How a zebra virus killed a polar bear

23 August 2012

Lars, a polar bear in the Wuppertal Zoo in Germany, survived an infection by a zebra virus. [Image: Zoo Wuppertal/Barbarar Scheer]
Lars, a polar bear in the Wuppertal Zoo in Germany, survived an infection by a zebra virus. [Image: Zoo Wuppertal/Barbarar Scheer]

Zoos bring together different animal species that would never encounter each other in the wild, with unforeseen consequences. When, in 2010, a polar bear died at a German zoo and another fell severely ill, veterinarians were at a loss to explain it.

Now new research by an international team, including Dr Simon Ho from the University of Sydney, has discovered that the mystery death and illness were caused by a virus found in zebras 'jumping' to the polar bears.

Their paper, recently published inCurrent Biologyprovides an insight into species-jumping diseases and their possible threat to the conservation mission of zoos.

It has rarely been considered that the species mix found in zoos can have unpredictable consequences in terms of transferring pathogens among animals. Generally, pathogens adapt to a specific host, but some are opportunistic and can spread to new hosts when encountered.

Two years ago a female polar bear, Jerka, died of encephalitis at the Wuppertal Zoo in Germany. Her male companion, Lars, exhibited similar symptoms but survived.

Encephalitis can be caused by a large number of viruses and bacteria, and identifying novel pathogens in wild animals is a huge challenge. However, the intensive investigation of Jerka, Lars, and nine additional polar bears, pointed to a zebra-derived herpesvirus as the pathogen responsible.

A surprising find was that a polar bear who died years earlier from renal failure in a different zoo, without any contact with Jerka or Lars, was also positive for the virus. This indicates that this virus has jumped independently before and might continue to do so.

"This work, led by Professor Alex Greenwood from the Leibniz-Institute for Zoo and Wildlife Research in Berlin, has important implications for zoos. There have been outbreaks of encephalitis in Australian zoos and earlier this year four white rhinos mysteriously died in the Taronga Western Plains Zoo in New South Wales."

"While there is currently no suggestion those illnesses were caused by species-jumping disease this research still presents a complex challenge for all zoos," said Dr Simon Ho, from theSchool of Biological Sciences.

"It can take a lot of work to detect the presence of a pathogen in a species, and it's even more difficult to predict which pathogens might make a fatal jump to another species. The best that zoos can currently do is to monitor pathogens and outbreaks of disease, and to provide this information to zoos worldwide."

As part of the international team Dr Ho analysed the viral genomes and is now part of the investigation into how the bears were infected.

Polar bears in Wuppertal are not cared for by the same zookeepers as zebras and direct contact is unlikely to be the route of transmission because the zebras are housed 68 metres away. In fact the parent hosts could be several other species including gazelles, guinea pigs or even wild mice or rats.

Interestingly, the virus which killed Jerka turned out to be what is called a recombinant - a combination of the genetic material of two different herpesviruses, both found in zebras.

It remains unclear whether this new type of recombined virus emerged recently in the zoo's zebra population or a long time ago in Africa, and whether the recombination event is responsible for the ability of the virus to jump to new hosts.