Molecular Epidemiology of HIV-1 Infection

The AIDS pandemic is caused by the HIV-1 and HIV-2 viruses which were transmitted to humans from different primate species. One of the main characteristics of HIV is its extensive genetic heterogeneity. Globally circulating HIV-1 strains have been divided into three main groups: (1) the major group M, which is further subdivided into ten phylogenetically distinct subtypes A-J; (2) the outlier group O; and (3) the newly described non-M/non-O group N. The group M is the most prevalent worldwide. Furthermore, intersubtype recombinants consisting of more than two different subtypes have been documented. The global distribution of HIV-1 subtypes varies greatly with geographic origin. The greatest genetic heterogeneity of HIV-1 has been noted in sub-Saharan Africa, whereas in Europe and in North America subtype B prevails. The introduction of many different non-B subtypes, mainly associated with heterosexual transmission, has also been described in Europe. Nevertheless, cases of infections with non-B subtypes that originated in Europe have been documented as well. In Asia, the HIV-1 epidemic is dominated by subtype E, which is mainly found in Thailand and possibly originated from Central Africa. The high genetic heterogeneity observed in Central Africa may be accounted for, at least in part, by the fact that the oldest HIV-1 infection was documented in that region. The global dispersion of the different HIV-1 subtypes appears to be related with socioeconomic changes, immigration, and international traveling rather than to differences in transmissibility. Distinct biological properties of strains belonging to different subtypes, such as coreceptor usage, drug susceptibility and rate of disease progression, have been described recently. The potential association between such biological properties of the virus and genetic subtypes has not been elucidated yet, and further studies are thus needed to clarify the issue.