Stochastic events, disease and climate change

Managers of even the remaining large lion populations have substantial evidence for concern over chance events leading to considerable population declines.  An epidemic of a morbillivirus closely related to canine distemper virus abruptly emerged in the Serengeti lion population in early 1994. The disease, most likely originating from domestic dogs, spread into the neighbouring Masai Mara Reserve and possibly other unmonitored lion populations in proximity to the Serengeti.  It is estimated that the epidemic claimed approximately 30% of the lion population of the Serengeti - Mara Lion Conservation Unit [1].  The Serengeti population was able to recover from the estimated 2000 surviving individuals, but a similarly devastating outbreak in smaller populations would likely entail compromised possibilities of population recovery as demonstrated in Ngorongoro [2}. Despite extensive domestic dog vaccinations, Serengeti lions were exposed to CDV again in 2006 [3]. 

Another large lion population largely defined by the Kruger National Park in South Africa is currently affected by an insidious and spreading occurrence of bovine tuberculosis; the impact on the population (some sources claim a likely 75 – 80% population loss by the year 2030) and what potential exists for subsequent recovery remain to be determined.  

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.  Current plans for corridors between isolated wildlife populations to promote gene flow might also include the unwanted consequence of disease (epidemic and endemic) spreading between sub-populations.  

Few lion populations would be able to recover from disease challenges similar to those faced by the Serengeti lions.  A single distemper outbreak in the only remaining wild population of 58 black-footed ferrets (Mustela nigripes) caused the species to become extinct in the wild [4]; almost half of all identified lion populations are estimated to contain less than 60 individuals.

Although lion populations can be affected by high mortality over brief periods caused by epidemic viruses such as feline coronavirus (FCoV), feline parvovirus (FPV), feline calicivirus (FCV), and canine distemper virus (CDV), endemic viruses such as feline herpesvirus (FeHV) and feline immunodeficiency virus (FIV) can be constantly prevalent and are thought to exhibit low pathogenicity [5]. 

Further, infection by more than one pathogen can change the expected transmission rates and virulence of a disease [6].  Environmental perturbations can also change the effect on the host by the pathogen/s [7].

100% of extant lion populations are infected with at least one, and most with multiple pathogens, often with multiple strains of the pathogen. Also, different populations, even those geographically close to each other, carry different viruses as seen when comparing the nearby lion populations of Ngorongoro Crater and Serengeti NP [8].

FIV, FeHV, and rarely FCoV, FPV, and CDV infections can persist in seropositive hosts and asymptomatic carriers can continue to transmit or shed the virus [9,10].  Translocation of shedding individuals into a susceptible population thus could entail serious consequences for overall population health [7]. 

Given the lack of data on pathogens infecting lions in the greatest percentage of LCUs, 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.

To discover more about the diseases impacting lions and to what extent they are infected click here.

References

[1] Roelke-Parker ME, Munson L, Packer C, Kock R, Cleaveland S, et al. (1996) A canine distemper virus epidemic in Serengeti lions (Panthera leo).  Nature 379: 441-5. (pdf)

[2] Packer C, Pusey AE, Rowley H, Gilbert DA Martenson J, O’Brien SJ (1991) Case study of a population bottleneck: lions of the Ngorongoro crater. Conservation Biology 5: 219–230 (pdf)

[3} Munson, L, Terio KA, Kock R, Mlengeya T, Roelke ME, et al. (2008) Climate Extremes Promote Fatal Co-Infections during Canine Distemper Epidemics in African Lions. PLoS ONE 3(6): e2545. doi:10.1371/journal.pone.0002545 (pdf)