Could an Unconventional Vaccine Program Prove a Roaring Success?

Canine distemper virus (CDV) is a potentially fatal virus caused by the paramyxovirinae family of viruses — the same family that causes measles and mumps in humans, and rinderpest virus in cattle. While CDV commonly affects domestic dogs, it can transmit and infect species of wild dogs such as foxes, coyotes, and wolves, as well as large wild cats like lions and tigers.

In this blog, we highlight the threat that CDV poses to the Amur tiger (Panthera tigris altaica) and how an unconventional strategy to vaccinate wild tigers against CDV might help protect them from extinction.

An Endangered Population Threatened by a Mystery Illness

The Amur tiger (also referred to as Siberian tiger) is an endangered species with only around 500 estimated to remain in the wild, located in two discrete populations: the Russian Far East and neighboring areas of China (Seimon et al. 2013). For conservation efforts to succeed, it is important to prevent a large drop in the numbers of breeding animals, meaning that any infectious disease could pose a significant threat to the remaining population.

As tigers are normally solitary and reclusive by nature, sightings of sick tigers are very rare. However, in 2001, serious neurological disease started to be observed in wild Amur tigers, suggesting that an infectious disease was emerging in this tiger population (Seimon et al. 2013).

CDV attacks the nervous system of animals, with symptoms like convulsions, seizures, jaw chewing movements, and paralysis observed in domestic dogs. In wild animals, the symptoms often resemble rabies. CDV was already known to affect many different wild species from raccoons and pandas to hyenas and wolves, but it had not yet been reported in wild tigers. The neurological nature of the disease and the prevalence in other wild species led researchers to postulate that distemper could be the source of the mystery illness.

CDV in a wild tiger was first confirmed in samples from an animal that died in 2003. It seems likely that the disease was introduced into the Amur tiger population sometime after 2000. Retrospective serological analysis from tigers found antibodies to CDV in 20/54 Amur tigers sampled after 2000, but none of the 18 serological samples from tigers prior to 2000 contained CDV antibodies (Gilbert et al. 2020).

The initial threat of CDV to the Amur tiger population was highlighted by Seimon et al. in research published in 2013. Through studying tissue from five tigers showing neurological disease, they were able to confirm CDV as the cause of the disease in three of these animals. Moreover, they established that in 2010 CDV directly or indirectly killed ~1% of Amur tigers, making it paramount to address this threat (Seimon et al. 2013).

More recent research from the Cornell Wildlife Health Center suggests that CDV, if left unchecked, could be devastating to the remaining small tiger population in north-eastern China (around 30 animals). They have predicted that the 50-year extinction probability could rise to between 50-65% in this small population, if the spread of CDV is not prevented (Gilbert et al. 2020). To reduce this likelihood, conservationists need to develop strategies to help protect wild tigers from infection. Disease transmission is an important part of understanding how to prevent infection, so Gilbert et al. wanted to build a picture of CDV epidemiology in the tigers’ habitat of the Russian Far East (Gilbert et al. 2020).

How Do Tigers Catch CDV?

Domestic dogs are often assumed to be a source of CDV infection in wild animals by conservationists and the general public (Gilbert et al. 2020) and as such a focus has been on the vaccination of domestic dogs to prevent the spread of CDV to other species.

Serological data (information about the presence of neutralizing antibodies and negative samples) can be used to understand prior infection of animals. By comparing samples obtained from domestic dogs, tigers, and other wild carnivores present in the natural habitats of Amur tigers, researchers were able to determine the likely reservoirs of infection for the tigers (Gilbert et al. 2020). Interestingly they found that domestic dogs are unlikely to be an important source of Amur tiger CDV infection; while CDV antibodies were detected among unvaccinated dogs, the population was too small to maintain the pathogen and very few dogs had the freedom of movement required to infect tigers (Gilbert et al. 2020).

Instead, it is likely that other wild animals are the primary source of infection which means that vaccinating domestic dogs alone is unlikely to help prevent CDV disease in Amur tigers and another approach is needed to protect them.

How Can We Prevent Disease Transmission?

Currently, there is no specific treatment available for CDV, so prevention of the disease is key. Blocking tactics, such as reducing interactions between tigers and potentially infected animals, like dogs and wild carnivores, is not practical as the tigers’ habitat is vast and often spans remote regions of land.

Vaccinating other animals to prevent prevalence in the area and reduce reservoirs of disease is another possibility. However, while it is possible to routinely vaccinate domestic animals, like dogs and ferrets, vaccinating wild animals is controversial and challenging. As there is no oral vaccine, approaches like lacing baited food are not viable and animals would need to be trapped and injected to administer the vaccine. This approach would require a lot of time and effort and is not feasible due to the abundant wildlife population (17 different species of wild carnivore) (Gilbert et al. 2020).

Can We Vaccinate Tigers?

It seems that the idea of tiger vaccination, while still controversial, may be plausible and help reduce the threat of extinction. In a recent study published in PNAS, researchers vaccinated tigers in captivity with a modified live vaccine and demonstrated that this could protect tigers from disease. The serum from vaccinated tigers contained antibodies that were able to neutralize the wild strain of CDV detected in the Russian Amur tiger population (Gilbert et al. 2020).

To further investigate the feasibility of vaccinating wild tigers, they used computer modeling to predict the numbers of tigers that would need to be vaccinated to have an impact on disease spread. Promisingly, this suggested that a low vaccination strategy, with as few as two tigers per year vaccinated, could be enough to reduce the spread of the disease significantly and help protect Amur tigers from extinction (Gilbert et al. 2020).

With such a low number of animals required to be vaccinated, this approach could even be undertaken when tigers are captured for routine radio collaring studies rather than requiring active trapping and vaccinating.

This study highlights how vaccinating a wild animal could help prevent the spread of a disease that threatens their extinction. However, because vaccination of wild animals is controversial (in part due to a now discredited hypothesis which suggested that rabies vaccination intervention may have contributed to the extinction of the African wild dog, Lycaon pictus, in the Serengeti), wildlife conservationists may remain wary of this approach (Gilbert et al 2020). It will be interesting to see if vaccination of Amur tigers forms part of their conservation strategy in the future.