Last updated 7 Mar 2012
Endemic and epidemic diseases, viruses and parasites impacting lion (Panthera leo) populations
Diseases can be classified as endemic or epidemic dependant on their persistence in a population. Although lion populations can be affected by high mortality over brief periods caused by epidemic viruses, endemic viruses can be constantly prevalent and are thought to exhibit low pathogenicity .
Epidemic disease risks for animals living in fragmented small populations become significantly higher as contact with human and domestic animal populations become more frequent and as a result of alterations in microclimate and landscape ecology. The tools to predict, prevent and respond to these risks are not well established in conservation management.
Co-infection by more than one pathogen can change the expected transmission rates and virulence of a disease  whilst also promoting parasite infection. Environmental perturbations can also change the effect on the host by the pathogen/s . 100% of extant lion populations are infected with at least one, and most with multiple pathogens, often with multiple strains of those pathogens.
Different populations, even those geographically close to each other, carry different viruses . Plans for corridors between isolated wildlife populations to promote gene-flow might also include the unwanted consequence of spreading diseases between sub-populations and promoting co-infections. This is especially of concern where one population may be naive to a disease carried by another and as such, have no acquired immunity to it. Many infections can persist in seropositive hosts and asymptomatic carriers can continue to transmit, or shed, the virus [5-6]. Translocation of shedding individuals into a susceptible population thus could entail serious consequences for overall population health .
Given the lack of data on pathogens infecting lions in the greatest percentage of lion populations, the consequences of infection upon the host species, the alterations to transmission and virulence of these diseases through combination of the viruses and their subtypes in a single host or how these combined pathogens respond to stochastic disease outbreaks, it should be considered that using any extant wild populations as a source for reintroduction programs should involve extreme caution.
It should also be noted that the pathogenicity and course of disease progression of many of the feline pathogens discussed in this section has only been carefully described among domestic cats. However, post-mortem studies of mortalities among exotic felids in zoos indicate that among many viral diseases at least, there are parallel consequences of infection. In addition, the occurrence of a particular disease among free-ranging lion populations is by necessity based on serum antibody presence, which is by definition retrospective – the animal survived the challenge. In only few cases, as perhaps in the Canine Distemper Virus outbreak among lions in the Serengeti in 1994, is progression of a disease recorded with any scientific rigour in a wild lion population.
Mostly, lions simply “disappear” from study populations. This is especially true of lion cubs, the most disease susceptible among the population. Carcasses, even if found intact by field biologists, cannot be given the careful post-mortem analysis to determine cause of death as such biologists neither have the means nor the expertise to do so. Multiple infections by a diversity of pathogens as is common among lions will also cloud a clear diagnosis of mortality. It is entirely likely that lions, like many other wildlife species, eventually succumb to an overload rather than a single pathogen.
What has become clear is that lion populations, even in protected areas like Kruger National Park (bovine tuberculosis) and the Serengeti (canine distemper), are fragile and susceptible to introduced diseases in addition to those that we assume occur naturally in their environment. The high number of individuals infected with the several strains of lion Feline Immunodeficiency Virus now identified is worrisome, as it has belatedly been recognized that this virus erodes immune competence over time as similar among domestic cats. Some strains seem more virulent than others, and individuals infected with multiple strains are potent sources for the evolution of new FIV viral strains.
Disease threats to wild animals have long been recognized and now become ever more important as we deal with small and isolated wildlife populations. The increasing rate of lion population declines has many causes, and we need to pay careful attention to pathogens as contributory agents in the future. We cannot medicate and inoculate wild lions. But we can more carefully identify sources of diseases from domestic animals and ensure they do not increasingly spread to wild carnivores.
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