While the world focuses on the development of new vaccines against COVID-19, biologists are building the case for using vaccines for the conservation of wildlife. Our own research on the impact of canine distemper virus (CDV) in the Russian Far East concludes that vaccination of Amur tigers is a potentially important strategy to avoid extinction of small populations.
Historically, wildlife biologists paid little attention to the impact of diseases on wildlife populations. Pathogens and parasites were regarded as a natural part of the ecosystem, which had evolved to co-exist with their wildlife hosts. Large outbreaks were infrequent and it seemed that there was little that could be done to alter their outcome, so they were simply left to run their course.
While that approach still holds today, the world and the threat posed by disease have changed. Once, large wildlife populations existed across vast landscapes in numbers of sufficient size to survive disease outbreaks. Today more species are relegated to small islands of habitat that support fewer individuals. In some cases, an outbreak could be the “straw that breaks the camel’s back,” driving these small populations to extinction locally.
The tiger is a classic example of this predicament. Once widely distributed across Asia, tigers are now mostly confined to protected areas and surrounding buffer zones. Even in the tiger’s Indian heartland one study projected that even under favorable scenarios, the majority of protected areas will soon support an average of just 14 individuals.
Only a few protected area networks have the capacity to hold larger, more stable populations. Under such conditions, each of these small populations is vulnerable to a host of unpredictable events, making a disease outbreak a serious threat.
We initiated our study following the deaths of several Amur tigers infected with CDV in the Russian Far East in 2003 and 2010. This region supports two populations of tigers, including one of the largest in the world with some 500 individuals in the Sikhote-Alin Mountains.
The other, much smaller population straddles the border of Russia and China and numbers perhaps 30 individuals (most in the SW corner of Primorski Province, Russia), but is particularly important as a source for recolonizing northeast China. Tigers with CDV have died in both populations.
Our recent study showed that while the risk that CDV posed to the large Sikhote-Alin population was extremely low, in a small population like the one in Southwest Primorski, CDV increased the likelihood of local extinction within 50 years by 65 percent.
Even robust tiger populations do not exist at sufficient numbers or density to allow a pathogen like CDV to persist in an ecosystem. Usually, there is another more abundant reservoir host species (or multiple species) that allows a pathogen to persist and act as a continual source of infection for rare hosts like tigers.
It is often assumed that domestic dogs are the main reservoir for CDV outbreaks in wildlife. A 1994 CDV outbreak in Serengeti lions began with a spillover from local dogs. A dog vaccination program helped contain future outbreaks.
In Russia, our research demonstrated that dogs were not the reservoir in the local ecosystem. Rather, small carnivores like sable, badgers, and raccoon dogs were passing the virus to the tigers. Since there is currently no oral bait-based vaccination for CDV, it is not feasible to control transmission in those species. Vaccination of tigers themselves is the only option.
Collecting a blood sample from a domestic dog. Image courtesy of Martin Gilbert.
Fortunately, existing injectable vaccines are safe and effective in tigers. Our simulations indicate that even vaccinating a small proportion of the Southwest Primorski tigers (e.g. 2 animals per year) would significantly reduce the probability of population collapse. This low coverage vaccination strategy is not intended to achieve “herd immunity” (the objective of COVID-19 vaccinations) but is simply an “insurance scheme” that allows an immune group of tigers to survive future outbreaks and enable population recovery.
The 2018 outbreak of CDV in the only remaining Asian lions should serve as a warning that other big cat populations are also likely at risk. Priority should be given to identifying tiger populations like those in Southwest Primorski with high strategic value despite their small size.
Research to identify local CDV reservoirs can inform control strategies before outbreaks occur. Proactive programs to vaccinate tigers in the wild should only be considered for high-risk and high-value populations where epidemiological research indicates that it is necessary.
However, there are a surprising number of tigers that are being captured each year across Asia for research, for dealing with human-tiger conflicts, and for rehabilitation of injured adults or abandoned cubs. In these circumstances, we recommend that delivery of CDV vaccines be incorporated as a routine part of tiger handling protocols.
Newly developed vaccines hold out the exciting promise of reducing the threat of COVID-19 in human populations. Similarly, if a simple injection of a vaccine known to be safe can likely increase the odds of a tiger population surviving a distemper outbreak, it is incumbent upon science-based conservation agencies to consider adopting this approach.
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