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This week the German press reported the results of an investigation into the cause of the unexpected death of a polar bear, Jerka.

Jerka had lived a relatively peaceful, uneventful life in the Wuppertal City Zoo, not far from Düsseldorf. Uneventful, that is, until in 2010 when she started to exhibit symptoms similar to epilepsy. The bear had encephalitis or inflammation of the brain and died within eight days of the initial onset of symptoms. Another polar bear in the zoo, a male called Lars, was infected by the same herpes virus. Lars also developed encephalitis but managed to recover.

Considerable scientific expertise was mobilised to try to understand the cause of this mysterious illness. An international team from the Berlin Leibniz Institute for Zoo- und Wild Animal Research has now published their results in the scientific journal ‘Current Biology’. An abstract of the report can be found using the link here. The results are worrying. It appears that the polar bears contracted the herpes virus from a zebra. However the polar bears and the zebras are separated in the zoo by about 70 metres and could not have come into direct contact with one another.

Although the virus might have been transmitted via zoo keepers this is unlikely because the animals have different keepers. It is much more likely that transmission was by rats or mice. These rodents live freely in the zoo and travel unhindered between different animal enclosures.

This means that a virus, the Equine Herpesvirus 1 (EHV-1) which had previously been identified in horses, zebras and guinea pigs has jumped into a very different kind of species. In addition to infecting the polar bears the same virus may also have infected the rodents which transmitted it between the different zoo enclosures.

Regular readers of this website will remember the article I wrote last week about the potentially deadly hantaviruses which pass from rodents to humans (you can find this article here). These viruses do not appear to seriously affect their rodent hosts but when transmitted to humans they can, in some cases, cause diseases for which there is no cure and lead to death in a high percentage of patients.

In Wuppertal we have a similar situation. The Equine HV1 virus (related to the family of viruses which cause chicken pox in humans) often exists in zebras without causing any serious health problems but it has led to the premature death of what was an apparently healthy polar bear.

There is a very serious issue to consider here. In the past there has been assumption that animal viruses tend to stick with certain host species. These viruses and their hosts have evolved together. When a virus infects a cell it often needs to take over certain items of cellular ‘equipment’ and this equipment is different in different species of animals. By evolving together the virus has been able to ensure it has the correct ‘operating instructions’ for the pieces of equipment needed from its own host’s cells (these operating instructions are contained in the viruses genes either as DNA, RNA or as a specialised protein molecule).

However we are discovering more and more examples of viruses and other pathogens which can jump from one animal species to another, sometimes with devastating consequences for the new host. Examples of viruses found normally in other animals which have serious health consequences for humans are SARS (presumed to be derived from bats), Ebola (whose host in the wild is still uncertain), Hantaviruses (carried by rodents) and Bird Influenza. (For more information on this subject you may be interested in reading an article I prepared explaining how these species ‘hopping’ pathogens might evolve – it can be found here).

There is a potential risk that in zoos we have created an ideal breeding ground for new killer viruses by keeping numerous animals in close proximity which would normally not be expected to come into contact with each other in nature. In its natural habitat the chances of a Polar Bear bumping into a Zebra and catching herpes from it are pretty remote. Zoos by bringing such disparate animals together could considerably increase the possibility of dangerous microorganisms finding and adapting to new host species.

In the polar beer/zebra example above we had a pathogen from one species which was introduced into another and led to a new and deadly disease. When such pathogens are also transmitted via another host (in this case the rodents) the potential for spreading the disease increases considerably.

In addition to simply jumping from one host species to another these pathogens may also exchange genetic material with other microbes which already reside in their new hosts. As a result a microbe which is relatively innocuous to its host may suddenly become highly virulent and cause a much more serious illness. This is the way that some new strains of influenza viruses are thought to evolve. They result from the recombination of DNA from multiple influenza strains, some of which are normally found in other animals like birds or pigs.

This idea of genetic material between different pathogens recombining also appears to have played a role in the Wuppertal zoo outbreak. In the case of Jerka, the polar bear, a detailed evaluation of the genetic material from the deadly herpes virus has been carried out by the scientist Alex Greenwood. Much to everyone’s surprise in addition to DNA from the equine virus EHV 1 he also found sections of DNA which have come from another virus, EHV 9, which is normally found in gazelles. It would appear that this version of the killer herpes virus had already undergone recombination before it was contracted by the polar bears.

At this point it is not known whether it was the addition of the hybrid DNA elements from the gazelle that made the EHV 1 herpes virus so deadly to the polar bear. It is also unknown how the genetic recombination between the zebra and the gazelle viruses took place. Was this also in a zoo? Were rodents also involved in transmission between the species? Could the genetic recombination have occurred in the rodents?

Finally what is the consequence of all of this for humans? Could the next deadly pandemic for humans result from genetic recombination of pathogens which occur naturally in animals kept in a zoo? If you look at the emerging strands of evidence this scenario may not be so far-fetched. The worst deadly pandemic, the plague (or Black Death) is also thought to have been caused by a pathogen which was transmitted by rodents. It is estimated that the plague killed about half of the human population. Perhaps it is time to think carefully about the potential dangers to both animals and humans arising from the way we manage, structure and populate our zoos.

Food for thought?

Chris Duggleby.