History of HIV/AIDS False-color scanning electron micrographof HIV-1, in green, budding from cultured lymphocyte. AIDSis caused by the human immunodeficiency virus ( HIV), which originated in non-human primatesin Sub-Saharan Africaand was transferred to humans during the late 19th or early 20th century. Two types of HIV exist: HIV-1and HIV-2. HIV-1 is more virulent, is more easily transmitted and is the cause of the vast majority of HIV infections globally. [ 1 ]The pandemic strain of HIV-1 is closely related to a virus found in the chimpanzeesof the subspeciesPan troglodytes troglodytes, which lives in the forests of the Central African nations of Cameroon, Equatorial Guinea, Gabon, Republic of Congo(or Congo-Brazzaville), and Central African Republic. HIV-2 is less transmittable and is largely confined to West Africa, along with its closest relative, a virus of the sooty mangabey(Cercocebus atys atys), an Old World monkey inhabiting southern Senegal, Guinea-Bissau, Guinea, Sierra Leone, Liberia, and western Ivory Coast. [ 1 ] [ 2 ] Transmission from non-humans to humans Most HIV researchers agree that HIV evolved at some point from the closely related Simian immunodeficiency virus(SIV), and that SIV or HIV (post mutation) was transferred from non-human primates to humans in the recent past (as a type of zoonosis). Research in this area is conducted using molecular phylogenetics, comparing viral genomic sequences to determine relatedness. HIV-1 from chimpanzees and gorillas to humans Scientists generally accept that the known strains (or groups) ofHIV-1 are most close ...subspeciesPan troglodytes troglodytes(SIVcpz), or to the SIV that infects Western lowland gorillas(Gorilla gorilla gorilla), called SIVgor. [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ]The pandemic HIV-1 strain (group M or Main) and a very rare strain only found in a few Cameroonian people (group N) are clearly derived from SIVcpz strains endemic inPan troglodytes troglodytes chimpanzeepopulations living in Cameroon. [ 3 ]Another very rare HIV-1 strain (group P) is clearly derived from SIVgor strains of Cameroon. [ 6 ]Finally, the primate ancestor of HIV-1 group O, a strain infecting tens of thousands of people mostly from Cameroon but also from neighboring countries, is still uncertain, but there is evidence that it is either SIVcpz or SIVgor. [ 5 ]The pandemic HIV-1 group M is most closely related to the SIVcpz collected from the southeastern rain forests of Cameroon (modern East Province) near the Sangha River. [ 3 ]Thus, this region is presumably where the virus was first transmitted from chimpanzees to humans. However, reviews of the epidemiological evidence of early HIV-1 infection in stored blood samples, and of old cases of AIDS in Central Africa have led many scientists to believe that HIV-1 group M early human center was probably not in Cameroon, but rather farther south in the Democratic Republic of the Congo, more probably in its capital city, Kinshasa. [ 3 ] [ 9 ] [ 10 ] [ 11 ] Using HIV-1sequences preserved in human biological samples along with estimates of viral mutation rates, scientists calculate that the jump from chimpanzee to human probably happened during the late 19th or early 20th century, a time of rapid urbanisation and colonisation in equatorial Africa. Exactly when the zoonosisoccurred is not known. Some molecular dating studies suggest that HIV-1 group M had its most recent common ancestor(MRCA) (that is, started to spread in the human population) in the early 20th century, probably between 1915 and 1941. [ 12 ] [ 13 ] [ 14 ]A study published in 2008, analyzing viral sequences recovered from a recently discovered biopsy made in Kinshasa, in 1960, along with previously known sequences, suggested a common ancestor between 1873 and 1933 (with central estimates varying between 1902 and 1921). [ 15 ] [ 16 ] Genetic recombinationhad earlier been thought to seriously confound such phylogenetic analysis, but later work has suggested that recombination is not likely to systematically bias [results], although recombination is expected to increase variance. [ 15 ]The results of a 2008 phylogenetics study support the later work and indicate that HIV evolves fairly reliably. [ 15 ] [ 17 ] H
Posted on: Tue, 21 Jan 2014 15:48:06 +0000
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