By Justin Kaiser
3rd year Veterinary Student
Colorado State University
Many people have heard of the frequent and recurring problems that numerous wildlife species in the world face in this day and age. More species are considered endangered than ever before by the International Union for the Conservation of Nature and the list just continues to grow. The main culprits for this struggle are frequently cited as habitat encroachment and poaching, but there exists a less frequently discussed area that damages wildlife species increasingly more in today’s world: infectious disease.
More specifically, infectious diseases originating in domestic animals and spilling into wildlife species that were previously naïve to these pathogens are creating a stir in wildlife news and conservation groups. As efforts increase to preserve natural habitats, civilization begins to surround and hug the borders of these natural areas. With this border-sharing comes the interactions of species that do not normally comingle. When these interactions occur, pathogens are exchanged and wildlife is threatened by a hazard arguably as harmful as poaching and habitat loss. This reverse transmission of disease from domesticated animals into wildlife populations is often termed 'spillback'.
There have been multiple documented examples of spillover and spillback in recent years, including pneumonia in bighorn sheep, tuberculosis in several species (for example - deer, opossums, and badgers), brucellosis in bison, and many others. Some of these have larger impacts than others on wildlife, environmental, and even human health. As such, it can be incredibly complicated to understand and control disease dynamics and transmission patterns once an outbreak begins due to wildlife acting as permanent reservoirs.
One particular area of research that I have chosen to focus in on as of late is global big cat conservation. Big cats have been battling the odds due to the previously mentioned effects of human encroachment and habitat loss. Now, threats have begun to emerge that standard conservation efforts may not be suited to address. One of these threats, canine distemper virus (CDV), is proving to be a greater risk to some of the planet’s most threatened and beloved animals than originally anticipated.
Belonging to the same genus of viruses as Rinderpest and Measles, CDV has a wide range of documented mortality and morbidity, depending on the species in question. In dogs and many other carnivores, the disease manifests as gastrointestinal, respiratory, and neurologic signs often resulting in death. If the animal does survive, lasting neurologic problems are often present. One aspect of the virus that is becoming more apparent, however, is its importance as one of the biggest infectious threats to carnivore species, second only to rabies.
Big cats have shown a particular susceptibility to the virus and first proved its lethal potential on the Serengeti plains in Africa, where it is believed to be responsible for the deaths of 1/3 of the wild lion population in the 1990s. The death toll was estimated to be over 1000 animals, and was characterized by grand mal seizures (warning: links to graphic video) and eventual death. Investigations suggested that the strain of CDV present in the lion populations matched that of the domestic and feral dogs surrounding the area. Vaccination programs were put into place for these dog populations, and though unclear its long term affects, it seemed to quell the source of CDV in the lion populations.
Recent research in Russia and some Asian countries has revealed a very similar problem in the Amur tiger (the largest cat in the world), the Bengal tiger, and several other large felid species of concern throughout the world. With habitat borders fighting to withstand the encroachment of human activity, domestic species interaction with big cats is becoming more common. In many situations, the buffer zones of national parks harbor animals associated with local people, and disease control in these populations is nearly nonexistent. For example, some areas in the US and other countries have nearly eradicated canine distemper with routine vaccination (however, we deal with the disease in raccoons and other wild carnivores in these areas which tend to act as reservoirs). As big cats predate smaller animals, both domestic and feral, they bring the diseases of their prey into the parks and ignite the spread of these infections that are difficult to measure in such naturally elusive species as the tiger.
Though vaccination is a possibility in big cats, it is not always practical to execute. New vaccine strategies are recommended, such as the use of oral bait vaccines such as those used to control rabies in the eastern US. Though an obvious area of growing concern, the complete impact of CDV on big cat populations has not been thoroughly studied. Research for vaccine implementation is currently underway for big cats, but without effective deployment strategies, vaccination of the cats will prove very difficult. Another more sustainable approach would be to implement vaccine programs for the feral and domestic animals that are in close proximity to big cat habitats. As the constant fight for natural space for wildlife continues, buffer zones to national parks must be kept clear of development and other anthropomorphic (caused by human) pressures that may cross the borders of the parks and silently kill the species within.
Packer, C., S. Altizer, M. Appel, E. Brown, J. Martenson, S. J. O'brien, M. Roelke-Parker, R. Hofmann-Lehmann, and H. Lutz. "Viruses of the Serengeti: Patterns of Infection and Mortality in African Lions." Journal of Animal Ecology J Anim Ecology 68.6 (1999): 1161-178.
Gilbert, Martin, Svetlana V. Soutyrina, Ivan V. Seryodkin, Nadezhda Sulikhan, Olga V. Uphyrkina, Mikhail Goncharuk, Louise Matthews, Sarah Cleaveland, and Dale G. Miquelle. "Canine Distemper Virus as a Threat to Wild Tigers in Russia and across Their Range." Integrative Zoology 10.4 (2015): 329-43.
Sharon L. Deem, Lucy H. Spelman, Rebecca A. Yates, and Richard J. Montali. Canine Distemper In Terrestrial Carnivores: A Review." Journal of Zoo and Wildlife Medicine 31.4 (2000): 441-51.
Photos acquired from Veterinary Initiative for Endangered Wildlife, a nonprofit organization dedicated to the conservation and veterinary care of endangered species worldwide.
About the author: Justin Kaiser is in his third year of veterinary school at Colorado State University. His passion is big cat conservation and it's currently focused on studying canine distemper in tigers. He may be reached at firstname.lastname@example.org.