Dog disease threatens Siberian tigers

Despite its name, canine distemper virus can infect plenty of non-dog species, including ferrets, seals and cats (though not the domestic variety). Though it’s been known that big cats can be infected with the virus since the 1980s, it wasn’t considered a threat to such species until 1994, when the virus swept through the Serengeti lion population, killing 1,000 animals, about 30 percent of the population.

Tigers have also proven vulnerable to the virus: Cases of the disease have been found among wild tigers in Russia in 2003 and 2010 and in India earlier this year. And with the tiger population having dwindled to around 1,000 breeding females worldwide — due mainly to poaching and human conflict — there is worry that the virus could add to the cats’ troubles.

Because the tiger population lives split into small groups, deadly diseases should theoretically have trouble gaining a foothold. For a pathogen to persist, there needs to be susceptible hosts to which an infected host can pass the disease. This is more difficult where there are fewer animals in a population. But because canine distemper virus can infect so many different species of animals — including many that coexist with tigers as well as the domestic dogs often found on the edges of tiger territory — the big cats can have many opportunities for exposure.

Martin Gilbert of the Wildlife Conservation Society in Bronx, New York, and colleagues studied tigers in Sikhote-Alin Biosphere Zapovednik, a nature reserve in the Russian Far East along the Sea of Japan. They looked at how the virus might spread among tigers in the reserve and then simulated what would happen if the virus infected populations of tigers in sizes ranging from three to 288 cats.

Under the most conservative infection scenario, populations of 25 tigers were 1.65 times more likely to go extinct within the next 50 years if canine distemper virus arrived than if the disease never infected a cat. About half the world’s tigers now live in populations of 25 or fewer cats, the researchers report in a study published October 29 in PLOS ONE.

The Serengeti lion population was able to recover from the 1994 event, but Gilbert and colleagues warn that tigers may have a tougher time. The big orange-and-black cats reach sexual maturity at an older age than cats such as leopards and cougars, and tiger populations take longer to recover from the loss of mature individuals.

“In lieu of a practical means of delivering [canine distemper virus] vaccines to wild tigers, the most viable strategy to ensure their conservation is the maintenance of large connected populations within protected areas that buffer the effects of local declines,” Gilbert and colleagues write. That would allow a population that loses some cats to the virus to recover with the addition of healthy tigers from farther away, improving the likelihood that the group survives the outbreak.