While global attention is currently focused on COVID-19, a disease that has jumped from animals to humans, it is important to remember that diseases that breach the species barrier also pass from people and their domestic animals to wildlife populations. And this poses a serious threat to endangered species like tigers who are already teetering on the brink of extinction.
The first episode in the Year of the Tiger series of WildCats Pawast covers the top three infectious diseases which are having an impact on tigers, including the indirect threats from African Swine Fever and Lumpy Skin Disease to the more direct impacts of Canine Distemper Virus.
One undeniable common theme between all three diseases is that the human disruption of natural systems is intensifying unnatural interactions between species and creating emerging infectious disease transmission pathways. But not only are we engaging in our natural environment in a more exploitative way, but we are also traversing it differently. The movement of people, animals and animal products has increased dramatically through international travel and trade which has effectively meant that diseases benefit from more freedom of movement than most of the human population.
Listen to the episode below
In this blog we take a deep dive into Canine Distemper Virus (CDV)
Wildlife Veterinarian and Epidemiologist, Dr Martin Gilbert, has worked for decades on understanding how CDV affects populations of tigers and other threatened carnivores. Currently a Senior Research Associate and Professor at Cornell University, Martin has published his research on the exposure and impact of canine distemper virus on Amur tigers in the Russian Far East and more recently Wild Sumatran Tigers in Indonesia.
Martin explained that while CDV is not that familiar in western countries where dog vaccination is practised regularly, it is still very common in a lot of the world where veterinary care is not as accessible.
CDV is a virus that is closely related to measles and while its name suggests it only affects dogs, it does, in fact, impact almost all carnivores and is very widespread within this order. The first case of CDV in big cats in the wild was recorded in the early 1990s when an outbreak was recorded in Tanzania and then later in the Kenyan lion populations. Those early outbreaks had a huge impact due to the social nature of lions making them much more exposed to transmission from animal to animal. This meant CDV spread really widely through the Serengeti reducing the lion population from 3,000 to 2,000.
While this was devastating for the lion population in Africa, the initial hope was that tigers would not be as easily impacted due to the fact they are not a social species so there is very little opportunity for transmission from individual to individual. However, Martin Gilbert explained that this has not shown to be true. In 2010 there were a number of cases of distemper over a very wide area within the tiger population. But how was this possible? Tigers are solitary animals and the only pathways for transmission between them are mothers to cubs and between breeding adults. Martin went on to describe how CDV is a multi-host-pathogen which is the secret to its success. It has a wide host range and so can spread very easily between species.
So now we know that tigers are at risk of contracting CDV the next question is what is the impact of this disease on the species as a whole? In short, we don’t know yet. We have a good amount of information on the tigers which have caught the disease and died but we don’t have the same amount of data for those who have been exposed to infection and survived. There is an estimated case fatality rate for infected individuals of between 30-50% depending on the strain of the virus.
But what is being done to control Canine Distemper Virus?
Safe and effective vaccines have been used in domestic dogs for decades and the immunity produced in this species from the vaccine tends to be very long-lasting and protective. However, when it comes to free-ranging animals, vaccines for conservation purposes are really in their infancy. It has been effectively practised in a few places, in particular for rabies, but for CDV it has only just been proposed as the logical next course of action.
If domestic dogs are the main reservoir then they would be the species that would need to be vaccinated as the most straightforward and cost-effective method of controlling exposure. But where wild carnivores are maintaining the virus and are the main source of infection for tigers, the only option would be to vaccinate the tigers themselves. This would be incredibly demanding but Martin explained the aim would not be to get very high coverage in the population as we have with COVID vaccinations. Instead, a very small proportion of the population would be vaccinated, enough that if there was an outbreak in the future that a small nucleus of immune individuals would be left for repopulation. Vaccination would therefore be preparation for the worst-case scenario to ensure the survival of the species in the wild and reduce the chances of extinction. If tiger populations were large and healthy the way they used to be then CDV is not likely to be an issue. But for small populations and source populations, CDV could lead to the extinction of the species.