Sexual Transmission of Zoonotic Pathogens
Sexual transmission is the main mechanism for the continued spread of the HIV-1 epidemic, with more than two-thirds of HIV-1 infections worldwide resulting from heterosexual intercourse. Viral particles or infected cells are transmitted in genital fluids or blood from an infected person, and are deposited on the mucosal surface of a recipient.
HIV-1 is a lentivirus that gradually attacks the immune system, targeting white blood cells known as CD4+ T cells, eventually leading to immunodeficiency, progression to AIDS, and death due to opportunistic infections.
Research has shown that HIV-1 is genetically similar to simian immunodeficiency virus (SIV)—a lentivirus that attacks the immune system of monkey species in the same way HIV-1 attacks the human immune system. One strain of SIV, found in chimpanzees, has been shown to be almost identical to HIV-1. This suggests that HIV-1 probably originated in chimpanzees and crossed into humans.
M strains of HIV-1 are the most prevalent strains of the virus. Despite the relatively low infectivity of these strains, several factors have facilitated their sustained H2H transmission. For example, M strains can overcome restriction factors in cells of the humans they infect. Restriction factors are proteins that provide an early line of defense against infection by helping to prevent viral replication.
M strains can also undergo genetic recombination and low-fidelity replication, allowing HIV-1 to evade the immune system and treatments. This accounts for “the nature of its long, ‘latent’, often sub-clinical infection, during which patients can transmit the virus,” the authors say.
Other factors that also promote H2H transmission include coinfection of the host with other sexually transmitted diseases, as well as promiscuous sexual behavior.