Epidemiology reveals Zhaotong City as the hub of human immunodeficiency virus type 1 transmission from the Yunnan province to other regions in China.

The Yunnan province in China has a high incidence of human immunodeficiency virus-1 (HIV-1) infection. Zhaotong City is located in the Yunnan province, a neglected 'important region'. In this study, we evaluated the unique molecular epidemiological characteristics of HIV-1 infection in Zhaotong City. We collected 305 serum samples from HIV-infected patients in Zhaotong City between May 2015 and April 2016. A total of 122 samples were selected for HIV-1 gag-pol gene amplification, of which 88 were successfully amplified and sequenced for phylogenetic and phylogeographic analysis. Circulating recombinant forms 07_BC (CRF07_BC, 23 cases, 26.14%) and CRF08_BC (49 cases, 55.68%) were the predominant subtypes; the high proportions of these two subtypes differed from those elsewhere in the Yunnan province. The other subtypes were CRF01_AE (11 cases, 12.5%), B (one case, 1.14%) and unique recombinant forms (four cases, 4.55%). Phylogeographic analysis of the CRF07_BC and CRF08_BC subtype strains revealed that Zhaotong was one of the regions in which CRF07_BC and CRF08_BC entered initially. The CRF08_BC strain originated from this region closer to the 'root' position of the phylogenetic tree. Thus, Zhaotong City may have been an important channel in the transmission route of HIV-1 from Yunnan to other parts of the country. Based on this unique distribution of HIV-1 subtypes in Zhaotong City, the epidemic outbreak in this area may have played an important role in the spread of CRF07_BC and CRF08_BC subtypes.