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HIV Types, Groups and Subtypes

At the present time HIV is classified on the bases of its genetic differences and similarities into types, groups and subtypes.

      There are two types of HIV :  HIV-1 and HIV-2
                                              
                                             
Both types are transmitted by sexual contact, through blood, and from mother to child, and the AIDS produces by them is clinically indistinguishable. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2.

Worldwide, the predominant virus is HIV-1, and generally when people refer to HIV without specifying the type of virus they will be referring to HIV-1. The relatively uncommon HIV-2 type is concentrated in West Africa and is rarely found elsewhere.

HIV GROUPS

  there are three groups of HIV based on their genetic materials

HIV-O      the "outlier" group HIV-N      the "new" group  HIV-M      the "major" group

These three groups of HIV strains may represent three separate sources of monkey immunodeficiency virus into humans.

Group O    appears to be restricted to west-central Africa and
Group N    discovered in 1998 in Cameroon and is extremely rare.
Group M   the most common of the three

Within group M there are known to be at least nine genetically distinct subtypes (or clades) of HIV-1. These are subtypes A, B, C, D, F, G, H, J and K.

Occasionally, two viruses of different subtypes can meet in the cell of an infected person and mix together their genetic material to create a new hybrid virus (a process similar to sexual reproduction, and sometimes called "viral sex").

^{Many of these new strains do not survive for long, but those that infect more than one person are known as "circulating recombinant forms" or CRFs.  For example, the CRF A/B is a mixture of subtypes A and B.}

The classification of HIV strains into subtypes and CRFs is a complex issue and the definitions are subject to change as new discoveries are made.

The HIV-1 subtypes and CRFs are very unevenly distributed throughout the world, with the most widespread being subtypes B and C.

Subtype C is largely predominant in southern and eastern Africa, India and Nepal. It has caused the world's worst HIV epidemics and is responsible for around half of all infections.

Historically, subtype B has been the most common subtype/CRF in Europe, the Americas, Japan and Australia. Although this remains the case, other subtypes are becoming more frequent and now account for at least 25% of new infections in Europe.

Subtype A and CRF A/G predominate in west and central Africa, with subtype A possibly also causing much of the Russian epidemic^{.
Subtype D is generally limited to east and central Africa; A/E is prevalent in south-east Asia, but originated in central Africa; F has been found in central Africa, south America and eastern Europe; G and A/G have been observed in western and eastern Africa and central Europe.
Subtype H has only been found in central Africa; J only in central America; and K only in the Democratic Republic of Congo and Cameroon.}

It is almost certain that new HIV genetic subtypes and CRFs will be discovered in the future, and indeed that new ones will develop as virus recombination and mutation continue to occur. The current subtypes and CRFs will also continue to spread to new areas as the global epidemic continues.

Initial tests for HIV are usually conducted using the EIA (or ELISA) antibody test or a rapid antibody test.

^{Most modern rapid HIV-1 tests are capable of detecting all the major subtypes of group M.}
EIA tests which can detect either one or both types of HIV have been available for a number of years. According to the US Centers for Disease Control and Prevention, current HIV-1 EIAs "can accurately identify infections with nearly all non-B subtypes and many infections with group O HIV subtypes.

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