When HIV first burst onto the scene in the 1980s, what it was and where it came from sparked huge debate among the public and scientific community, with ideas ranging from some unknown “contagion” to a “gay disease” to some kind of man-made pathogen released by the government.
Of course, we now know that the causative agent of AIDS is a virus and that HIV has its origins in viruses found naturally in various non-human primate species. These viruses jumped the species barrier and entered the human population on several separate occasions, giving rise to the different groups of HIV. While this much was known, what has remained hazy is which apes harbored the viruses that resulted in two of the four known HIV-1 groups. Now, after decades of confusion, scientists have finally pinpointed their origins: western lowland gorillas.
AIDS is caused by two related viruses, HIV-1 and HIV-2, which came from a group of viruses that infect more than 40 primate species in Africa, the simian immunodeficiency viruses. HIV-1 is responsible for the majority of infections worldwide, whereas HIV-2 has remained largely restricted to West Africa.
HIV-1 is further subdivided into four distinct groups, or clades, known as M, N, O and P—each of which resulted from a separate cross-species transmission event of SIV into the human population, probably as a consequence of man hunting primates in Africa and butchering them for food. But what is particularly interesting about the origins of HIV is that each of these events have had very different outcomes.
Groups M and N, for example, have both been traced to geographically distinct chimp populations in Cameroon, but while N has resulted in fewer than 20 human infections, M—the cause of the AIDS pandemic—has infected some 40 million people worldwide. O and P are also poles apart in terms of documented infections; O has resulted in a substantial epidemic in West Africa, but P is known from just two cases. But where did these two very different groups come from? While scientists managed to trace the origins of M and N, they struggled to do the same for O and P, which is why researchers headed to Africa to investigate.
As described in Proceedings of the National Academy of Sciences, they started off by collecting fecal samples from western lowland gorillas, eastern lowland gorillas and mountain gorillas living in Cameroon, Gabon, the Democratic Republic of Congo and Uganda. They then sequenced these samples and looked for evidence of infection with the gorilla SIV. Interestingly, they found this virus only in western lowland gorillas residing in four sites in southern Cameroon, but not in the other countries or gorilla subspecies. And while the viruses they did find exhibited high levels of diversity, two were strikingly similar to the HIV-1 group P and another appeared to be very closely related to group O. From these data, the scientists concluded that these groups originated in western lowland gorillas.
Alongside solving a long-standing mystery, understanding the origin of emerging diseases has important implications for gauging human infection risks, the authors note. For example, studies such as this highlight the fact that chimps and gorillas harbor viruses that are capable of jumping into humans and triggering significant disease outbreaks.