Image caption Canine distemper virus has been confirmed in a number of Amur tigers

Some of the world's rarest big cat species are facing a potentially deadly threat from a virus carried by domestic dogs, a wildlife expert has warned.

John Lewis, director of Wildlife Vets International, said there was evidence that Indonesian tigers were at risk.

Canine distemper virus has evolved in recent decades from infecting only dogs to affecting other animal groups.

Dr Lewis plans to work with Indonesian vets to develop a strategy to protect the nation's tigers from the virus.

A close relative of measles, Canine distemper virus (CDV) was first described at the beginning of the 20th Century and has been cited as contributing to the demise of the thylacine (commonly known as the Tasmanian tiger as a result of the black stripes on its back).

"If you wind the clock back about 30 or 40 years, it was a dog disease - it was a canine virus and only affected dogs," Dr Lewis explained.

"But in the intervening years, the virus has evolved and has changed its pattern of animals it can infect to include marine mammals (such as seals) and big cats."

Reservoir dogs

He told BBC News that CDV needed a reservoir, like a population of dogs, to remain effective as a pathogen.

Image caption Anecdotal evidence suggests CDV is already affecting critically endangered Sumatran tigers

These conditions were present when the first case of the disease affecting wild big cats was documented, he recalled.

"In the mid-1990s, in the Serengeti, Africa, about 30% of the lions died from CDV, which came from dogs in surrounding villages.

"It has also been recognised in the Asian big cat populations," he added.

"Since 2000, in the Russian Far East, there have been a few cats reported as behaving strangely and coming into villages, apparently not showing much fear towards people.

"In the past few years, tissue from at least a couple of those cats have now been confirmed as showing the presence of CDV infection.

"There have not been too many cases at the moment, we think about three or four, but we think there could have been more that have gone undiagnosed."

While some tigers appear as if they are able to build up a reasonable immunity response, most of the animals do succumb to the disease if they are exposed to the virus.

Dr Lewis explained that symptoms manifested themselves in a number of ways:

The big threats facing tigers is habitat loss and degradation and poaching, but I think the third big threat now is likely to be disease Dr John Lewis, Wildlife Vets International

"Some will die as a result of respiratory problems, such as pneumonia for example.

"Some will have neurological problems, such as losing the fear of people or having seizures."

But, he added: "We do not have enough information on CDV in tigers to know what percentage go on to die; we just have a little bit of data from zoos and a little bit of data from the wild.

"There are a lot of cases of distemper in the region and tigers are partial to eating dogs.

"For a tiger to take a dog on the periphery of a village is not usual at all, so you do have the circumstances that would bring tigers into contact with CDV."

Although it was assumed the cause of CDV infection in tigers was a result of coming into contact with dogs carrying the virus, Dr Lewis said that a research project was under way to look at the source of CDV in Amur tigers (also known as Siberian tigers) in the Russian Far East.

Worrying signs

The behaviour change in tigers was particularly worrying, Dr Lewis observed.

"This puts them at big risk because they lose their fear of poachers or they bring themselves in situations of conflict, such as playing with traffic."

On a recent visit to the Indonesian island of Sumatra, he said conversations with local wildlife vets seemed to indicate that CDV could already be present in the population of the critically endangered Sumatran tiger.

They told him that they had seen strange behaviour displays by tigers, such as the big cats coming into villages and losing their fear of people.

"To me, that suggests that distemper is already beginning to have an impact on tigers in Sumatra," he warned.

"But before you say 'yes, that is definitely the result of CDV', you need diagnostic testing of brain tissue.

"The big threats facing tigers are habitat loss and degradation and poaching, but I think the third big threat now is likely to be disease, particularly one like CDV."

The Sumatran tiger is only found on the island and population estimates suggest that there are fewer than 700 left in the wild, of which only 40% are viable mature individuals.

Dr Lewis is returning to Sumatra in September to bring together all the vets from all the different areas that come into contact with tigers.

"The goal is to thrash out a very simple way of deciding what samples need to be taken from all tigers that are handled by humans throughout Sumatra, in order to help us with diagnostics," he explained.

"We also need to thrash out what samples need to be taken from domestic dog populations.

"We need to work out where we can send these samples for laboratory testing. We need to work out how we are going to store and move these samples.

"Once we have got that nailed down then we start work and try to design some sort of mitigation strategy, and that won't be easy."