Origin and Spread of HIV-1 Subtype B among Heterosexual Individuals in Bulgaria

Human immunodeficiency virus (HIV) was originally introduced in Bulgaria through heterosexual transmission (HET) and later transferred to other vulnerable groups along with numerous more recent introductions from outside Bulgaria. To define the diversity, origins, and dynamics of the HIV-1 subtypes prevalent in HET population in Bulgaria, we applied phylogenetic and phylodynamic analyses using polymerase (pol) sequences from HET individuals to infer the spatiotemporal evolutionary history of the HIV-1 epidemic in this population in Bulgaria. High genetic diversity was found, including 13 different HIV-1 subtypes: 45.7% subtype B, 19.9% CRF01-AE, 7.5% CRF02-AG, 7.5% sub-subtypes A1 and A6, 7.1% subtype C, 5.3% subtype F1, 4.0% URFs, 1.2% CRF05-DF, 0.6% subtype G, 0.3% CRF04-cpx, 0.3% CRF29-BF, 0.3% CRF14-BG, and 0.3% subtype H. The estimated root of the subtype B in the phylogenetic tree dated back to the year 1980 largely due to multiple introductions of subtype B from outside the country. Several significant clades have been identified highlighting six different main epidemic entrances of subtype B dating from 1989 to 2007. The Bayesian skyline plot showed two different exponential growth periods starting in the 1980s to 1990 followed by a constant phase up to about 2008, with another exponential growth period from 2008 to the year 2012. The migration analysis identified dynamic pattern of gene flow and demonstrated that many HET probably acquired the infection abroad (14.6%), while only (6.6%) of non-HET were infected outside country. The phylogenetic analysis showed an intermixing between sequences from Bulgarians with sequences from other countries, suggesting different HIV introduction in this country followed by the internal spread through local transmission networks.