The epidemic emergence of HIV: what novel enabling factors were involved?


Humans acquired retroviruses from simians, mainly through bushmeat handling. All epidemically successful HIV groups started to spread in early 20th century, contrasting with the antiquity of T-cell lymphotropic viruses, implying that novel enabling factors were involved in HIV emergence. Here we review the Parenteral Serial Transmission and the Enhanced Heterosexual Transmission hypotheses for the adaptation and early spread of HIV. Epidemic start roughly coincides in time with peak genital ulcer disease in cities, suggesting a major role for sexual transmission. Only ill-adapted and rare HIV groups emerged after approximately 1950, when injections and transfusions attained their maximal levels, suggesting that if parenteral serial transmission was necessary for HIV adaptation, it had to be complemented by sexual transmission for HIV to reach epidemic potential.

Both human immunodeficiency virus types (HIV-1 and HIV-2) have been acquired by humans from nonhuman primates infected with simian immunodeficiency virus (SIV). HIV-1 comprises four known groups, each demonstrably derived from an independent simian-to-human transmission [1]. The pandemic group M (HIV-1-M), and the rare group N (HIV-1-N) originated from SIV infecting central chimpanzees (Pan troglodytes troglodytes; SIVcpz) living in southeast and south-central Cameroon, respectively [1–3]. HIV-1 group O (HIV-1-O, which infects <20,000 people in Central Africa) and the rare group P (HIV-1-P) originated from SIVgor, endemic in western lowland gorillas (Gorilla gorilla gorilla) [4,5]. HIV-2 is divided into nine known groups (A–I), all independent crossovers from SIVsmm, infecting West African sooty mangabeys (Cercocebus atys atys) [6–9]. Of these, only groups A and B have spread widely, probably infecting together more than one million people [10]; HIV-2 groups D and F were found in two individuals each [6,8,9]; the remaining HIV-2 groups (C, E, G–I) were identified in just one person each [6,7], and might therefore not deserve the classification of a group.

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