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HIV-1 AND HIV-2: WHAT IS THE DIFFERENCE?

What is the difference between HIV-1 and HIV-2? There are currently two types of HIV: HIV-1 and 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. Both HIV-1 and HIV-2 are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2. How many subtypes of HIV-1 are there? HIV-1 is a highly variable virus which mutates very readily. So there are many different strains of HIV-1. These strains can be classified according to groups and subtypes and there are two groups, group M and group O. In September 1998, French researchers announced that they had found a new strain of HIV in a woman from Cameroon in West Africa. The strain does not belong to either group M or group O, and has only been found in three other people, all in the Cameroon. Within group M there are currently known to be at least 10 genetically distinct subtypes of HIV-1. These are subtypes A to J. In addition, Group O contains another distinct group of very heterogeneous viruses. The subtypes of group M may differ as much between subtypes as group M differs from group O. Where are the different subtypes found? The subtypes are very unevenly distributed throughout the world. For instance, subtype B is mostly found in the Americas, Japan, Australia, the Caribbean and Europe; subtypes A and D predominate in sub-Saharan Africa; subtype C in South Africa and India; and subtype E in Central African Republic, Thailand and other countries of southeast Asia. Subtypes F (Brazil and Romania), G and H (Russia and Central Africa), I (Cyprus), and group O (Cameroon) are of very low prevalence. In Africa, most subtypes are found, although subtype B is less prevalent. What are the major differences between these subtypes? The major difference is their genetic composition; biological differences observed in vitro and/or in vivo may reflect this. It has also been suggested that certain subtypes may be predominantly associated with specific modes of transmission: for example, subtype B with homosexual contact and intravenous drug use (essentially via blood) and subtypes E and C, with heterosexual transmission (via a mucosal route). Laboratory studies undertaken by Dr Max Essex of the Harvard School of Public Health in Boston have demonstrated that subtypes C and E infect and replicate more efficiently than subtype B in Langerhans cells which are present in the vaginal mucosa, cervix and the foreskin of the penis but not on the wall of the rectum. These data suggest that HIV subtypes E and C may have a higher potential for heterosexual transmission than subtype B. However, caution should be exercised in applying in vitro-studies to real-life situations. Other variables which affect the risk of transmission, such as the stage of HIV disease, the frequency of exposure, condom use, and the presence of other sexually transmitted diseases (STDs), must also be taken into consideration before any definite conclusions can be drawn. Are some subtypes more infectious than others? Some recent studies have suggested that subtype E spreads more easily than subtype B. In one study conducted in Thailand (Mastro et al., The Lancet, 22 January 1994), it was found that the transmission rate of subtype E among female commercial sex workers and their clients was higher than that for subtype B found among a general population in North America. In a second study conducted in Thailand (Kunanusont, The Lancet, 29 April 1995), among 185 couples with one partner infected with HIV subtypes E or B, it was found that the probability of both partners in a couple becoming infected was higher for subtype E (69%) than for subtype B (48%). This suggests that subtype E may be more easily transmissible. However, it is important to note that neither study was designed to fully control for multiple variables which may affect the risk of transmission. Is subtype E a new subtype? Subtype E is not new. Stored blood samples show that subtype E was already identified at the beginning of the epidemic in Central Africa and as early as 1989 in Thailand. Do conventional HIV antibody tests detect all subtypes? Routine HIV antibody tests which are currently being used for blood screening and diagnostic purposes detect virtually all subtypes of the HIV. (Most companies have modified their assays so that they detect the newly identified HIV-1 group O strains.) Are more subtypes likely to "appear"? It is almost certain that new HIV genetic subtypes will be discovered in the future, and indeed that new subtypes will develop as virus mutation continues to occur. The current subtypes will also continue to spread to new areas as the global epidemic continues. However, in some countries there is very little monitoring undertaken to detect new sub-types. For example, in Britain, the government's Public Health Laboratory Service which is responsible for monitoring the spread of HIV in Britain, only analyses 2 new infections a month for sub-type information. What are the implications of HIV variability for research on treatment? More research needs to be undertaken. Some HIV subtypes have been observed in the laboratory to have different growth and immunological characteristics; these differences need to be demonstrated in vivo. It is not known whether the genetic variations in subtype E or other subtypes actually make a difference in terms of the risk of transmission or the response to antiviral therapy.