Methods
Mortality rates typical of disease outbreaks in
wild apes were characterized from data compiled from sixteen previously published outbreaks, wherein community size, number infected and the mortality rate were explicitly reported. We used a demographic modeling exercise to demonstrate the resilience of populations to disease outbreaks. To describe population growth in gorillas we used a discrete,
logistic model:
N(t+1)=N(t)+R*N(t)*(K-N(t)/K)
We parameterized the model using an estimated demographic rate (R) for gorillas in the Virunga Mountains of Rwanda. To be conservative, we used highly optimistic estimates which tended to overestimate gorilla reproductive potential. They yielded a Leslie matrix estimate for the annual rate of increase of R= 3.7% (Walsh and Caillaud, unpublished). The estimated population size was 320
gorillas, and the carrying capacity estimate (K) was 300 to 500, so we used a midpoint of K = 400.
We considered a series of five scenarios in which proportional mortality rate, a, corresponded to the mortality rate observed in a real outbreak. In each scenario, we seeded the logistic growth model with an initial, post-outbreak population size of
N(0)=N-a*N
then iterated the logistic model in annual times steps until gorilla population size reached the initial population size, N, as a measure
of recovery time. To examine the resilience of gorillas to disease we considered five disease mortality scenarios and the outputs of these simulations are reported here.
We examined human vaccination rates and reported cases (where available) for five exemplar great ape range countries using the UNICEF/WHO 2009 global immunization summary and the WHO 2009 WHS (World Health Statistics), for seven diseases known to be communicable to great apes. We also included data from the United Kingdom, a leading source country for ape tourists. We conducted a literature review of human vaccines for pathogens that were either already known to infect wild apes or presented a high risk of infection (e.g. respiratory pathogens likely to be carried by tourists). For each pathogen we scored whether at least one vaccine was licensed (L) or under development; in the advanced stage of development (A) if the most advanced vaccine was in human clinical trials; or in the early stage of development (E), if the most advanced vaccine was not yet in human clinical trials but had protected captive non-human primates from pathogen challenge. We also identified mode of transmission, the identity of the reservoir host, and the likely duration of vaccineinduced immunity.
