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HIV type O and viral diversity
   Last updated: 18.06.04
 
In 1994 researchers in Cameroon reported that they had identified a new variant of HIV, named type O, which was not picked up by standard antibody tests for HIV–1. This is now seen as representing a distinct branch of HIV-1 from the main "M" group of viruses, as a result of a separate transmission of the virus to people from its original host, the common chimpanzee.

A further group of isolates from the same area have been described as an "N" group, which is more closely related to known chimpanzee viruses than either O or M, and is therefore likely to represent a more recent inter-species transmission event.

These variants account for only a minority of infections even in Cameroon and Gabon and are extremely rare elsewhere.

Antibody testing kits for use in initial screening, including rapid tests, have increasingly been adapted to include antigens from type O viruses as well as type M and from HIV-2.

Other variants of HIV–1 are less divergent, and are already detectable by standard antibody testing kits. References to these variants, or sub–types, are becoming more frequent in scientific literature. The sub–types each predominate in different parts of the world, and the occurrence of the same sub–type in countries far apart provides clues about the global dissemination of HIV.
  • Sub-type A is found in Central Africa

  • Sub-type B is the predominant strain found in the developed world amongst injecting drug users and gay men. It is common in the Americas, Europe, Australia and Thailand (predominantly amongst injecting drug users and their sexual partners, although subtype E is now increasingly dominant among both groups).

  • Sub-type C is found in India, Brazil, Ethiopia, Tanzania, China and southern Africa and is now the most widespread virus on a global scale

  • Sub-type D is found in Central Africa

  • Sub-type E is found in the Central African Republic and in Thailand

  • Sub-type F is found in Brazil, Romania and the Democratic Republic of the Congo

  • Sub-type G is found in the Democratic Republic of the Congo, Gabon and Taiwan

  • Sub-type H is found in the Democratic Republic of the Congo and Gabon.


Recombinant forms of the virus, between different sub-types, are increasingly important in some parts of the world, especially West Africa, where most people with HIV have viruses described as 'AG' recombinants - though these often include sections of other subtypes too.

Sub-type E is in fact a recombinant between subtype A and another virus. It has therefore been reclassified as a 'circulating recombinant form' and given the alternative name of 'CRF 01-AE'.

The large number of variants in Africa strongly suggests that HIV has been present in humans in Africa for longer than in other parts of the world.

It has been suggested that some variants of HIV may be transmitted more easily by vaginal intercourse, whilst some may be better adapted to transmission through blood, but research is still going on to answer this question. Recent work in Kinshasa has identified a number of viruses in the M group that cannot be classified in existing sub-types. This suggests that the whole sub-type phenomenon is nothing more than a sampling effect from the human population in which the virus was first established on a large scale, and in which it then diversified at random.

References


HIV-2 infections identified in the UK. CDR Weekly:11:21, 2001.

Kuiken CL et al: Epidemiological significance of intra- and inter-person variation of HIV-1. AIDS 8 (Supplement 1) pp S73-S84, 1994.