Environmental and demographic risk factors for respiratory illness in endangered wild mountain gorillas (Gorilla beringei beringei) in Rwanda

Morbidity and mortality from infectious diseases are an important health consideration for free ranging great apes, and respiratory illness (RI) is of particular concern in mountain gorillas (Gorilla beringei beringei), accounting for 25% all of mortalities. In addition to ongoing efforts to determine the etiology of RI in mountain gorillas, an improved understanding of environmental and demographic risk factors for the development of RI is critical for informing a disease prevention strategy for this species. To evaluate potential risk factors for RI, data were compiled from medical and life history records on free-ranging human-habituated mountain gorillas in Rwanda over a 7-year period from December 2005-December 2012. Associations between demographic, environmental and management risk factors and the presence of signs of clinical RI were evaluated using a multivariable mixed-effects logistic regression model. Clinical records from 21 habituated gorilla family groups, including 346 individuals, were included in analyses. Seventy-one percent of groups (n=15/21 gorilla groups) and 55% of individuals (n=192/346 gorillas) experienced at least one RI event. Rainy season (odds ratio [OR]=2.30, 95% CI = 1.74-3.03, P-value<0.001) and group size (OR = 1.15, 95% CI = 1.11-1.18, P-value<0.001) were associated with increased odds of RI in mountain gorillas, and there was a significant trend of increasing odds of RI with increasing group size for groups larger than 16 individuals and a more than 3-fold increase in odds of RI for groups larger than 38 individuals compared to groups of ten or less.