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A
peculiar fact of HIV is its capacity to quickly mutate. Therefore is a
highly variable virus and there are many different strains of HIV, even
within the body of a single infected person.
At the present time HIV is
classified on the bases of its genetic differences and similarities into
types, groups and subtypes.
HIV TYPES
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
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HIV-O
the "outlier" group |
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HIV-N
the "new" group
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HIV-M
the "major" group |
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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
More than 90% of HIV-1 infections belong to HIV-1 Group M
HIV SUBTYPES
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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