Keys
Published on January 04, 2017
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More than 130 endangered deer have been killed by a decades-absent parasite, now a resurgent organism in the Florida Keys.

The deaths from the New World screwworm, a type of fly larvae that eats living flesh of warmblooded animals, had slowed by mid-November 2016. But U.S. Fish and Wildlife Service officials are preparing measures to save the Key deer if their population falls to a critical level.

Prior to the 2016 infestation, the Key deer population was estimated at 800 to 1,000. Found only in the Florida Keys, they are the smallest subspecies of white-tailed deer, the largest males standing 1 meter tall at the shoulder, according to FWS information.

A Key deer (Courtesy of U.S. Fish and Wildlife Service)

The FWS received increasing reports of sick Key deer, some with maggot-infested wounds, starting in early July 2016. Authorities from the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service announced Oct. 3 they had confirmed screwworm infections among deer on Big Pine Key in the National Key Deer Refuge, and some pets in the area may have had infestations over the preceding two months.

Since then, two dogs, two cats, one pig, and one raccoon in the Keys also had positive test results for screwworms. No human infestations have been reported.

Infestations had been confined to the Keys through early December. Dr. Michael Short, Florida state veterinarian, said the state has investigated 15 reports of potential infestations on the mainland in animals ranging from horses to a rat, all with negative test results.

Self-sustaining screwworm populations had been eradicated in the southeastern U.S. by 1959 and the entire U.S. by 1966, although re-infestations occurred until 1982, APHIS information states. Two isolated infestations were identified in two dogs in 2007 and 2010, and both had traveled to or through Florida.

Eradication and re-eradication

FWS officials announced Nov. 24 that Key deer reproductive organs were being collected from roadkill in an effort to preserve the deer’s genetic material, which could be used to save the species. Population surveys would be used to decide if and when the remaining deer should be placed in enclosures as a last resort.

Since that announcement, three deer have had confirmed infestations, according to state information.

Key deer are the only large herbivore in the Keys, FWS information states. Poaching and habitat loss reduced the population to a few dozen deer by the 1950s. The Key deer refuge was established in 1957, and the deer were listed as endangered in 1967.

Between Oct. 6 and Dec. 2, 2016, the USDA released more than 45 million sterile screwworm flies in the Keys, according to Tanya Espinosa, an APHIS spokeswoman. A female screwworm fly mates one time, and sterile fly releases have been used to eradicate screwworms in North America and Central America.

Screwworm larvae (Courtesy of USDA APHIS)

U.S. and Panamanian governments together produce and release more than 2 billion sterile pupae each year near the Panama-Colombia border to prevent spread of the flies into Central America. The Panama USDA facility is producing sterilized pupae that are flown to Florida and placed in chambers that are hung from trees in areas where the developed flies can search for mates, Espinosa said in a message.

“We will continue eradication efforts until we are assured that screwworm is no longer here in the Florida Keys,” she said. “We will continue releasing sterile flies until we are sure that the only screwworm flies in the Florida Keys are sterile screwworms.”  

Dr. Short said he has received no indication how long the eradication program will last, but it would be “months, not weeks.” His department has tentative plans through May, and the state will rely on expert assessment and surveillance findings. 

Information from the Florida Department of Agriculture and Consumer Services indicates state and federal authorities were trapping flies to test the extent of infestation, performing surveillance for infested animals, and maintaining a quarantine zone with a checkpoint for inspecting animals leaving the Keys.

About 7,500 animals, mostly dogs, had been examined at the checkpoint by Dec. 19 without any observed signs of infestation. Cats, chickens, parrots, horses, and an ape were among other animals checked.

On the mainland, state agriculture officials are educating animal owners about the risks of screwworm infestation, promoting a hotline for reporting suspected infestations (1-800-HELP-FLA), and conducting surveillance at agricultural markets and other animal concentration points, Dr. Short said. 

Cattle live as far south as Miami-Dade County, but Dr. Short said surveillance has indicated those cattle are at low risk of infestation. Far more cattle live to the north in central Florida, according to USDA statistics.  

Screwworm development 

Espinosa of APHIS said it is important that veterinarians contact their state veterinarian if they see suspicious or unusual clinical signs, including maggots in the tissue of a living animal. 

Dr. Charles M. Hendrix, a professor of parasitology at Auburn University, said fly larvae are common in animal wounds, although houseflies, blowflies, and flesh flies tend to lay eggs in contaminated wounds, while screwworms lay eggs in clean and uncontaminated ones and mucous membranes. He teaches his students that screwworms are easily identified by examining the dorsal surface of the posterior end of third-stage larvae, which are distinct for their blackened parallel tubes ending in the spiracular plate. 

“If you do see those two tubes—those two tubes with the spiracular plate at the end of them—you do have to report it,” he said. 

Screwworm flies (Courtesy of Pam Manns/USDA APHIS)

Screwworm flies are metallic blue, blue-green, or gray, with orange eyes and three dark stripes on their backs, APHIS information states.

Eggs hatch within a day into larvae that feed on the animal for up to one week, APHIS information states. Mature larvae drop to the ground to tunnel and re-emerge as adults.

Screwworms in their early stages can be difficult to see.

“The most obvious sign is a change in the wound’s appearance—as larvae feed, the wound gradually enlarges and deepens,” APHIS information states. “An infested wound also gives off an odor and some bloody discharge.

“Even if the actual wound on the skin is small, it could still have extensive pockets of screwworm larvae beneath it.”

Infestation can kill an untreated animal in one or two weeks.

Related JAVMA content:

Screwworms found in Florida (Nov. 15, 2016)