In 2001 a few tigers in Russia started to show signs of obvious distress. Endangered Amur (or Siberian) tigers (Panthera tigris altaica) were underweight, weak, disoriented and incapable of hunting as a result. At least four of the big cats had to be put down after they wandered into towns in search of food. One mother left behind her three-week-old cubs, which had already starved to death.

Three years ago the cause of this mysterious behavior was uncovered: The tigers had contracted canine distemper (CDV), a usually fatal viral disease that causes a wide range of symptoms including fever, diarrhea, labored breathing, dehydration and seizures. The tigers probably caught the disease from eating infected domesticated dogs or from any of the other carnivores present in the Russian wilderness.

Vaccinating wild tigers is next to impossible, as I wrote in 2011, but efforts quickly began to vaccinate dogs and other carnivores. Even so, the problem quickly got worse. By 2013 it had been estimated that canine distemper had already killed 1 percent of the world's 450 or so Amur tigers.

It's going to get worse, according to a study published October 29 in PLoS ONE. Scientists from the Wildlife Conservation Society (WCS), Panthera and other organizations used computer models to examine all manner of possible disease transmission including tiger-to-tiger, dog-to-tiger and other carnivore-to-tiger scenarios. They found that in tiger populations of 25 or fewer individuals the disease increased the likelihood of that group dying out by as much as 55.8 percent. Most Amur tigers live in very small, isolated groups of 25 or fewer cats, so the disease will spread much more quickly inside those groups.

Even larger groups may be at risk. Russia's Sikhote–Alin Biosphere Zapovednik (SABZ), located about 200 kilometers from the border with North Korea, had 38 tigers in 2007. By 2012 that number had declined to nine. Canine distemper was discovered in two dead tigers from the biosphere in 2009 and 2010. The scientists used the data from SABZ as the basis for their models.

Overall, the new study shows that canine distemper threatens not just individual tigers but the subspecies as a whole. The disease adds to the already long list of reasons why Amur tigers are endangered. "Tigers face an array of threats throughout their range, from poaching to competition with humans for space and for food," WCS Russia program director Dale Miquelle explained in a prepared statement. "Consequently, many [Amur] tiger populations have become smaller and more fragmented, making them much more susceptible to diseases such as CDV. While we must continue to focus on the primary threats of poaching and habitat destruction, we now must also be prepared to deal with the appearance of such diseases in the future."

Photo: WCS Russia Program