Simian immunodeficiency virus

Simian immunodeficiency virus (SIV) is a retrovirus that is found, in numerous strains, in primates; the specific strains infecting humans are HIV-1 and HIV-2, the viruses that cause AIDS.

The origin of HIV is now generally attributed to SIV from African primates. HIV-2 is most closely related to SIVsm, the SIV strain that primarily infects Sooty Mangabeys, while HIV-1 is closely related to the chimpanzee strain of SIV, designated SIVcpz. The most likely route of transmission of HIV-1 to humans involves contact with the blood of chimps that are often hunted for bushmeat in Africa.

SIV monkey strains are transmitted sexually and usually do not cause AIDS in their natural hosts, Sooty mangabeys and African Green Monkeys, even when the infected hosts carry large viral loads. SIV strains may cause an AIDS-like immune deficiency known as SAIDS (simian acquired immunodeficiency syndrome) if they cross species boundaries. For example, SIVagm, the SIV strain from African Green Monkeys causes SAIDS in Pig-Tailed Macaques. Unlike African Green Monkeys, macques are native to Asia and do not carry their own strain of SIV. Interestingly, yellow and chacma baboons, African monkeys, do not develop SAIDS when they contract SIVagm.

SIV was first discovered in 1985, in captive Rhesus macaques suffering from SAIDS. This observation was made shortly after HIV-1 had been isolated as the cause of AIDS and led to the discovery of the HIV-2 strains in West Africa that same year. HIV-2 was more similar to the then-known SIV strains than to HIV-1, suggesting for the first time the simian origin of HIV. Further studies indicated that HIV-2 is derived from the SIVsm strain found in sooty mangabeys whereas HIV-1, the predominant virus found in humans, is derived from SIV strains infecting chimpanzees (SIVcpz).

The monkey SIV strains do not infect humans and HIV-1 does not infect monkeys. In 2004, this tropism was partly explained by different variants of the protein TRIM5α in humans and monkeys. This intracellular protein recognizes the capsid of various retroviruses and blocks their reproduction. Other proteins such as APOBEC3G/3F may be important in restricting cross-species transmission.

To better study HIV/AIDS in animal models, researchers have created various HIV-SIV chimeras, viruses whose genome partly comes from HIV and partly from SIV. These are often referred to as SHIV.

The ICTVdB code of SIV is 61.0.6.5.003.