Subtypes of HIV
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|Human immunodeficiency virus|
|Phylogenetic Tree of the |
Group VI (
|International Statistical Classification of Diseases and Related Health Problems Codes|
|Classification and external resources|
One of the obstacles to treatment of the
HIV-1 is the most common and pathogenic strain of the virus. Scientists divide HIV-1 into a major group (Group M) and two or more minor groups, namely Group N, O and possibly a group P. Each group is believed to represent an independent transmission of
With 'M' for "major", this is by far the most common type of HIV, with more than 90% of HIV/AIDS cases deriving from infection with HIV-1 group M. This major HIV virus which was the source of pre-1960 pandemic viruses originated in the 1920s in Kinshasa, which is now known as the capital of the Democratic Republic of Congo (DRC). The M group is subdivided further into clades, called subtypes, that are also given a letter. There are also "circulating recombinant forms" or CRFs derived from recombination between viruses of different subtypes which are each given a number. CRF12_BF, for example, is a recombination between subtypes B and F.
The spatial movement of these subtypes moved along the railways and waterways of the Democratic Republic of Congo (DRC) from Kinshasa to these other areas. These subtypes are sometimes further split into sub-subtypes such as A1 and A2 or F1 and F2.[
The 'N' stands for "non-M, non-O". This group was discovered by a Franco-Cameroonia team in 1998, when they identified and isolated the HIV-1 variant strain, YBF380, from a Cameroonian woman who died of AIDS in 1995. When tested, the YBF380 variant reacted with an envelope antigen from SIVcpz rather than with those of Group M or Group O, indicating it was indeed a novel strain of HIV-1. As of 2015, less than 20 Group N infections have been recorded.
The O ("Outlier") group has infected about 100,000 individuals located in West-Central Africa and is not usually seen outside of that area. It is reportedly most common in Cameroon, where a 1997 survey found that about 2% of HIV-positive samples were from Group O. The group caused some concern because it could not be detected by early versions of the HIV-1 test kits. More advanced
In 2009, a newly analyzed HIV sequence was reported to have greater similarity to a simian immunodeficiency virus recently discovered in wild
As of 2010, there are 8 known HIV-2 groups (A to H). Of these, only groups A and B are pandemic. Group A is found mainly in West Africa, but has also spread globally to
HIV-2 is closely related to
There are six additional known HIV-2 groups, each having been found in just one person. They all seem to derive from independent transmissions from
HIV-2 diagnosis can be made when a patient has no symptoms but positive blood work indicating the individual has HIV. The Multispot HIV-1/HIV-2 Rapid Test is currently the only FDA approved method for such differentiation between the two viruses. Recommendations for the screening and diagnosis of HIV has always been to use enzyme immunoassays that detect HIV-1, HIV-1 group O, and HIV-2. When screening the combination, if the test is positive followed by an indeterminate HIV-1 western blot, a follow up test, such as amino acid testing, must be performed to distinguish which infection is present. According to the NIH, a differential diagnosis of HIV-2 should be considered when a person is of West African descent or has had sexual contact or shared needles with such a person. West Africa is at the highest risk as it is the origin of the virus.
HIV-2 has been found to be less pathogenic than HIV-1. The mechanism of HIV-2 is not clearly defined, nor the difference from HIV-1, however the transmission rate is much lower in HIV-2 than HIV-1. Both infections can lead to AIDS in affected individuals and both can mutate to develop drug resistance. Disease Monitoring in patients with HIV-2 includes clinical evaluation and CD4 cell counts, while treatment includes
Choice of initial and/or second-line therapy for HIV-2 has not yet been defined. HIV-2 appears to be resistant to NNRTIs intrinsically, but may be sensitive to NRTIs, though the mechanism is poorly understood. Protease inhibitors have shown variable effect, while integrase inhibitors are also being evaluated. Combination regimens of the above listed therapies are being looked into as well, also showing variable effect depending on the types of therapies combined. While the mechanisms are not clearly understood for HIV-1 and HIV-2, it is known that they use different pathways and patterns, making the algorithms used to evaluate HIV-1 resistance-associated mutations irrelevant to HIV-2.
Each virus can be contracted individually, or they can be contracted together in what is referred to as co-infection. HIV-2 seems to have lower mortality rates, less severe symptoms and slower progression to AIDS than HIV-1 alone or the co-infection. In co-infection, however, this is largely dependent on which virus was contracted first. HIV-1 tends to out compete HIV-2 for disease progression. Co-infection seems to be a growing problem globally as time progresses, with most cases being identified in West African countries, as well as some cases in the US.
If a pregnant mother is exposed, screening is performed as normal. If HIV-2 is present, a number of perinatal ART drugs may be given as a prophylactic to lower the risk of mother-to-child transmission. After the child is born, a standard 6-week regimen of these prophylactics should be initiated. Breast milk may also contain particles of HIV-2; therefore, breastfeeding is strictly advised against.