When a polar bear suddenly takes ill and dies, the natural inclination is not to suspect zebras as the cause. But according to research published August 16 in Current Biology, that's what happened at Wuppertal Zoo in Germany in 2010.

The strange saga started on June 8, 2010, when Jerka, a 20-year-old female polar bear, started experiencing epilepsy-like seizures and foaming at the mouth. She died eight days later. A second bear, 16-year-old Lars, also suffered the same symptoms, but he was treated with antiseizure medications and survived.

A necropsy performed on Jerka revealed that she was suffering from encephalitis, a swelling of the brain that can be caused by viral infections. A team of scientists from Leibniz Institute for Zoo and Wildlife Research Berlin (IZW), the University of Berlin, the University of Sydney and Wuppertal Zoo set out to find exactly what virus could have been the cause.

The result was shocking: Jerka's killer was a recombinant of two equine herpes viruses (EHV9 and EHV1) normally found in zebras. Specifically, the EHV9 virus transferred the portion of its DNA that causes neurological diseases into the EHV1 virus, which typically only triggers respiratory distress.

Jerka wasn't the only polar bear infected. Tests on Lars and nine additional Wuppertal polar bears, plus one that died in 2006, revealed that they all carried the virus as well.

The researchers don't know when the recombination of the viruses occurred. Perhaps more important, they have yet to figure out how and when the virus could have jumped from the zoo's zebras to its polar bears. Wuppertal's enclosures for the two species are 68 meters apart, so the animals never came into direct contact. They do not share the same zookeepers, so transmission by humans seems unlikely. The authors theorize that mice or rats could have transmitted the virus from one enclosure to the other.

"These viruses do not seem to respect species boundaries and, in fact, we don't really know whether they have any," co-author Klaus Osterrieder, a professor of veterinary medicine at the University of Berlin, said in a prepared release. "One conundrum is that these viruses are not particularly stable in the environment, so it is important to figure out how they move between species."

The authors warn that zoos should be on the watch for disease outbreaks potentially caused by bringing together animals from different parts of the world and putting them in relatively confined spaces. As lead author Alex Greenwood, professor of virology at IZW, told BBC News, "One of the missions of zoos is conservation of animals, and species-jumping viruses like the one in this study suggest that mission can be threatened if they are undetected."

Photo: Lars, the polar bear that survived the equine herpes virus at Wuppertal Zoo, courtesy of Wuppertal Zoo/Barbarar Scheer