When the polar bears started dying, nobody suspected the zebras.
Jerka was the first. The 20-year-old polar bear was born in captivity, and had lived in Germany’s Wuppertal Zoo since the age of two. In the summer of 2010, she started suffering from epileptic seizures and eight days later, on the 16th of June, she finally passed away. Lars, a male bear who lived in the same enclosure, also became seriously ill. He was hooked up to an IV drip and treated with anti-seizure medicine. It took several weeks, but he eventually made a full recovery.
When the zookeepers dissected Jerka’s body, they found signs of inflammation in her brain. The pattern of damage pointed to a viral infection, but no one knew which virus was responsible. A team of scientists led by Alex Greenwood from the Leibniz-Institute for Zoo and Wildlife Research searched Jerka’s brain tissue for the genetic material of many possible viruses, from rabies to canine distemper virus. They found only one hit, and it looked a lot like EHV1 – a virus that infects horses.
EHV1, or equine abortion virus, is a herpesvirus that’s related to the ones that cause herpes and chickenpox in humans. It affects the lungs, airways and brains of horses and donkeys, and it’s widespread among zoo zebras. Greenwood thinks that the virus probably jumped into Jerka from Wuppertal’s zebras, but it’s not clear how this happened since the zebras live 68 metres away from the bears and never came into direct contact. Maybe the zookeepers ferried the virus between them, or perhaps rodents did by sneaking in and out of the two enclosures.
The virus that killed Jerka wasn’t a pure strain of EHV1. One of its genes contained DNA from a close relative called EHV9. It’s what is known as a “recombinant virus”. At some point, EHV1 and EHV9 infected the same zebra and fused to form a hybrid virus that went on to infect both Jerka and Lars.
This isn’t the first time that EHV1 has caused problems in zoos. In another German zoo, it killed four black bears. In yet another, it finished off two Thomson’s gazelles and 18 guinea pigs, all from brain damage. Meanwhile, EHV9 killed a polar bear at San Diego zoo, which had been housed around 200 feet away from a herd of Grevy’s zebras. These catholic tastes are unusual, especially since herpesviruses usually stick to one specific host.
To make things worse, herpesviruses can infect hosts without any of the obvious symptoms that killed Jerka and sickened Lars. Greenwood found that another polar bear called Struppo, who died of an unrelated kidney disease in 2006, was also infected with EHV1. His strain was identical to Jerka’s strain, albeit without the extra fragment of EHV9 DNA. He was also only carrying the virus in his blood rather than his brain, which may explain why he never developed any fatal symptoms.
So, we don’t know how common EHV1 and EHV9 are among captive animals, how they spread, or how to control them. All we know is that they can infect mammals from at least five distinct groups. That spells big trouble for zoos, which offer up a platter of new and unexpected hosts to these promiscuous viruses.
Greenwood’s study highlights one of the many problems that zoos face. By bringing together animals from different continents and habitats, they create breeding grounds for new viruses that could undo the zoos’ valuable conservation work.
Reference: Greenwood, Tsangaras, Ho, Szentiks, Nikolin, Ma, Damiani, East, Lawrenz, Hofer & Osterrieder. 2012. A Potentially Fatal Mix of Herpes in Zoos. Current Biology http://dx.doi.org/10.1016/j.cub.2012.07.035
Images by Muhammad Mahdi Karim and Alan Wilson