PANOS London, December 20, 1995
James Deane, director of the AIDS programme at Panos

This theory, if correct, would help explain why HIV has spread so rapidly in developing countries, where 90 per cent of all HIV infection is found, and why it has spread more slowly in the industrialised world.

It would also mean that the industrialised world is still vulnerable to a large-scale AIDS epidemic and it could pose problems in developing an effective AIDS vaccine.

The controversial theory also raises questions around the widely-held belief that the rapid spread of HIV in developing countries is due largely to the presence of untreated sexually transmitted diseases.

Essex's theory, discussed in Kampala at the recent International Conference on AIDS in Africa, suggests that there might be important differences among existing strains of HIV.

Scientists have identified as many as nine different strains circulating in different parts of the world. Essex has studied five of these strains and argues that one - the so-called B strain - is comparatively difficult to spread through heterosexual intercourse. It is this B strain that is found mainly in Europe and North America, where HIV has been spread mainly through intravenous drug use and through male-to-male sexual contact.

In contrast, the other main sub-types - A, C, D and E - are found mainly in those developing countries in Africa and Asia where HIV is believed to spread predominantly through heterosexual intercourse. Key characteristics of these sub-types allow them to be spread particularly easily by sexual intercourse, Essex says.

His argument is that the virus from these sub-types grows particularly well in what are called langerhans cells, which exist in high density in the vagina and foreskin of the penis. This makes it easier for the virus to be transmitted during sex. By contrast, the B strain of the virus does not grow as well in these cells. All types of the virus seem to grow equally well in blood cells known as monocytes.

To illustrate his theory, Essex points to countries such as India where the epidemic of the B strain of HIV has been comparatively limited despite having reached the country first. The C strain, though, has spread very rapidly, infecting well over one million people in less than 10 years. In Thailand, the B strain appeared first but has spread relatively slowly; the E strain has spread very rapidly since it appeared in the late 1980s.

Conventional explanations for the AIDS epidemic in heterosexual populations of Africa and Asia include socio-economic factors and widespread sexually transmitted diseases, the sores and ulcers of which are believed to facilitate HIV infection.

If Essex is right, then the sub-types that have so far not been prevalent in industrialised countries could, once established, spread much more rapidly than the B strain.

Essex's ideas provoked scepticism among some conference delegates.

"These theories can seem convincing in the laboratory," said Dr Philippe Mayaud of the African Medical Research Foundation, "but we know from practical experience on the ground that sexually transmitted diseases are a key factor in the spread of HIV. When we treated those diseases, the number of new HIV infections fell."

Mayaud has been closely involved in a recent project in Mwanza, Tanzania, where the number of new HIV infections was almost halved after services to treat other sexually transmitted diseases were improved. While Essex acknowledges that sexually transmitted diseases play a significant role in the spread of HIV - most studies point to a three- to five-fold increase in vulnerability of HIV infection if someone already has a sexually transmitted disease - he argues that this is not enough to explain the speed at which HIV is spreading in the developing world compared with industrialised countries.

"The differences are much more than three- to five-fold in the transmission of heterosexual cases," he argues. They are, he says, more like 100-fold or 200-fold or even 400-fold.

"Furthermore, in some places such as Thailand, the appearance of sexually transmitted diseases appear to be less important than in sub-Saharan Africa."

If Essex's theory is correct, it could have important implications for the development of an AIDS vaccine. The vaccines at the most advanced stage of testing are currently being tried out in Thailand. However, most of these are based on sub-type B only, although some preliminary trials with a vaccine based on a combination of B and E strains are also underway.

Dr Jose Esparza, director of vaccine development at the World Health Organisation, says that some experts believe a vaccine produced against one sub-type may protect against other sub-types.

"There are two candidate vaccines currently being tested in Thailand," he says. "These are the only - and perhaps last - opportunity to show whether this particular type of vaccine can induce some protection.

The vaccine trials "should be conducted for three reasons," Esparza says. "First they may work and we can't know if they work unless we try them. Second, the vaccines may not be very good but we may learn something about the type of immune response that may protect against HIV infection. Third, those vaccines may not work, but we had better know that and the only way to find out is through experimentation."

Essex is optimistic that a vaccine that will be effective in developing countries will be found, and hopes it can be done within five to seven years.

"But to do that, leaders at the highest levels of both Western and developing nations will have to get together to agree some kind of formula, not only for vaccine design and testing but also for ultimate distribution of a vaccine."

The most compelling argument for many Western countries to invest in such a vaccine may be Essex's claim that they have escaped a major AIDS epidemic so far because the most efficient strains of HIV have not been present there.

It is, Essex says, in the "best interests of Western governments and leaders to recognise this problem because I believe that it is likely that other strains of HIV will be moving into the West - indeed they are already."

951220
PS951201

Copyright © 1995 - Panos Institute. All rights reserved. Reproduced with permission. Reproduction of this article (other than one copy for personal reference) must be cleared through the Panos Institute, Permissions Desk, 9 White Lion St., London N1 9PD, UK TEL: (+44) 171 278 1111 FAX: (+44) 171 278 0345  http://www.oneworld.org/panos/
Email:panos@panoslondon.org.uk