Feline leukemia virus threatens endangered panthers

Published on May 15, 2004
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The Florida panther, a subspecies of the mountain lion, is one of the most endangered mammals on earth. 

A new threat has emerged in the swamps and forests of southern Florida to challenge the state's endangered panthers.

Feline leukemia virus threatens endangered panthers

Feline leukemia has been diagnosed in four panthers since November 2002. The infectious, often fatal disease is common to domestic cats but practically nonexistent in large, wild feline species. Two of the diagnoses were made this year alone.

Now, state and federal wildlife teams are hunting the elusive predator in its southern Florida habitat, not to eradicate the panther as in days past. Rather, their aim is to erect an ecologic disease barrier against feline leukemia by vaccinating as many panthers as they can locate.

Among the dangers confronting Florida panthers, infectious diseases are relatively low on the list, said wildlife veterinarian, Dr. Mark Cunningham. Intraspecific aggression—attacks by other, usually male, panthers—tops the threat list, followed by motor vehicle collisions, he said.

For the past three years, Dr. Cunningham has been part of the Florida Fish and Wildlife Conservation Commission's ongoing efforts to protect the panthers and preserve their habitat.

A subspecies of the mountain lion, the Florida panther has been on the U.S. Endangered and Threatened Species List for more than three decades. With a mere 70 to a hundred panthers remaining, the panther is one of the most endangered mammals on earth.

Like its cousins in the western United States and parts of Canada, the Florida panther is a large, carnivorous cat with a long, dark-tipped tail and relatively small head. The upper parts of its body vary in color from light brown to tawny, with a dull white underside.

Mature males examined in the wild in Florida have weighed as much as 154 pounds and measured nearly 7 feet from nose to tail tip. Females are smaller, weighing up to 108 pounds and measuring almost 6 feet.

The panthers' habitat begins just south of Lake Okeechobee. It encompasses roughly a million acres of swamp, marsh, and forest, and includes Big Cypress National Preserve and Everglades National Park.

One reason for the panther's precarious status is its confinement to a limited region bounded by human settlements. When an infectious disease emerges in a naive population of animals living in proximity to one another, there is cause for alarm.

"It's very concerning to us, especially since the Florida panther is such an isolated species," said Brian Call, president of the Friends of the Florida Panther Refuge. Call worries that the feline leukemia outbreak will spread, and a large number of the rare panthers will fall prey to the disease.

Dr. Cunningham said the panther population has been routinely tested for feline leukemia virus since 1983. The panthers tested were seronegative until November 2002, when the first infection was diagnosed.

The most recent infection was discovered this March. Dr. Cunningham described the four feline leukemia cases in two male and two female panthers ranging from 2 to 11 years of age; two succumbed to disease complications, whereas panthers killed the others.


U.S. Fish and Wildlife Service personnel band a tranquilized panther for research purposes.

Although no one knows for certain how the disease was introduced, several Texas cougars released in southern Florida in 1995 as part of preservation efforts were suspected. But they had tested seronegative for the virus before their release, Dr. Cunningham said, adding that the most likely culprit was an infected domestic cat. 

A retrospective study of antibody titers from archived panther serum indicated significant viral exposure beginning in the mid- to late '90s and steadily increasing. Nearly all the exposed panthers are concentrated in Hendry County within the northern portion of the panthers' range, Dr. Cunningham observed.  

"So the virus was brewing out there for at least five years before it showed up in 2002," he said.  

Several institutions are lending a hand in researching the feline leukemia virus responsible for the outbreak. Viral samples are being cultured at The Ohio State University, the National Cancer Institute is sequencing the virus, and antibody titers are being studied at a veterinary laboratory in Dixon, Calif.  

Dr. Cunningham explained that the state Fish and Wildlife Conservation Commission has been capturing panthers annually for several years. The cats are fitted with radio collars to track their movements and are vaccinated for rabies and other feline viruses, but not feline leukemia.  

The disease outbreak prompted state wildlife officials last year to order a trial of feline leukemia vaccine on captive panthers. The biologic was successful, and the vaccine was added to the cats' health care regimen, with vaccinations beginning in Hendry County. 

"We're vaccinating to the south in a band between the positives and the rest of the population," Dr. Cunningham said. "The first step is to create a boundary of vaccinated (panthers). As we get a higher proportion vaccinated there, then we'll move to other areas."  

Two capture teams of five members apiece—one team state, the other federal, since the panthers also roam on federal lands—use trained dogs to track the panthers, which prefer hiding in a tree to confrontation. The risks to the animal are considerable, so each team includes a veterinarian and a hundred pounds of emergency equipment, including water, drugs, and oxygen. 

Once a panther is treed, it is shot with a tranquilizer dart. The anesthetized panther falls into a net suspended by team members waiting at the base of the tree. The cat is then examined, measured and weighed, vaccinated, and fitted with a radio collar. That's when everything goes according to plan, however. 

"There are a lot of risks in running a 150-pound carnivore up a tree and then darting it and catching it in a net. There's a lot that go wrong," Dr. Cunningham said. 

"A lot of times, we'll dart them and then they'll jump out of the tree and run. Their body temperature can go way up before they become immobilized, and then they're not blowing off the heat." 

In addition to being injured or suffering from hyperthermia, there's a chance of a darted panther drowning. "We're working in swamps a lot, so there's a risk there of the cat jumping out (of the tree) and going down in water," he said. 

Still, less than 2 percent of the panthers have suffered major injuries as a result of the capture program, Dr. Cunningham said. "It's dangerous, but we try to take every precaution possible," he said. 

The human members of the capture teams are in little danger. It's a different story for the tracking dogs, though. In a few cases, a panther has bitten a dog, but by far, the greatest threats to the dogs are alligators and snakes. 

Eighteen panthers had been vaccinated as of mid-March. "From here on out, this is going to be part of our standard capture procedure, to vaccinate for feline leukemia. We are doing boosters as well," Dr. Cunningham said. 

He thinks the chances that the outbreak is contained are good. "Right now, I'm pretty optimistic about it," Dr. Cunningham said. "I don't want to say that we've turned the corner, but I'm really pleased we've vaccinated as many cats as we have."