HIV-2 in the UK

HIV–2 is extremely rare in the UK. To the end of September 2002 only 83 cases of HIV-2 had been confirmed in the UK. Most of the cases have been identified by anonymous testing, so it has been impossible to identify their origin.

Sixty-eight of these in the UK have been diagnosed as having HIV-2 infection only.

A further 15 cases were reported to have had both HIV-1 and HIV-2 infection.

Between 1985 and 2003 inclusive, 1324 individuals diagnosed and reported with HIV had probably been infected in West Africa, with 222 diagnoses made in 2003. 917 (69%) were HIV-1 infected and 52 (6%) HIV-2 or HIV-1/HIV-2 co-infected. For 355 (27%) the HIV type was not reported.

The proportion of HIV-2 and HIV-1/HIV-2infections varied by country of infection (p<0.001): ranging from the Gambia (11.7%–15.2%) to Nigeria (0.7%–1.0%). A further 130 individuals were probably infected through heterosexual intercourse within the United Kingdom by a heterosexual partner infected in West Africa. 89 (68%) were HIV-1 infected and three(2%) HIV-2 infected or HIV-1/HIV-2 co-infected. For 38 (29%)HIV type was not reported.

Current testing strategies do not always seek to distinguish between HIV-1 and HIV-2 infection.

Five children born to HIV-2 infected mothers have been reported: two children have been shown to be uninfected, two are still HIV-2 indeterminate, and one has been lost to follow-up.

Three of the five surveys in the unlinked anonymous (UA) testing programme identified HIV-2 positive specimens. From 1990 to 2000, the programme identified 36 specimens positive for HIV-2 alone, and a further 17 positive for both HIV-1 and HIV-2, out of 1,730,573 specimens tested. There were 11,131 specimens positive for HIV-1 infection only.

In some cases the same individual may have been included more than once, and if they have had a diagnostic test they may also be included among the 83 cases described in the first paragraph of this section. The UA programme has shown, however, that HIV-2 is making a very small contribution to the numbers of HIV-infected individuals in the UK: the ratio of HIV-1 infections to HIV-2 is over 250:1.

HIV-2 globally

HIV–2 is highly concentrated in West African countries such as Senegal, Ivory Coast, Cape Verde, Gambia, Guinea-Bissau, Liberia, Ghana and Nigeria. It has tended to spread only to countries with strong links to these West African countries – France, Portugal, Angola and Mozambique. Very few cases have been reported outside these countries.

A number of factors suggest that HIV–2 is unlikely to spread in the same way as HIV–1:

• A number of strains of HIV-2 have been identified, classified into four clades (A, B, C, D) which are no more closely related to each other than they are to different strains of an SIV (simian immunodeficiency virus) found in wild sooty mangabey monkeys in West Africa. It therefore appears that the different clades represent separate transmissions of the virus at some time in the past from sooty mangabeys to people.
• HIV-2 infection appears to be less pathogenic than HIV-1. It is much more difficult to isolate HIV-2 from the blood of infected individuals than in cases of HIV-1 infection.
• Immunodeficiency caused by HIV-2 infection takes much longer to develop. Individuals infected with HIV-2 are asymptomatic for much longer, and may not develop high levels of virus in their blood for fifteen to twenty years after infection, by which time they may be much less sexually active or past child-bearing age.
• HIV-2 is not easily transmitted during the lengthy asymptomatic phase of infection due to the very low levels of virus found during this time, which may explain why the virus is less widespread. Studying sex workers in Senegal, researchers found that whilst HIV-1 spread very rapidly (the number of new cases of HIV-1 was greater during each year of the study than the preceding year), HIV-2 spread much more slowly (the number of new cases each year was almost identical). Researchers estimated that it would take 5 years for the number of HIV-1 cases to double, but 31 years for the number of HIV-2 cases to double (Kanki).
• HIV-2 is less easily transmitted from mother to baby, perhaps because of much lower viral load in HIV-2 infected mothers (Gayle).

HIV type O and viral diversity

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 subtypes, are becoming more frequent in scientific literature. The subtypes each predominate in different parts of the world, and the occurrence of the same subtype in countries far apart provides clues about the global dissemination of HIV.

• Subtype A is found in Central Africa, especially in Kenya.
• Subtype 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).
• Subtype C is found in India, Brazil, Ethiopia, Tanzania, China and southern Africa and is now the most widespread virus on a global scale.
• Subtype D is found in Central Africa.
• Subtype E is not found as a separate subtype in itself but is only ever found in a recombinant form, as half of a ‘spliced’ virus where its genes are mixed with subtype A virus. It is found in the Central African Republic and in Thailand. It has been given the alternative name of 'CRF 01-AE'.
• Subtype F is found in Brazil, Romania and the Democratic Republic of the Congo.
• Subtype G is found in the Democratic Republic of the Congo, Gabon and Taiwan.
• Subtype H is found in the Democratic Republic of the Congo and Gabon.

Recombinant forms of the virus, between different subtypes, 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.

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 subtypes. 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.