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Origins of HIV
Sunday, February 1, 2009/ 1:23 AM
The Origins of HIV
There are two lineages of HIV-1, both of which are related to SIV isolated from chimpanzees(cpz). Of the two lineages of HIV-1, the M lineage, is found worldwide and is responsible for the majority of human infections. The O lineage, is found only in western Africa and in France. It is not known if these two lineages represent two different introductions of HIV into humans or whether they represent a single introduction that has subsequently diverged. Very few isolations of SIV from chimps have been made, and the prevalence of SIV in chimps is not have been made, and the prevalence of SIV in chimps is not known. In fact, it has been demonstrated that SIVcpz is a natural infectious agent in wild chimp populations.
There are two lineages of HIV-1, both of which are related to SIV isolated from chimpanzees(cpz). Of the two lineages of HIV-1, the M lineage, is found worldwide and is responsible for the majority of human infections. The O lineage, is found only in western Africa and in France. It is not known if these two lineages represent two different introductions of HIV into humans or whether they represent a single introduction that has subsequently diverged. Very few isolations of SIV from chimps have been made, and the prevalence of SIV in chimps is not have been made, and the prevalence of SIV in chimps is not known. In fact, it has been demonstrated that SIVcpz is a natural infectious agent in wild chimp populations.
HIV-2 represent a distinct lineage that is closely related to SIV of sooty mangabey monkeys (smm) and of macaques (mac). SIV of African green monkeys (agm) and of mandrills (mnd) form other lineages that are more closely related to HIV-2 than to HIV-1. It is clear that SIVmm and SIVagm are naturally occurring infectious agents that are widespread in Africa and have coevolved with their monkey hosts. Sequence comparisons have shown that different isolates of SIVagm group with their hosts rather than by geography, and they are therefore adapted to their hosts. They cause no disease in their natural host, but SIVsmm does cause AIDS when transferred to Asian macaques in captivity. HIV-2 is found primarily in western central Africa, where its distribution is almost coincident with that of mangabey monkeys. It seems clear that HIV-2 represents a separate introduction of SIV from a different monkey host into humans.
Thus, SIV must be the source of HIV, with different viruses serving as the source of HIV-1 and HIV-2. When the viruses crossed the species barrier and became established in man as HIV is not clear. HIV-1 has been isolated from serum collected in 1959 in Zaire and antibodies to HIV have been found in serum collected in 1963 in Burkino-Faso, so HIV-1 has been in the human population at least that long. From sequencing studies of the glycoprotein gene and examination of the rate of divergence, one estimate is that the virus might have entered the human population about 70 years ago, although estimates of divergence rates are controversial. A likely scenario is that the viruses have been introduced into human populations many times in the past but failed to become epidemic. Infections in humans either died out because of the low transmissibility of the virus or smoldered in only a small fraction of the population. The different lineages of HIV-1 and, especially, of HIV-2 suggest, in fact, that they may result from independent introductions into humans, consistent with the hypothesis that multiple introductions of both viruses have occurred.
Recent changes in human behaviour, including more extensive travel by truck, bus, and plane, changes in sexual practices, and the use of injectable drugs, as well as the increase in the human populations, could have allowed the virus to spread more extensively than in the past and to become epidemic worldwide. The spread of the virus could also have been aided by the appearance of mutants that were more easily transmissible from person to person. The large increase in population during the last century has certainly resulted in more opportunities for the introduction and spread of the virus in humans, and therefore for the selection of such transmissible mutants.
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