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Rare Amur tigers in Rus­sia are suc­cumb­ing to infec­tion with canine dis­tem­per virus (CDV), a pathogen most com­monly found in domes­tic dogs, accord­ing to the authors of a study pub­lished on 13 August in mBio®, the online open-​access jour­nal of the Amer­i­can Soci­ety for Microbiology.

Pres­sure from poach­ing, dec­i­ma­tion of their prey base, and habi­tat frag­men­ta­tion have dimin­ished the pop­u­la­tion of Amur tigers (or Siber­ian tigers) to fewer than 500. In the study, a team of sci­en­tists from the US and Rus­sia show that CDV infected and caused fatal neu­ro­log­i­cal dis­ease in the Crit­i­cally Endan­gered Amur tigers (Pan­thera tigris altaica). They esti­mate that the virus has killed at least 1% of Amur tigers since 2009.

Since 2001, sev­eral rare Amur Tigers have exhib­ited a set of strange behav­iours. Nor­mally a reclu­sive species, tigers have been seen enter­ing vil­lages and wan­der­ing onto roads in the Russ­ian Far East, stum­bling, ema­ci­ated, and unafraid of humans. This has also been noticed in Indone­sia with the Suma­tran tigers which became infected. It is a wor­ry­ing symp­tom of canine dis­tem­per: that tigers seem to lose their fear of man. As it increases the chance of human-​tiger con­flict and leaves them easy prey for poachers.

A very wor­ri­some exam­ple can be seen in this video, shot on the road between the vil­lage of Vyazemsk and the City of Bikin, Khabarovsk Krai:

In each of the doc­u­mented cases, the tiger even­tu­ally died or was destroyed after its con­di­tion wors­ened. Early find­ings showed that at least one of the tigers was infected with a mem­ber of the mor­bil­livirus fam­ily of viruses, such as CDV, but con­clu­sive answers had evaded sci­en­tists and wildlife man­agers until now.

Using tis­sue sam­ples from five wild Amur tigers that died or were destroyed due to neu­ro­log­i­cal dis­ease in 2001, 2004, or 2010, McAloose and her col­leagues proved that infec­tion with CDV is to blame for the deaths of two of the tigers and caused a seri­ous infec­tion in a third. Under the micro­scope, the brains of the two tigers that died of CDV infec­tion were rid­dled with lesions, indi­cat­ing they suf­fered from severe viral encephali­tis, con­sis­tent with their clumsy, abnor­mal behav­iour. Mol­e­c­u­lar analy­ses to iden­tify CDV-​specific pro­teins and immuno­la­belling with CDV-​specific anti­bod­ies con­firmed that CDV was present in these tis­sues. A gene for a CDV-​specific gene was detected in the third tiger.

The prob­lem isn’t lim­ited to one loca­tion, says McAloose. The three tigers that tested pos­i­tive for CDV were dis­trib­uted across the Russ­ian Far East. “That tells us this is a dis­ease that is dis­trib­uted all across Amur tiger range,” he says. “And it also appears to be a rel­a­tively new threat to tigers since blood sam­ples from wild tigers prior to 2000 tested neg­a­tive for anti­bod­ies to the virus”.

But how do tigers con­tract a CDV infec­tion? Rel­a­tively few domes­tic dogs in the Russ­ian Far East are vac­ci­nated against CDV, McAloose says, and tigers do kill and eat dogs, so they rep­re­sent one pos­si­ble source. But domes­tic dogs aren’t the only sus­pects. “In the Russ­ian Far East, domes­tic dogs are one of the biggest con­cerns, but other species, like rac­coon dogs or foxes, can also har­bour the dis­ease,” says McAloose.

McAloose and her col­leagues are now work­ing on col­lect­ing sam­ples from dogs and small wild car­ni­vores in the Russ­ian Far East to get a more com­plete pic­ture of the var­i­ous strains of CDV in cir­cu­la­tion in the hopes of link­ing tiger infec­tions to a source, knowl­edge that would hope­fully aid in pre­vent­ing more infec­tions among tigers.

“The sit­u­a­tion is quite seri­ous”, says McAloose, and when asked if CDV could spell the demise of Amur tigers, she says, “It’s pos­si­ble. It’s the first infec­tious dis­ease that we know is a sig­nif­i­cant risk to Amur tiger survival.

(Source: Amer­i­can Soci­ety for Micro­bi­ol­ogy press release, 13.08.2013)