Two species of HIV infect humans: HIV-1 and HIV-2. HIV-2 may have originated from the Sooty Mangabey , an Old World monkey of Guinea-Bissau, Gabon, and Cameroon.
HIV-1 is more virulent. It is easily transmitted and is the cause of the majority of HIV infections globally. HIV-2 is less transmittable and is largely confined to West Africa. HIV-1 is the species described below
Likely spread from animal to human populations
Most HIV researchers agree that HIV evolved from the closely related Simian Immunodeficiency Virus (SIV), and that HIV was transferred from non-human primates to humans in the recent past (as a type of zoonosis). The details of how and where this occurred remain controversial, and no single hypothesis has been unanimously accepted..
Cameroon chimpanzees hypothesis
One hypothesis is the so-called 'Hunter' Theory, according to which a human bushmeat hunter was bitten or cut while hunting or butchering an ape, resulting in infection.
Exactly where and when this occurred has been a matter of debate in the scientific community. Researchers announced in May, 2006 that HIV-1 most likely originated in wild chimpanzees in the southeastern rain forests of Cameroon (modern East Province).
Others place the origin of HIV-1 closer to Kinshasa. Seven years of research and 1,300 chimpanzee genetic samples led Dr. Beatrice Hahn, of the University of Alabama at Birmingham, to identify chimpanzee communities near Cameroon's Sanaga River as the most likely original reservoir of HIV-1.
Calculations based on viral mutation rates and HIV-1 sequences preserved in biological samples in 1959 ("ZR59") and 1960 (DRC60) suggest that the jump from chimpanzee to human likely happened during the late 19th century or the early 20th century, a time of rapid urbanisation in the colonial period.Comparative primatologist Jim Moore suggests that colonial practices may have created conditions conducive to the spread of the virus. The hardships of forced labour could have suppressed the immune system of the initial hunter, allowing the virus to infect and take hold in a new host. Rapid urbanisation brought infected people into close contact with others, and colonial commerce provided opportunities for further geographical spread.
Vaccination campaigns against illnesses such as sleeping sickness may have sped the initial spread of HIV-1 when immunisation needles were re-used, and needles were also shared in the booming colonial city of Kinshasa.
Other technological and social disruptions, especially those that affected the food supply and the hunting of bushmeat, are thought to have promoted the cross-over from chimpanzees and the spread amongst humans.
Method of spread
After the initial transfer of SIV from a non-human primate to humans, and the mysterious and sudden change or possible mutation into HIV, the virus ultimately spread via contact among humans to the rest of the world. Since a cross species jump is most likely the origin of HIV, and since HIV became a true epidemic, transmissible from human to human, then the following conditions were needed:
1. a large human population;
2. a large nearby population of the appropriate host animal;
3. an infectious pathogen in the host animal, that eventually produces a mutation that can spread from animal to human;
4. interaction between the species to transmit enough of it to humans to establish a human foothold, which may take millions of individual exposures;
5. a mutation of same pathogen that can spread from human to human;
6. some method that allows the pathogen to disperse widely. This prevents the infection from "burning out" by either killing off its human hosts or provoking immunity in a local population of humans.
Such requirements existed in the remote past with smallpox, and also with the 20th century Spanish flu, despite Spanish flu's New World origin at Fort Riley, Kansas. There the animal reservoir seems to have been two species, chickens and pigs, which were of Old World origin.
Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is more virulent and more easily transmitted. HIV-1 is the source of the majority of HIV infections throughout the world, while HIV-2 is less easily transmitted and is largely confined to West Africa.
Both species of the virus (HIV-1 and HIV-2) are believed to have originated in West-Central Africa and jumped species (zoonosis) from a non-human primate to humans. HIV-1 somehow came from Simian Immunodeficiency Virus (SIVcpz) found in the chimpanzee subspecies Pan troglodytes troglodytes. DNA sequencing indicates that HIV-1 (group M) entered the human population in the early 20th century, probably sometime between 1915 and 1941.
A 2008 paper using a recently-discovered 1960 biopsy suggested a common ancestor between 1884 and 1924.[6] HIV-2 crossed species from a different strain of SIV, this one found in sooty mangabeys (an Old World monkey) of Guinea-Bissau.
SIV in non-human primates tends to cause a non-fatal disease. Comparison of the gene sequence of SIV with HIV should therefore give us information about the factors necessary to cause disease in humans. The factors that determine the virulence of HIV as compared to most SIVs are only now being elucidated. Non-human SIVs contain a nef gene that down-regulates CD3, CD4, and MHC class I expression; most non-human SIVs therefore do not induce immunodeficiency; the HIV nef gene however has lost its ability to down-regulate CD3, which results in the immune activation and apoptosis that is characteristic of chronic HIV infection.
No comments:
Post a Comment