What are the Different Types of HIV?

One of the obstacles to treatment of the Human Immunodeficiency Virus is its high genetic variability.

HIV mutates very readily resulting in the formation of many different strains of HIV, even within the body of a single infected person. Based on genetic similarities, the numerous virus strains may be classified 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 they appear to cause clinically indistinguishable AIDS.

HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in sooty mangabeys. HIV-2 seems to be less easily transmitted, and the period between initial infection and illness is longer in the case of HIV- 2.

The relatively uncommon HIV-2 type is concentrated in West Africa and it is rarely found elsewhere.

HIV-1 is predominant virus all over the world. When people refer to HIV without specifying the type of virus they will be referring to HIV-1.

The strains of HIV-1 can be classified into four groups: the “major” group M, the “outlier” group O and two new groups, N and P. These four groups may represent four separate introductions of simian immunodeficiency virus into humans.

Group M: 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. Group M can be further subdivided into subtypes based on genetic sequence data.

Some of the subtypes are known to be more virulent or resistant to different medications. The subtypes are as follows:

Subtype A – Subtype A and CRF A/G predominate in west and central Africa, also causing much of the Russian epidemic.

Subtype B – subtype B has been the most common subtype/CRF in Europe, America, 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 C – 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.

Subtype D – generally seen in Eastern and central Africa only.

Subtype F – found in central Africa, South America and Eastern Europe.

Subtype G – found in Africa and central Europe.

Subtype H – limited to central Africa.

Subtype J – primarily found in North, Central and West Africa, and the Caribbean, Subtype K – limited to the Democratic Republic of Congo and Cameroon.

Subtype E has never been identified as a nonrecombinant only recombined with subtype A as CRF01_AE.

Group N:

The ‘N’ stands for “non-M, non-O”. This group was discovered in 1998 and has only been seen in Cameroon. As of 2006, only 10 Group N infections had been identified.

Group O:

The O (“Outlier”) group is not usually seen outside of West-central Africa. It is reportedly most common in Cameroon. The group caused some concern because it could not be detected by early versions of the HIV-1 test kits. More advanced HIV tests have now been developed to detect both Group O and

Group P:

In 2009, a newly-analyzed HIV sequence was reported to have greater similarity to a simian immunodeficiency virus discovered in wild gorillas (SIVgor) than to SIVs from chimpanzees (SIVcpz).

The virus had been isolated from a Cameroonian woman residing in France who was diagnosed with HIV-1 infection in 2004. The scientists reporting this sequence placed it in a proposed Group P “pending the identification of further human cases”.

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.

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.

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.