Five endangered Florida panthers have died with a disease that was unknown in the wild cats before state scientists discovered it less than two years ago.

About half of 21 cats captured since November 2002 have tested positive for contact with the potentially fatal disease, feline leukemia. Scientists estimate only 80 to 100 of the cats still exist in the wild, exclusively in southern Florida.

Feline leukemia virus damages a cat's immune system, similar to the way HIV affects human health.

However, there are big differences between HIV in humans and feline leukemia, which is believed to have spread to wild cats from domestic breeds.

For one, there is a vaccine for the disease that appears to be effective, so far.

And even if they are not inoculated, about half of panthers that come into contact with the disease appear to be able to overcome it with their own antibodies.

Scientists think Florida's panther population eventually would defeat the disease on its own. But given as few as 80 of the big cats with which to gamble, they aren't taking any chances. They hope that by the end of the coming cool season they will have vaccinated about half of the total panther population — a level of protection that computer modeling predicts will eradicate the disease.

"We're concerned about it," said Mark Cunningham, the panther veterinarian for the Fish and Wildlife Conservation Commission.

"We're hitting it as hard as we can to prevent the loss of as few panthers as we can, and we want to prevent it from becoming established in the population. It's something we need to take seriously, that's for sure," Cunningham said from the FWC Wildlife Research Laboratory in Gainesville.

Scientists found the disease in November 2002 during routine tests on panthers being tracked by the state. They are convinced the disease didn't come from eight Texas cougars introduced to strengthen the Florida gene pool. The outbreak may have resulted from a panther eating a domestic cat, and panthers may spread it among each other by fighting, or through breeding contacts.

One healthy male panther in its prime is known to have killed an infected male, and died within three months. Of panthers with the disease that have died, two appear to have been killed by bacterial infections — pneumonia — that took advantage of the panthers' weakened condition. Two diseased male panthers were killed by other males, and one panther appears to have succumbed to the disease itself.

Panther researchers are watching and waiting until cool weather comes, and water from unprecedented hurricanes recedes, before vaccinating more panthers.

Chasing the cats with dogs, knocking them out of trees with tranquilizer guns, and putting them through the process of blood and tissue samples and inoculations can kill the animals with kindness. Panthers lose their ability to regulate body temperature when tranquilized, so the catch team carries water and ice for first aid. But they can't risk partially anesthetized panthers temporarily getting away and drowning in the swamp.

They prepare to catch the cats with nets and "crash bags" — a layer of air-filled trash bags sandwiched between tarps laid out under trees that may be as high as 45 feet — although that process is not without danger. Biologist Mark Lotz once suffered a torn anterior cruciate ligament — a blown-out knee — when a cold-cocked cat fell from a tree onto his leg.

Nineteen-year panther biologist Darrell Land, the panther team leader who works out of the wildlife commission office in Naples, declined to speculate what percentage of the population could die from the newly acquired virus.

"I don't like crunching numbers like that. It's hard to say what percentage of the population has been affected," Land said.

"But I'm pleased that a new supply of cats is coming up, and we've not seen the disease move beyond the OK Slough population," he said. "If we don't see the disease in any new cats this year, then I'll feel a whole lot better."

Okaloacoochee Slough Wildlife Management Area in Hendry and Collier counties is a 32,000-acre tract that is flanked by the 7,500-acre Spirit of the Wild and 21,700-acre Dinner Island Ranch WMAs on the west and east sides. In all the Panther Team estimates 10 to 15 panthers live in that area at any given time.

That 95 square miles is the northern limit of breeding panther populations — about half of all Florida panthers are mature females — although a few males have territories north of the Caloosahatchee River.

As for the new disease, panther managers aren't panicking, but they are concerned.

"This isn't going to wipe out the panther population, but it could become a significant threat. It's something we want to jump on right away," Cunningham said.