Human immunodeficiency virus (HIV) infection is a common sexually transmitted disease that may be caused by retroviruses HIV-1 and HIV-2. With regard to HIV-2, the highest prevalence rates are reported in West Africa, where this type of HIV may account for up to 20% of HIV infections. Even though HIV-2 is generally considered to be less virulent than HIV-1, it may still induce life-threatening acquired immunodeficiency syndrome (AIDS) and lead to death.
The classification system used by the World Health Organization comprises four hierarchical clinical stages of HIV infection, ranging from stage 1 (asymptomatic) to stage 4 (AIDS) . Patients showing at least one symptom attributed to a higher stage are assigned this higher stage. Also, they cannot be re-assigned a lower stage if symptoms are relieved by adequate treatment. This classification is to be understood as a one-way street and is described in detail in another article available on this platform, namely in the article dealing with HIV infection in general.
The clinical presentation of early-stage HIV infection is non-specific, and this applies to infections with either type of the virus. The majority of patients remains asymptomatic for years or develops persistent generalized lymphadenopathy. Individuals with HIV-2 infection tend to remain in this stage for even longer periods of time than those who acquired HIV-1 . What's more, large shares of HIV-2 patients may be classified as long-term non-progressors who are not expected to show signs of immunodeficiency or disease for more than a decade  .
Those who do progress to advanced-stage HIV infection and AIDS develop similar symptoms, irrespective of the causative agent. Slight differences have been observed regarding the relative frequency of certain symptoms, but the clinical relevance of such data may be doubted. For the sake of completeness, they shall be briefly summarized here :
Routine testing doesn't aim at an immediate confirmation of HIV-2 infection but rather strives for the clarification of an individual's general HIV status. Both direct and indirect assays may be used to confirm HIV infection, and they primarily differ in their window periods :
HIV type discrimination ideally relies on the results of both nucleic acid amplification and immunological tests . This approach allows for a certain compensation of limitations in sensitivity (due to low HIV-2 viral loads and antigenic differences) and specificity (due to the cross-reactivity of antibodies against HIV-2).
The confirmation of HIV-2 infection entails an extensive laboratory workup to assess the patient's immune status, and to check for co-infections. This workup follows the general recommendations for HIV infection and includes :
Adequate treatment should be provided as soon as HIV infection is diagnosed, irrespective of the presence of symptoms. This applies to both genders and all ages, including pregnant women. The World Health Organization has published Consolidated Guidelines on the Use of Antiretroviral Drugs for Treating and Preventing HIV Infection , but these guidelines are mainly focused on the management of the much more common HIV-1 infection. Similarly, most antiretroviral drugs have been designed for the treatment of HIV-1 infection, and HIV-2 carries intrinsic resistance to several compounds.
Neither non-nucleoside reverse transcriptase inhibitors (e.g., efavirenz and nevirapine) nor the fusion inhibitor enfuvirtide should be administered to HIV-2 patients , although these drugs are part of most first-line regimens as described in the aforementioned guidelines . Reduced susceptibility to protease inhibitors like amprenavir has also been shown for HIV-2 and may be confirmed by means of drug-resistance testing . Taking into consideration these peculiarities of HIV-2, clinical trials have been carried out to assess the effects of integrase-inhibitor-based regimens:
The authors of both studies concluded these regimens to be effective, safe, and well-tolerated. Alternative single-tablet regimens with expected effectiveness in HIV-2 infection are listed elsewhere , and some of them may possibly be employed in pregnant women, adolescents, or children. However, reliable data is lacking to establish a preferred antiretroviral therapy regimen in these patient groups.
Lifelong treatment is required, and patients should be monitored for their response to ART. Six and twelve months after the initiation of ART, and annually thereafter, HIV loads should be measured, and CD4+ T-cell counts should be evaluated to assess the patient's immune status. These tests may be complemented by standard analyses of blood samples to detect potential signs of organ failure .
Patients with HIV-2 infection have a better prognosis than those with HIV-1 infection, although the reasons for these difference remain poorly understood . There is a tendency towards lower viral loads and higher CD4+ T-cell counts in HIV-2 patients, even in advanced stages of the disease. These parameters may thus not reliably predict the outcome if compared with reference values mainly obtained from HIV-1 patients. In general, both long-term non-progressors and those with higher viral loads who receive effective ART have a near-to-normal life expectancy.
Upon the onset of AIDS-defining illnesses, median survival times of patients infected with HIV-1 and HIV-2 are six and twelve months, respectively .
The Simian Immunodeficiency Virus of sooty mangabeys (Cercocebus atys) has been identified as the origin of HIV-2 . In detail, two independent transmission events, which possibly occurred at the beginning of the 20th century, yielded HIV-2 groups A and B, which differ in genetics, pathogenicity, and virulence. According to a study realized in Portugal, HIV-2 group A accounts for up to 97% of cases . Another seven groups of HIV-2 have been described in single patients only. The hunting of sooty mangabeys, the consumption of their meat, and their use as pets may have provided multiple opportunities for cross-species transmission .
The routes of transmission of HIV-2 are not different from those described for HIV-1 and mainly comprise the exposure to blood and body fluids during sexual intercourse, needle sharing, and the transfusion or transplantation of blood and organs, as well as vertical transmission in utero, peripartum, or during breastfeeding  . The overall risk of transmission is significantly lower for HIV-2, which is due to lower viral loads in blood and body fluids as well as lower genital shedding  . For instance, the risk of mother-to-child transmission has been estimated at <2% and 10-40% for HIV-2 and HIV-1, respectively .
The origin of HIV-2 has been located in the Taï forest in Ivory Coast, West Africa . And while HIV-2 infection may be diagnosed worldwide, the majority of cases is still reported in West Africa. HIV-2 group A is distributed throughout the entire region, and HIV-2 group B is mainly found in Ivory Coast, Ghana, Burkina Faso, and Mali. In sum, about 1-2 million people may have been infected in 2007, with more recent figures not being available. The percentage share of HIV-2 infections may reach 20% in West African nations, but is estimated at <5% in Portugal and France, and <1% in all other countries.
The detrimental consequences of HIV infection are due to the gradual loss of CD4+ T-cells and imbalance in CD4+ T-cell homeostasis, resulting in progressive immunodeficiency and ultimately death. CD4+ T-cells are implicated in cellular and humoral immune responses against infections, and they are selectively infected by HIV. The virus binds to CD4 and enters the target cell, replicates inside the lymphocyte and eventually induces host cell lysis or the formation of syncytia with uninfected T cells. Either way, additional cells will become infected with the virus.
The spread of the infection is associated with decreasing quantities of CD4+ T-cells and a reduction in the number of functional cells, but this process seems to run rather slow in HIV-2 infection. CD4+ T-cell counts diminish at a smaller rate than those observed in HIV-1 patients, which may be due to a more efficient immunological response and lower replication efficiency in primary cells. Dendritic cells, for instance, do not normally get activated and are not efficiently infected by HIV-1 but by HIV-2 .
There is no vaccine to prevent HIV-2 infection, and little work is done to this end. Considerable research is underway to develop an effective and safe HIV-1 vaccine, but even if these efforts come to fruition, such a product is unlikely to provide cross-protection for HIV-2 . Accordingly, information campaigns and sex education remain the pillars of prophylaxis of this sexually transmitted disease. The correct and consistent use of condoms largely reduces the risk of infection, as does the reduction of the number of sexual partners. Before abstaining from condoms, both partners should get tested for HIV infection. At the community level, condoms may be provided to those who are unable or unlikely to purchase them, needle and syringe programs may be implemented to diminish the spread of the virus among drug addicts, and blood, tissues, and organs to be transplanted to another person should always be screened for the presence of HIV .
Additionally, people at very high risk of exposure to HIV may be provided pre-exposure prophylaxis containing tenofovir disoproxil fumarate . This may apply to serodiscordant couples, heterosexual men and women, men who have sex with men, transgender people, sex workers, people who inject drugs, and those in prisons and other closed settings. Although there are no trials regarding the effectivity of pre-exposure prophylaxis of HIV-2 infection, HIV-2 is susceptible to all nucleoside reverse transcriptase inhibitors currently approved for HIV-1 treatment, including tenofovir disoproxil fumarate . Similarly, post-exposure prophylaxis as applied to prevent HIV-1 infection is likely to be effective in the
prevention of HIV-2 transmission.
HIV infection may be caused by either of two related, yet distinct retroviruses: by HIV-1 and HIV-2. HIV-2 has first been isolated in 1986, namely from two patients originating from West Africa . It has been speculated to have been circulating in West Africa since at least 1966, and the reduced virulence of this type of HIV is commonly mentioned as the main reason for it remaining undetected for decades. HIV-2 infection usually involves a longer asymptomatic stage and the decline in CD4+ T-cell counts is slower. The risk of horizontal and vertical transmission is lower, as is AIDS-related mortality .
Human immunodeficiency virus (HIV) infection is possibly the most widely known sexually transmitted disease. It follows a progressive course and is associated with a gradual impairment of the immune system, eventually leading to life-threatening acquired immunodeficiency syndrome (AIDS) and death. There are two types of HIV, namely HIV-1 and HIV-2. The latter accounts for a minority of infections and is most commonly found in West Africa. HIV-2 is generally considered to be less virulent than HIV-1: Patients infected with HIV-2 may remain in the asymptomatic stage of the disease for decades. Still, progression to AIDS is possible and should be avoided by adequate treatment.
The routes of transmission don't differ between HIV-1 and HIV-2, and neither does the clinical presentation. Specific tests need to be run in order to distinguish between both types of the virus and to choose the most effective treatment regimen. In sum, the general recommendation to seek medical advice in case of suspected exposure to HIV should be followed irrespective of the type of the virus.