AIDS is caused by a human immunodeficiency virus (HIV), which originated in non-human primates in Central and West Africa. While various sub-groups of the virus acquired human infectivity at different times, the present pandemic had its origins in the emergence of one specific strain – HIV-1 subgroup M – in Léopoldville in the Belgian Congo (now Kinshasa in the Democratic Republic of the Congo) in the 1920s. There are two types of HIV: HIV-1 and HIV-2. HIV-1 is more virulent, more easily transmitted, and the cause of the vast majority of HIV infections globally. The pandemic strain of HIV-1 is closely related to a virus found in chimpanzees of the subspecies Pan troglodytes troglodytes, which live in the forests of the Central African nations of Cameroon, Equatorial Guinea, Gabon, the Republic of the Congo, and the Central African Republic. HIV-2 is less transmissible 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.
Transmission from non-humans to humans
Research in this area is conducted using molecular phylogenetics, comparing viral genomic sequences to determine relatedness.
HIV-1 origins: cross-species transmission from chimpanzees and gorillas Scientists found that the closest relatives of HIV-1 are simian immunodeficiency viruses (SIVs) infecting wild-living chimpanzees and gorillas in West Central Africa. In particular, each of the known HIV-1 strains is either closely related to the SIV that infects the chimpanzee subspecies Pan troglodytes troglodytes (SIVcpz) or closely related to the SIV that infects western lowland gorillas, called SIVgor. The pandemic HIV-1 strain (group M or Main) and a rare strain found only in a few Cameroonian people (group N) are clearly derived from SIVcpz strains endemic in Pan troglodytes troglodytes chimpanzee populations living in Cameroon. Another very rare HIV-1 strain (group P) is clearly derived from SIVgor strains of Cameroon. Finally, the primate ancestor of HIV-1 group O, a strain infecting 100,000 people mostly from Cameroon but also from neighbouring countries, was confirmed in 2006 to be SIVgor. Based on this, scientists believe that Cameroon might be the initial location where HIV-1 transmitted from simians to humans. Later, through retrospective analysis of the co-prevalence of HIV-1 and genital ulcer disease in Africa, researchers speculated that the epicenter of the early HIV-1 epidemic could have been Kinshasa in the Democratic Republic of the Congo, further south than Cameroon, which was known as Léopoldville, Belgian Congo. Using molecular clock and phylogenetic analysis methods, scientists have revealed the origins of HIV-1 group M and group O. The most recent common ancestor (MRCA) (that is, started to spread in the human population) of HIV-1 group M is around 1931 (1915 - 1941), while the MRCA of group O is estimated to be around 1920 (1890 - 1940). Another study amplified and analyzed HIV-1 sequences from a 1960 lymph node biopsy in Kinshasa, positioning it close to the ancestral node of subtype A and, through relaxed molecular clock analysis with another early sequence, dated the MRCA of HIV-1 group M to the early 20th century. Genetic recombination had earlier been thought to affect the accuracy of phylogenetic analysis, but subsequent studies have confirmed the reliability of this approach. However, due to the critically endangered status of primates, the sample sizes in existing studies are generally very small, making it nearly impossible to further utilize them for research on the origins of HIV.
HIV-2 origins: cross-species transmission from sooty mangabeys
SIV of sooty mangabeys (SIVsmm) is believed to be the progenitor of HIV-2 and has been transmitted to humans on multiple occasions. A molecular epidemiological survey of SIVsmm in a community of approximately 120 free-ranging sooty mangabeys in the Taï Forest of Ivory Coast revealed that the two strains of HIV-2 (groups A and B) that spread widely in humans likely originated from this region. There are six additional known HIV-2 groups, each having been found in just one person. They all seem to derive from independent transmissions from sooty mangabeys to humans. Groups C and D have been found in two people from Liberia, groups E and F have been discovered in two people from Sierra Leone, and groups G and H have been detected in two people from the Ivory Coast. These HIV-2 strains are probably infections of dead-end hosts, and each of them is most closely related to SIVsmm strains from sooty mangabeys living in the same country where the human infection was found. HIV-2 groups A and B originated in the first half of the 20th century, with their MRCAs estimated at 1940 ± 16 and 1945 ± 14, respectively, stemming from zoonotic transmission of SIVsmm. The rapid epidemic expansion of HIV-2 subtype A in Guinea-Bissau began around 1955–1970, coinciding with sociocultural changes during the independence war (1963–1974), while the SIV common ancestor in sooty mangabeys (around 1809) suggests a cross-species transmission to humans occurred only a few hundred years before the HIV-2 outbreak.
Bushmeat practice According to the natural transfer theory (also called "hunter theory" or "bushmeat theory"), the virus was transmitted from an ape or monkey to a human when a hunter was cut or otherwise injured while hunting or butchering an infected animal. The resulting exposure to blood or other bodily fluids of the animal can result in SIV infection. In rural Africa, due to food shortages, it is common to rely on bushmeat as a primary source of meat. Scientists believed that this situation may increase the probability of cross-species virus transmission. A study revealed that the more people come into contact with bushmeat, the higher their risk of SIV infection: the percentage of people showing seroreactivity to antigens—evidence of current or past SIV infection—was 2.3% among the general population of Cameroon, 7.8% in villages where bushmeat is hunted or used, and 17.1% in the most exposed people of these villages. However, the mechanism by which SIV evolved into HIV after crossing species to humans remains unclear.
Emergence
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