Extinction Watch

Vaccines for a common pet problem could help save this big cat from extinction

We are not the only species in need of vaccinations.

ANO Amur and the Zoological Society of London

In the midst of the global pandemic, vaccines are at the forefront of everyone's minds. And scientists think a vaccine could help pull back another creature from the brink.

In a new study published Monday in the Proceedings of the National Academy of Science, researchers show how vaccinating Amur tigers — one of the world's rarest big cats — against a common pet problem could help these tigers survive. Without further intervention, these tigers face the threat of extinction.

Disease Transmission — Canine distemper virus occurs mostly in domestic dogs, but it also affects other wildlife, including the rare Amur tiger. Scientists detected the first Amur tiger death from CDV in 2003, and it has since grown as a threat to the nearly 550 Amur tigers living in the far east of Russia and neighboring China, according to the study.

It was thought dogs in this region were spreading the virus to wild animals like the Amur tiger. If that were true, then vaccinating dogs against the virus would make sense. But this new research upends all prior assumptions.

"Rather like Covid-19, if dogs had been acting as an important reservoir of infection we’d have expected to see more virus in areas where there are larger numbers of dogs mixing in larger communities," Martin Gilbert, lead author on the study and Senior Research Associate at Cornell Wildlife Health Center, tells Inverse.

Samples collected from Amur tigers including Boris and Svetlaya (pictured) enabled the researchers to assess the population's exposure to canine distemper virus

Credit: Wildlife Conservation Society

Gilbert and his team compared canine distemper virus outbreaks in dogs from more densely populated areas to outbreaks in dogs from more remote regions.

"In reality, we found that fewer dogs were exposed and outbreaks occurred less frequently in our densely populated study site compared to our more remote sparsely populated areas," Gilbert says.

The findings suggest that although domestic dogs can theoretically infect tigers, they aren't the most common source or "reservoir" of virus transmission. Rather, other wild carnivores are the more likely culprits for transmission.

"To take the Covid-19 analogy further, the equivalent would have been for the virus to take hold in upstate New York while leaving Manhattan comparatively unaffected," Gilbert says.

"The most likely explanation for this pattern of infection would be that more remote communities are experiencing a greater transmission from another reservoir, namely wildlife."

Time to Vaccinate — With a better understanding of the disease transmission, the scientists used a computer simulation model to test three different strategies to reduce canine distemper virus in the Amur tiger population.

These were:

  • Reduce transmission of the virus in other animals
  • Block transmission of the virus from other animals to tigers
  • Reduce transmission of the virus among tigers

Rather than try stop the virus at the source, Gilbert and his team concluded that vaccinating tigers was the most feasible and cost-effective way to save the species.

"Since we have shown that wildlife is an important reservoir of infection for tigers, and since we have no way of delivering vaccine to wild carnivores at levels that would be sufficient to reduce transmission, and no way to keep tigers separate from other wildlife then our only option left is to vaccinate the tigers themselves," Gilbert says.

The scientists concluded that vaccinating two tigers annually would reduce the tiger's chance of extinction from 15.8 percent to 5.7 percent over a 50-year period. The average cost to do this comes in at $30,000 — cheaper than other strategies, the researchers found.

The big challenge: catching the tigers and trapping them long enough to administer a vaccine.

"Obviously the more tigers we can vaccinate the better. But tigers are elusive and catching them to vaccinate requires a lot of people working for extended periods of time, which inevitably costs money," Gilbert says.

Amur tigers share their taiga forest habitat with wild carnivores that act as a reservoir of canine distemper virus.

Credit: ANO Amur and the Zoological Society of London

According to Gilbert, canine distemper virus is a multi-host pathogen, which functions differently from a pathogen that only infects a single species, like measles or polio in human populations.

For these single-host pathogens, "we have a realistic opportunity to eradicate the virus, but to do so we need to immunize a large proportion of the population — equivalent to the 70 or 80 percent -herd immunity- that we hear about so much with COVID-19," Gilbert says.

It's different with the infected tigers, though.

"We have to accept that there will always be a source of infection in other wild carnivores, and so the best that we can do is to design a tiger vaccination program that immunizes a sufficient proportion of the population to make it very unlikely that [the virus] will ever cause the extinction of the whole population," Gilbert says.

Light at the end of the tunnel — Tigers around the world face threats to their existence on a daily basis, and the Amur tiger is no exception.

The study's findings hone in on wildlife as a major source of the canine distemper virus pathogen — but they also show a way to beat it.

"It is still quite possible that Russian dogs may be capable of acting as a reservoir on their own and we just weren’t able to detect this," Gilbert says.

"But crucially, the weight of evidence pointing to a wildlife reservoir really dictates our options for managing infection in tigers."

Abstract: Canine distemper virus (CDV) has recently emerged as an extinc-tion threat for the endangered Amur tiger (Panthera tigris altaica).CDV is vaccine-preventable, and control strategies could requirevaccination of domestic dogs and/or wildlife populations. How-ever, vaccination of endangered wildlife remains controversial,which has led to a focus on interventions in domestic dogs, oftenassumed to be the source of infection. Effective decision makingrequires an understanding of the true reservoir dynamics, whichposes substantial challenges in remote areas with diverse hostcommunities. We carried out serological, demographic, and phylogenetic studies of dog and wildlife populations in the Russian FarEast to show that a number of wildlife species are more importantthan dogs, both in maintaining CDV and as sources of infection fortigers. Critically, therefore, because CDV circulates among multiplewildlife sources, dog vaccination alone would not be effective atprotecting tigers. We show, however, that low-coverage vaccina-tion of tigers themselves is feasible and would produce substan-tive reductions in extinction risks. Vaccination of endangeredwildlife provides a valuable component of conservation strategiesfor endangered species.
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