Population dynamics of HIV-2 in rural West Africa: comparison with HIV-1 and ongoing transmission at the heart of the epidemic.

Objectives: To compare the population dynamics of HIV-2 and HIV-1, and to characterize ongoing HIV-2 transmission in rural Guinea-Bissau.
Design: Phylogenetic and phylodynamic analyses using HIV-2 gag and env, and HIV-1 env sequences, combined with epidemiological data from a community cohort.
Methods: Samples were obtained from surveys in 1989-1991, 1996-1997, 2003 and 2006-2007. Phylogenies were reconstructed using sequences from 103 HIV-2-infected and 56 HIV-1-infected patients using Bayesian Evolutionary Analysis by Sampling Trees (BEAST), a relaxed molecular clock and a Bayesian skyline coalescent model.
Results: Bayesian skyline plots showed a strong increase in the 1990s of the HIV-1 effective population size (Ne) in the same period that the Ne of HIV-2 came into a plateau phase. The population dynamics of both viruses were remarkably similar following initial introduction. Incident infections were found more often in HIV-2 transmission clusters, with 55-58% of all individuals contributing to ongoing transmission. Some phylogenetically linked sexual partners had discordant viral loads (undetectable vs. detectable), suggesting host factors dictate the risk of disease progression in HIV-2. Multiple HIV-2 introductions into the cohort are evident, but ongoing transmission has occurred predominantly within the community.
Conclusion: Comparison of HIV-1 and HIV-2 phylodynamics in the same community suggests both viruses followed similar growth patterns following introduction, and is consistent with the hypothesis that HIV-1 may have played a role in the decline of HIV-2 via competitive exclusion. The source of ongoing HIV-2 transmission in the cohort appears to be new HIV-2 cases, rather than the pool of older infections established during the early growth of HIV-2.