An Unwelcome Gift from Gorillas

July 3, 2015

By Medical Discovery News


You probably know that Acquired Immunodeficiency Syndrome (AIDS), which has affected 79 million people and killed 39 million since 1981, is the result of Human Immunodeficiency Virus (HIV). What you may not know is that there are several different types of this virus and they did not all come from the same source, making the search for HIV’s origins lengthy and complicated.

There are four groups of HIV-1: M, N, O, and P. Each of them was transmitted between African primates as simian immunodeficiency viruses (SIVs) before infecting humans, and each crossed species to humans independently. More than 40 African primates carry SIVs, which emerged up to 6 million years ago. It is likely that transmission to humans has occurred many times when hunters where exposed to the blood and tissues of infected animals. However the isolation of humans in Africa limited the spread of SIVs that crossed into humans until the last century.

It was not until modern travel allowed infected humans to move from the bush to cities and from there to other cities and countries that an HIV strain such as M took hold among humans, leading to a global pandemic. Group M causes more than 90 percent of AIDS cases and currently affects 40 million people worldwide. We already know that it came from chimpanzees in southern Cameroon. Group N also came from chimpanzees, but has infected less than 20 people.

Group O has infected about 100,000 people in Cameroon, Chad, Gabon, Niger, Nigeria, Senegal, and Togo. Although anti-retroviral drug combinations have made HIV infections survivable, many in Africa and the developing world do not have access to these treatments. Group P has only been isolated from two people. The origins of groups O and P were previously unknown, but now their source has been definitely confirmed: gorillas.

Scientists gathered fecal samples from western lowland, eastern lowland, and mountain gorillas, screening them for SIV antibodies and genetic information. Despite testing many wild troops of gorillas throughout Cameroon, Gabon, the Democratic Republic of Congo, and Uganda, the virus was identified at only four sites. Two strains of SIVs from southwestern Cameroon resembled HIV Group P and one from central Cameroon resembled Group O.

Not only does this data prove that gorillas were the immediate source of groups O and P, but the genetic information revealed that the viruses originated through a cross-species transmission from chimpanzees to gorillas. These are the same chimpanzees that infected humans, leading to groups M and N. Chimpanzees and gorillas share the same habitat, so the virus could have infected a gorilla if it bit a chimpanzee with SIV or was exposed to its blood or tissues.

Understanding the origins of HIV in humans is crucial if we want to prepare for additional viruses, especially SIV variants, entering the human population in the future, which will remain a risk as long as humans continue to hunt and eat primates.

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