Monkeys do not carry Human Immunodeficiency Virus (HIV). Instead, many non-human primate species in Africa naturally host Simian Immunodeficiency Virus (SIV), a related virus. While HIV causes Acquired Immunodeficiency Syndrome (AIDS) in humans, SIV typically does not lead to disease in its natural monkey hosts. Understanding SIV provides significant insights into the historical connection between these viruses and human health. This knowledge is also important for scientific advancements in the prevention and treatment of HIV/AIDS.
Understanding Simian Immunodeficiency Virus (SIV)
Simian Immunodeficiency Virus (SIV) is a diverse group of retroviruses that persistently infect at least 45 species of non-human primates. As a retrovirus, SIV carries its genetic information in RNA rather than DNA. When SIV infects a host cell, it uses an enzyme called reverse transcriptase to convert its RNA into DNA, which then integrates into the host cell’s genome. This integration allows the virus to use the host cell’s machinery to replicate.
SIV is closely related to HIV. Despite these genetic similarities, SIV generally causes non-pathogenic infections in its natural hosts. This means that while infected monkeys carry high viral loads, their immune systems effectively manage the infection without developing severe immune deficiency. This contrasts sharply with HIV-1 and HIV-2 infections in humans, which typically lead to AIDS.
The long co-evolution between SIV and its natural monkey hosts has resulted in this unique coexistence. However, if SIV is introduced into a non-natural host, such as Asian macaques, it can cause a disease similar to AIDS, known as simian AIDS (SAIDS). This difference highlights the specific adaptations that natural hosts have developed to tolerate the virus. Researchers study SIV to understand HIV/AIDS, as SIV infection in primates serves as a model for analyzing vaccine potency and disease progression.
SIV Among Primate Populations
SIV is widespread among many non-human primate species across Africa. These include sooty mangabeys, African green monkeys, and chimpanzees. SIV has been present in monkeys and apes for at least 32,000 years.
Natural transmission of SIV within primate populations primarily occurs through contact with infected bodily fluids, such as blood. Sexual transmission is considered the main route of SIV spread in African green monkeys. Aggression and fights involving biting can also contribute to transmission. Mother-to-offspring transmission is negligible in some natural hosts like African green monkeys.
In most of their natural hosts, SIV causes persistent infections but rarely leads to severe illness. For example, sooty mangabeys infected with SIVsmm do not develop disease despite high levels of circulating virus. However, SIV infection in chimpanzees can provoke a disease resembling AIDS, similar to HIV in humans.
The Origin of Human Immunodeficiency Virus (HIV)
Human Immunodeficiency Virus (HIV) originated from SIV through a process called zoonotic transmission, where a virus jumps from an animal host to humans. Scientists have identified two main types of HIV, HIV-1 and HIV-2, each stemming from different SIV strains found in African primates.
HIV-1, which is responsible for the global AIDS pandemic, is closely related to SIVcpz, a strain found in chimpanzees in West Central Africa. Specifically, HIV-1 group M, the most common strain, likely arose from SIVcpz. HIV-2, which is less transmissible and primarily found in West Africa, originated from SIVsmm found in sooty mangabeys. There have been at least four independent transmissions of SIV from chimpanzees and gorillas leading to HIV-1 groups M, N, O, and P, and at least eight or nine independent transmissions of SIVsmm to humans resulting in HIV-2.
The most likely mechanism for these cross-species transmissions involved humans coming into contact with infected primate blood. This often occurred during hunting and butchering of primates for bushmeat. If a hunter was cut or had an existing injury while handling an infected animal, the virus could enter the human bloodstream. The initial transmission of SIVcpz to humans, leading to HIV-1 group M, is estimated to have occurred around 1908, with a likely range between 1884 and 1924.
Current Transmission Risks
Direct transmission of SIV from wild primates to humans is generally rare but remains possible under specific circumstances. The risk is highest for individuals who hunt primates and prepare their meat for consumption. Contact with infected blood or tissues during these activities can facilitate spillover. Inoculation with SIV-containing material, such as through accidental needle sticks in research settings, also represents a potential route of exposure.
While SIV can still cross into humans, the primary mode of HIV transmission globally is human-to-human. The emergence of new HIV strains from SIV cross-species transmission is a concern, given the ongoing exposure of humans to a variety of SIVs in sub-Saharan Africa. However, only a fraction of documented cross-species transmission events have resulted in viruses that spread widely within human populations.