Cats and dogs don't usually mix. But a domestic dog virus is posing a new threat to endangered tigers in the wild, experts say—partly by making them less fearful of people. (See tiger pictures.)

Forced into increasingly smaller habitats, tigers are sharing more space with villagers and their dogs, many of which carry canine distemper virus (CDV), an aggressive, sometimes fatal disease that is usually found in dogs but is also carried by other small mammals.

Based on odd tiger behavior on the Indonesian island of Sumatra, scientists suspect the virus is a problem there and in other countries. Many of these potentially CDV-infected tigers seem to be unfazed by people, wandering onto roads and into villages.

John Goodrich, now senior tiger program director of the conservation group Panthera, found the first known tiger with distemper in 2003 in Pokrovka, Russia: "This tiger just walked into a town and sat down. She was absolutely beautiful—a healthy-looking young tigress."

Even so, she had a fixed stare and did not respond to stimuli. "The lights were on, but no one was home," said Goodrich, who was then with the Wildlife Conservation Society.

Goodrich and colleagues anesthetized the tigress and found her positive for distemper. They cared for her in captivity for six weeks before she died.

Such fearless behavior is likely a symptom of brain damage caused by distemper, which also causes respiratory disease, diarrhea, seizures, loss of motor control, and sometimes death.

Veterinarians still don't know much about tiger distemper. It seems that the tigers "can get a mild infection that doesn't cause any problem—conversely it can be more serious than it is in the natural host," said Andrew Greenwood, a zoo and wildlife veterinarian at WVI.

Concerned by the development, WVI plans to work with the Indonesian government and veterinarians to launch the world's first tiger-disease surveillance program, which aims to find out how tigers catch distemper, identify the likely source of the virus, and determine how to best tackle it.

"If we get it right, it could help us forestall a major problem, which is the last thing tigers need in their precarious state," WVI director John Lewis said in a statement.

Conservationists estimate that only 3,200 tigers exist in the wild in 13 Asian countries—a 93 percent reduction of their historic range. (See a National Geographic magazine interactive of big cats in danger.)

Deadly Distemper

Scientists first realized that canine distemper virus can cross over to felines when it broke out in captive big cats living in California in the 1980s, Greenwood said. (Learn about National Geographic's Big Cats Initiative.)

In recent decades, lions in the Serengeti have caught the virus from domestic dogs that live with Maasai herders, and a 1994 outbreak wiped out a third of the Serengeti's lion population.

Luckily, because tigers aren't as social as other big cats, the animals don't seem to be spreading distemper—an airborne virus—among themselves. It's likely that the tigers are eating dogs infected with the virus. An outbreak akin to what happened in 1994 in the Serengeti would be "catastrophic" for tigers as a species, Greenwood said.

Brain-damaged tigers who don't die from distemper and approach human settlements can be easily killed by poachers or by villagers concerned for their safety, he said.

To prevent tigers from getting killed, in addition to the surveillance program, WVI plans to create a rabies and distemper vaccine campaign in Indonesia, which would encourage people to vaccinate their dogs against the deadly viruses, Greenwood said.

Such a campaign was successful in Zimbabwe—where domestic dogs were sickening the rare African wild dog—when people realized that vaccinating their dogs would also help protect their children against disease. (Read about infectious animals in National Geographic magazine.)

Additionally, a vaccination program among the Maasai herders after 1994 likely prevented another outbreak of distemper in the Serengeti lions.

Last Straw?

Barney Long, head of Asian species conservation at WWF-US, said by email that "the threat of the distemper virus in tigers is real, and we should monitor it closely.

"I am glad a veterinary organization has done the research, so the correct expertise is on the case," he said.

"However, poaching remains ... the most immediate threat to tigers in the wild. Every part of the tiger—from whisker to tail—is traded in illegal wildlife markets."

Panthera's Goodrich added that distemper in tigers is a "definite concern," but it's not as worrisome as it is for lions.

Overall, disease is a conservation issue "that tends to get forgotten," WVI's Greenwood emphasized.

In some cases, isolated populations of endangered species besieged by other threats finally fall to an outbreak of a disease.

For instance, canine distemper would've killed off the formerly endangered black-footed ferret in the U.S. if it weren't for a captive-breeding program.

Russia's tigers are dispersed across a large-enough area that distemper would likely not be their death knell.

But if canine distemper broke out in more isolated tiger populations, such as in India, it could be, as WVI put it, "the final straw that makes extinction inevitable."