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UF study shows mosquitoes bite manatees and may give them viruses

Tyler Treadway
Treasure Coast Newspapers

Speeding boats, freezing temperatures and fishing nets: Florida's manatees already have enough threats on their lives.

Now add to the list mosquito-borne viruses.

The first record of mosquitoes biting manatees is revealed in a new study by Lawrence Reeves, an entomologist at the Florida Medical Entomology Laboratory in Vero Beach.

There's no proof any manatees have died from mosquito-borne viruses, Reeves said, "but this study paints the picture that they are susceptible to these viruses."

Among the various forms of wildlife that can be seen at Round Island Riverside Park are manatee, which frequent the park during the cooler months.

Mosquito-borne viruses "should be considered suspects" in cases where the cause of manatee deaths haven't been determined, Reeves said.

No resistance

Manatees generally have strong immune systems, Reeves said. But viruses such as West Nile and Eastern equine encephalitis are recent arrivals in America, and manatees haven't had the chance to build up resistance to them yet.

Among the 415 manatees that died this year as of Aug. 14, the causes of 68 deaths have not been determined, according to the most recent data from the Florida Fish and Wildlife Conservation Commission. Last year, it was 74 out of 373, FWC data shows.

The FWC lab in St. Petersburg, where necropsies are performed on manatee carcasses, never has found animals with mosquito-borne viruses, said Dr. Martine de Wit, a veterinarian at the lab, but it doesn't routinely test for them.

"I cannot 100% rule out that it does not occur or will be a threat in the future," she said, "because ... we have a significant number of carcasses with undetermined cause of death." 

A mosquito bites into a manatee Dec. 15, 2015, at Flamingo Marina in Everglades National Park.

Undetermined causes of death usually result because the carcasses are too decomposed, she said.

Still, the carcasses the lab does examine "are a good representation of what is going on health wise in the manatee population," she said, "so I would be surprised if (mosquito-borne viruses were) a significant health threat that we have missed so far."

Wide-ranging threat

Mosquito-borne viruses pose a threat not only to people, but have caused disease and deaths in birds, horses and wildlife. Captive orcas reportedly have been killed by mosquito-borne viruses, and there is evidence of infections found in wild dolphins in the Indian River Lagoon.

Mosquitoes are known to bite alligators, crocodiles and turtles, Reeves said.

Manatees frequently come to the surface of the water, sometimes with just their snouts out to breathe, but often with their backs exposed to the air, at least for short periods of time. It's enough time for mosquitoes to strike.

While studying mosquitoes in the Everglades, Reeves shot photos of mosquitoes biting the backs of manatees basking in the water near the Flamingo Marina.

"Manatees have pretty tough skin and a layer of blubber," Reeves said, "but if they can bite through crocodile skin, they can get through manatee skin."

A mosquito uses the sharp tip of its strawlike mouth to pierce the skin, "looks around for a blood source," Reeves said, and draws blood up through its mouth. As it does this, it injects saliva containing an anticoagulant to keep the blood flowing.

Whether it sucks blood or not, an infected mosquito can put the virus into the host's body along with the saliva, Reeves said.

"I hope the study raises a red flag saying, 'Here's one more thing that can kill manatees,'" Reeves said.

Save the manatees

The Florida manatee population has grown to at least 7,500 animals. The species was reclassified in May 2017 from "endangered" to "threatened" under the federal Endangered Species Act.

Manatees are protected by the Florida Manatee Sanctuary Act and federally by both the Marine Mammal Protection Act and the Endangered Species Act.

Here are some FWC tips for boaters and manatee watchers:

  • Obey posted speed zone signs while in areas known to be used by manatees.
  • Wear polarized sunglasses to reduce glare on the surface of the water so you can see manatees more easily.
  • Look for manatee "footprints," swirls or flat spots on the water created by a manatee’s tail.
  • Stay in deep-water channels whenever possible.
  • Avoid boating over sea grass beds and shallow areas where manatees are often found.
  • Keep powerboats at least 50 feet away from manatees.
  • Don’t throw monofilament line, hooks or any other litter in the water; ingesting or getting entangled in debris can be fatal to manatees and other wildlife.
  • Don't feed or give freshwater to manatees.
  • Report dead or injured manatees at 888-404-3922.

Tyler Treadway is an environment reporter who specializes in issues facing the Indian River Lagoon. Support his work on TCPalm.com.  Contact him at 772-221-4219 and tyler.treadway@tcpalm.com.