What type of virus is HIV?

What type of virus is HIV?

HIV is a lentivirus, and like all viruses of this type, it attacks the immune system. Lentiviruses are in turn part of a larger group of viruses known as retroviruses. The name 'lentivirus' literally means 'slow virus' because they take such a long time to produce any adverse effects in the body. They have been found in a number of different animals, including cats, sheep, horses and cattle. However, the most interesting lentivirus in terms of the investigation into the origins of HIV is the Simian Immunodeficiency Virus (SIV) that affects monkeys, which is believed to be at least 32,000 years old.¹

So did HIV come from an SIV?

It is now thought that HIV came from a similar virus found in chimpanzees.

It is now generally accepted that HIV is a descendant of a Simian Immunodeficiency Virus because certain strains of SIVs bear a very close resemblance to HIV-1 and HIV-2, the two types of HIV.

HIV-2 for example corresponds to SIVsm, a strain of the Simian Immunodeficiency Virus found in the sooty mangabey (also known as the White-collared monkey), which is indigenous to western Africa.

The more virulent, pandemic strain of HIV, namely HIV-1, was until recently more difficult to place. Until 1999, the closest counterpart that had been identified was SIVcpz, the SIV found in chimpanzees. However, this virus still had certain significant differences from HIV.

What happened in 1999?

In February 1999 a group of researchers from the University of Alabama² announced that they had found a type of SIVcpz that was almost identical to HIV-1. This particular strain was identified in a frozen sample taken from a captive member of the sub-group of chimpanzees known as Pan troglodytes troglodytes (P. t. troglodytes), which were once common in west-central Africa.

The researchers (led by Paul Sharp of Nottingham University and Beatrice Hahn of the University of Alabama) made the discovery during the course of a 10-year long study into the origins of the virus. They claimed that this sample proved that chimpanzees were the source of HIV-1, and that the virus had at some point crossed species from chimps to humans.

Their final findings were published two years later in Nature magazine³. In this article, they concluded that wild chimps had been infected simultaneously with two different simian immunodeficiency viruses which had "viral sex" to form a third virus that could be passed on to other chimps and, more significantly, was capable of infecting humans and causing AIDS.

These two different viruses were traced back to a SIV that infected red-capped mangabeys and one found in greater spot-nosed monkeys. They believe that the hybridisation took place inside chimps that had become infected with both strains of SIV after they hunted and killed the two smaller species of monkey.

They also concluded that all three 'groups' of HIV-1 - namely Group M, N and O (see our strains and subtypes page for more information on these) - came from the SIV found in P. t. troglodytes, and that each group represented a separate crossover 'event' from chimps to humans.