Monkeys do not carry the human immunodeficiency virus (HIV), but they harbor a closely related pathogen. This distinction is important because the history of HIV is a story of viral evolution and transmission across species barriers, a process known as zoonosis. The virus found in primates is the ancestor of the human virus, and understanding this relationship provides insight into how new diseases emerge. The origins of the human pandemic are directly linked to natural infections in African non-human primates.
Simian Immunodeficiency Virus
The virus circulating in African non-human primates is called Simian Immunodeficiency Virus (SIV). Over 40 different species of African monkeys and apes are infected with unique, species-specific strains of SIV, which have been present for thousands of years. For example, SIV strains are found in African green monkeys and sooty mangabeys.
SIV is non-pathogenic, meaning it does not cause illness in its natural primate host, despite high levels of viral replication. This lack of disease progression results from long-term co-evolution between the virus and the primate immune system. Natural hosts have developed specific adaptations, such as lower expression of the viral entry receptor CCR5, which prevents the severe immune activation and cell depletion seen in humans.
Defining the Difference Between SIV and HIV
SIV and HIV are both retroviruses in the Lentivirus genus. They share fundamental characteristics, such as using RNA as genetic material and relying on reverse transcriptase to copy their genome into DNA. The primary distinction is the host they infect and the resulting disease. HIV causes Acquired Immunodeficiency Syndrome (AIDS) in humans, while SIV is the ancestral virus found in non-human primates.
The human virus exists in two main types: HIV-1 and HIV-2. Genetic analysis confirms these human viruses originated from separate SIV strains in different primate species. HIV-1, the strain responsible for the global pandemic, is related to SIV found in chimpanzees (SIVcpz) and gorillas (SIVgor). HIV-2, which is mostly confined to West Africa, originated from SIV found in sooty mangabeys (SIVsmm).
How SIV Crossed Species to Become HIV
The transmission of SIV from primates to humans is an example of zoonosis, or cross-species jump. This event occurred when humans came into direct contact with the blood or other bodily fluids of infected primates. The most plausible route involves hunters or butchers being exposed to the animal’s blood during hunting and preparation of bushmeat in Central and West Africa.
Multiple, independent transmission events were necessary to establish the viruses in the human population. For HIV-1, the most common strain (Group M) is estimated to have jumped from chimpanzees to humans sometime before 1931, likely in the Democratic Republic of Congo. In total, HIV-1 arose from at least four separate transmissions from chimpanzees and gorillas to humans. Similarly, HIV-2 resulted from at least eight to nine independent cross-species transmissions from sooty mangabeys. The spread was facilitated by factors like urbanization and changing transportation routes in the 20th century.
Using SIV to Understand HIV
Research into SIV remains relevant because studying the virus in its natural, non-pathogenic hosts offers a unique perspective on controlling the infection. Scientists focus on the differences between SIV infection in natural hosts, which do not develop AIDS, and in non-natural hosts like humans and rhesus macaques, which do. This research seeks to understand the mechanisms that allow natural hosts to tolerate high viral loads without the immune system collapse characteristic of AIDS.
Non-human primate models, specifically rhesus macaques infected with SIV, are used extensively for preclinical testing of HIV vaccines and treatments. Since HIV does not effectively infect macaques, researchers often use a hybrid virus called Simian-Human Immunodeficiency Virus (SHIV). SHIV contains the outer coat protein of HIV combined with the core of SIV. These models have been instrumental in identifying correlates of protection and refining vaccine strategies before they move into human clinical trials.