HIV-1 and HIV-2 are closely related, but distinct viruses. They are both retroviruses belonging to the genus Lentiviridae and they are both transmitted in the same way.
Each of these viruses is thought to have arisen as a result of simian immunodeficiency virus (SIV) being introduced into the human population, although the origin for HIV-2 was the sooty mangabey (SIVsm), while for HIV-1, it was the chimpanzee (SIVcpz).
Globally, HIV-1 is the most prevalent type of HIV and is generally the virus that people are talking about if they mention HIV without specifying a type. HIV-2 is relatively uncommon. It is mainly concentrated in West Africa, where it is currently epidemic, although it has been reported in other countries. Cases of HIV-2 have been reported in France, Portugal, and in countries with colonial links to these nations as a result of immigration from and commercial ties to West Africa.
HIV-2 is associated with lower viral loads and is less infectious than HIV-1. The cells that HIV infects and destroys, called CD4+ cells, therefore decline in number at a slower rate than with HIV-1 and disease progresses more slowly. Around 90% of people with this infection are long-term, clinical non-progressors and recent estimates suggest that people with an undetectable HIV-2 viral load have similar survival chances to that of the general population. However, HIV-2 can suppress the immune system and lead to the development of AIDS, in which case a person develops the same symptoms and infections that are seen with HIV-1.
Management of HIV-1 and HIV-2
In contrast to HIV-1, there is little detailed knowledge about the management of HIV-2. Research into the immune system’s response to commonly used forms of antiretroviral therapy (ART) has shown that ART is active against HIV-2. However, the increase in CD4+ cell count as a response to ART is greater among individuals infected with HIV-1 than among those infected with HIV-2. Furthermore, no controlled clinical trials of ART for HIV-2 have been carried out and no recommendations exist to guide decision making in the management of the immunosuppression and disease progression that can occur as a result of this condition. Together, these factors would mean that the optimum treatment approach to HIV-2 remains poorly understood.
Diagnosis of HIV-1 and HIV-2
The type of ART used to treat people with HIV-2 differs from that used to treat HIV-1, meaning it is essential to differentiate between the two viruses when testing people who are at risk of having HIV-2. Any samples that are submitted for HIV diagnostic testing should be screened using an enzyme immunoassay that is able to detect both HIV-1 and HIV-2 and any laboratory performing this screening should include algorithms for differentiating between the two viruses in repeatedly reactive samples. In the case of specimens that test negative for HIV-2 antibody but might have indeterminate HIV-1 antibody present, a Western blot can be used to confirm the presence of HIV-1 antibody.