Ronald Gilbert / Cornell Wildlife Health Center

Amur tiger in grass.

December 17, 2020

Cornell-Led Study Finds Vaccinating Endangered Tiger Subspecies Significantly Reduces Extinction Rate

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With three subspecies already extinct, global tiger populations have decreased from 100,000 to only 3,500, representing a pattern of mass wildlife extinction further exacerbated by humans in the last century.  

But in the midst of these dramatic population declines, new research has found a way to reduce the threat of extinction to the Amur tigers of the Russian Far East. 

Like other tiger subspecies, Amur tiger populations — also known as the Siberian tiger and now numbering at around 500 to 600 tigers — have been greatly reduced over the last century due to habitat loss, poaching and demand for cubs for zoos and circuses. The loss of tiger populations threatens the health of ecosystems in the Russian Far East, as tigers keep herbivore populations in check, maintaining a healthy balance of vegetation in the area. 

A team led by Dr. Martin Gilbert, a senior research associate in the Department of Population Medicine and Diagnostic Sciences, has shown that vaccinating endangered Amur tigers is the only viable method of protecting the species from canine distemper virus, which causes respiratory and neurological infections in tigers and other carnivores.

CDV adds yet another threat, especially for the smaller subpopulation of Amur tigers that are beginning to repopulate northeast China, which Gilbert contends is a key step in the species’ fight against extinction. According to Gilbert, about one-third of wild Amur tigers have been exposed to CDV since the turn of the century. 

“When you have tiger populations that are down to maybe 20 or 30 individuals, that’s when [CDV] becomes a risk,” Gilbert said. “Because tigers have been so badly knocked back by all these other pressures  — hunting, habitat destruction, human infrastructure — the tiger populations that remain are increasingly at risk from things like infectious disease.” 

Managing CDV requires vaccinating either the tigers themselves or the diseases’s reservoir, other animals whose populations maintain the virus and pass it along to tigers. Wildlife managers have traditionally shied away from directly vaccinating endangered wildlife, partly due to previous suspicions that rabies vaccinations were a cause of the extinction of African wild dogs in the Serengeti in Tanzania. Instead, domestic dogs — a common reservoir for CDV — have been vaccinated. 

However, Gilbert’s team found that wild carnivore populations in the Russian Far East such as racoon dogs and badgers, are the major reservoir and conveyer of CDV to tigers. 

Gilbert’s team used antibody tests, genetic analyses and surveys to understand the nature of the CDV epidemic in Amur Tigers, as no single piece of evidence provided clear conclusions as to where and how the tigers were getting infected. 

Genetic sequencing revealed that wild carnivore CDV strains were similar to that of the Amur tiger, whereas the domestic dog virus was fairly distinct. Gilbert’s team concluded that wild carnivores, rather than domestic dogs, are likely to be important in maintaining and transmitting CDV to tigers. 

But to vaccinate wild carnivore populations, baited food must be combined with an oral vaccine, yet no oral CDV vaccine is currently available, leaving only one viable option: vaccinating the tigers themselves. Researchers conducted laboratory tests which showed that the CDV vaccine currently used for domestic dogs can successfully trigger an antibody response for Amur tigers when they are vaccinated instead.  

Even with an effective vaccine, the small population and remote geographic ranges of Amur tigers make administering vaccines difficult. Injecting vaccines would require time-consuming capture operations or adapting remote darting systems used on other wildlife populations. However, even with these practical constraints, computer models show that vaccinating just two tigers per year could reduce the 50-year extinction probability of the smaller Amur subpopulation by over 60 percent.

“For someone [whose] main motivation is conservation, I think we now know enough about CDV in this system to be able to act,” Gilbert said. “We know that the virus is a problem for tigers. We know where tigers are getting it, and we know how to control it. So the idea would be to just start moving ahead with vaccinating tigers to greatly reduce the threat it poses to their conservation.” 

Being able to confidently move forward in managing CDV in Amur tigers is a major milestone for the subspecies’ conservation, which Gilbert has been studying since he was a Ph.D. student. 

“One of the biggest lessons is probably the approach that we’ve taken,” Gilbert said. “We had a series of questions to answer. Firstly, is [CDV] a problem? If it is a problem, how and where are tigers getting it? And how do we use that to inform how we manage it?” 

The success of this step-by-step approach can inform how scientists investigate and manage other viruses in tiger populations, including in other endangered carnivores like Asian lions and Ethiopian wolves. 

“It’s quite unusual for a study to look at an issue as holistically as that,” Gilbert said. “And so [that approach] could apply to all sorts of wildlife health issues that impact endangered species.”