Impact of simian immunodeficiency virus infection on chimpanzee population dynamics
Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL, United States; Department of Anthropology, Stanford University, Stanford, CA, United States; Woods Institute for the Environment, Stanford University, Stanford, CA, United States; Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN, United States; Department of Medicine, University of Alabama at Birmingham, Birmingham, AL, United States; Fauna and Flora International, Monrovia, Liberia; Africa Program, Wildlife Conservation Society, Bronx, NY, United States; Gombe Stream Research Centre, The Jane Goodall Institute, Kigoma, Tanzania; The Jane Goodall Institute, Arlington, VA, United States; Lester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL, United States; The AIDS and Cancer Virus Program, SAIC-Frederick Inc., National Cancer Institute-Frederick, Frederick, MD, United States; Department of Evolutionary Anthropology, Duke University, Durham, NC, United States; Department of Anthropology, University of Minnesota, Minneapolis, MN, United States
Like human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus of chimpanzees (SIVcpz) can cause CD4+ T cell loss and premature death. Here, we used molecular surveillance tools and mathematical modeling to estimate the impact of SIVcpz infection on chimpanzee population dynamics. Habituated (Mitumba and Kasekela) and non-habituated (Kalande) chimpanzees were studied in Gombe National Park, Tanzania. Ape population sizes were determined from demographic records (Mitumba and Kasekela) or individual sightings and genotyping (Kalande), while SIVcpz prevalence rates were monitored using non-invasive methods. Between 2002-2009, the Mitumba and Kasekela communities experienced mean annual growth rates of 1.9% and 2.4%, respectively, while Kalande chimpanzees suffered a significant decline, with a mean growth rate of -6.5% to -7.4%, depending on population estimates. A rapid decline in Kalande was first noted in the 1990s and originally attributed to poaching and reduced food sources. However, between 2002-2009, we found a mean SIVcpz prevalence in Kalande of 46.1%, which was almost four times higher than the prevalence in Mitumba (12.7%) and Kasekela (12.1%). To explore whether SIVcpz contributed to the Kalande decline, we used empirically determined SIVcpz transmission probabilities as well as chimpanzee mortality, mating and migration data to model the effect of viral pathogenicity on chimpanzee population growth. Deterministic calculations indicated that a prevalence of greater than 3.4% would result in negative growth and eventual population extinction, even using conservative mortality estimates. However, stochastic models revealed that in representative populations, SIVcpz, and not its host species, frequently went extinct. High SIVcpz transmission probability and excess mortality reduced population persistence, while intercommunity migration often rescued infected communities, even when immigrating females had a chance of being SIVcpz infected. Together, these results suggest that the decline of the Kalande community was caused, at least in part, by high levels of SIVcpz infection. However, population extinction is not an inevitable consequence of SIVcpz infection, but depends on additional variables, such as migration, that promote survival. These findings are consistent with the uneven distribution of SIVcpz throughout central Africa and explain how chimpanzees in Gombe and elsewhere can be at equipoise with this pathogen.
Africa; article; chimpanzee; demography; genotype; growth rate; habitat quality; infection rate; mathematical model; nonhuman; population dynamics; population growth; Simian immunodeficiency virus; survival; Tanzania; virus infection; virus transmission; Animals; CD4-Positive T-Lymphocytes; Computer Simulation; Feces; Female; Humans; Male; Models, Statistical; Pan troglodytes; Phylogeny; Population Dynamics; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; RNA, Viral; Simian Acquired Immunodeficiency Syndrome; Simian immunodeficiency virus; Tanzania; Human immunodeficiency virus 1; Pan; Simian immunodeficiency virus