Origin and diversity of human retroviruses.

Simian immunodeficiency viruses, simian T­cell lymphotropic viruses, and simian foamy viruses from nonhuman primates have crossed the species barrier to humans at several time points, leading to the HIV and human T lymphotropic virus epidemic and to sporadic cases of human infections with simian foamy viruses, respectively. Efficient infection and spread in humans differs between simian foamy virus, simian lymphotropic virus, and simian immunodeficiency virus, but seems also to differ among the different viruses from the same simian lineage, as illustrated by the different spread of HIV­1 M, N O, P or for the different HIV­2 groups. Among the four HIV­1 groups, only HIV­1 group M has spread worldwide, and the actual diversity within HIV­1 M (subtypes, circulating recombinants) is the result of subsequent evolution and spread in the human population. HIV­2 only spread to some extent in West Africa, and similarly as for HIV­1, the nine HIV­2 groups have also a different epidemic history. Four types of human T lymphotropic virus, type 1 to 4, have been described in humans and for three of them simian counterparts (simian T lymphotropic virus­1, ­2, ­3) have been identified in multiple nonhuman primate species. The majority of human infections are with human T lymphotropic virus­1, which is present throughout the world as clusters of high endemicity. Humans are susceptible to a wide variety of simian foamy viruses and seem to acquire these viruses more readily than simian immunodeficiency viruses or simian T lymphotropic viruses, but neither signs of disease in humans nor human­to­human transmission of simian foamy virus have been documented yet. The current HIV­1 M epidemic illustrates the impact of a single cross­species transmission. The recent discovery of HIV­1 P, HIV­2 I, new human T lymphotropic virus­1 and ­3 variants, as well as simian foamy virus infections in humans in Central Africa, show that our knowledge of genetic diversity and cross­species transmissions of simian retroviruses is still incomplete.