Aetiology and pathology of HIV and AIDS
The HIV pandemic has far exceeded projections over the past decade. The World Health Organization estimates that, all over the world, the number of people living with the human immunodeficiency virus (HIV) or acquired immune deficiency syndrome (AIDS) at the end of 2000 was 36.1 million. During that year, there were 5.3 million new cases of HIV infection and three million AIDS deaths. A total of 21.8 million people with AIDS have died since the start of the pandemic.1 The major burden of the pandemic and deaths has been in the developing world.
In the UK, there have been approximately 43,000 cases of HIV infection, with 17,400 AIDS diagnoses and 13,780 deaths attributable to HIV.2 In 1999, for the first time, the number of new infections acquired through heterosexual intercourse exceeded those acquired through homosexual intercourse. This trend continued into 2000 (1,315 in heterosexuals, compared with 1,096 in homosexuals), when the number of people who acquired HIV in the UK was the highest for any year since 1985.3
In 1981, several clusters of a distinct immunodeficiency syndrome were reported in homosexual men in the US.4,5 Two years later, a retrovirus, subsequently named HIV-1, was isolated by independent researchers from a number of individuals with this acquired immunodeficiency syndrome.6,7 Antibodies to HIV can be identified in all individuals with AIDS.7 Subsequent reviews of medical literature have discovered reports of AIDS-like illnesses as far back as the 1940s. However, some of these, such as the case of the Manchester sailor reported to have died from AIDS in 1959, are doubtful.8 The exact origins of HIV are unknown, but HIV-1 is a descendant of the simian immune deficiency virus (SIV), which has been isolated from central African chimpanzees.9 HIV-2 is closely related to SIV and is isolated from macaques and sooty mangabeys.10 It is possible that these viruses somehow crossed over into human populations. Theories as to how this happened include inadvertent introduction using contaminated oral polio vaccines in the late 1950s,11 and malaria experiments with blood transfusions.12 However, more recent studies suggest that the time of this crossover to humans was much earlier (nearer to the 1930s) and that the virus then mutated, leading to the modern day HIV.13
HIV can be found in many body fluids, although in some of these fluids it is present in such low concentrations as not to constitute a risk. The predominant fluids of transmission are semen, cervical and vaginal secretions, breast milk and blood. HIV can also be transmitted by the transfusion of blood or blood products, and by the sharing of injecting equipment among drug users. In other words, transmission can occur via sexual intercourse, injectable drug use, transfusion of infected blood and from mother to child. HIV can be transmitted as the free virus or as virus-infected cells, and several factors determine the likelihood of transmission. HIV ribonucleic acid (RNA) has been isolated from blood,14 and measurements of HIV RNA levels, known as viral load, are now used in monitoring those infected with HIV. Viral load is higher during primary HIV infection and in late-stage disease, and loads have been shown to correlate with HIV transmission.15 Concurrent illnesses, such as tuberculosis (TB) may also increase the viral load.16
Many studies have shown the presence of HIV in the semen, particularly in patients with lower CD4 counts and symptomatic disease.17,18 However, in one study, HIV was isolated in 43 per cent of a group of asymptomatic men on at least one occasion over six months.19 In women, HIV can be isolated from the cervix and the vagina, although swabs from the former yield HIV more readily.20 Levels of HIV RNA in the plasma and vaginal fluid have been shown to correlate very closely.21 It is logical that levels of HIV in the vagina also increase at the time of menstruation.22 The presence of other sexually transmitted infections (STIs) has been shown to increase the amount of HIV in genital secretions, therefore increasing transmission. A study in Malawi of 86 HIV-positive men with urethritis showed a significant reduction in concentration of HIV in semen, following treatment for urethritis.23 A more recent study in HIV-positive women in Kenya showed that treatment for cervicitis reduced HIV shedding in cervical secretions.24
Some individuals remain HIV negative despite repeated exposure to the virus. There have now been several studies in such people, suggesting possible mechanisms of HIV resistance. A study in Gambia found HIV specific cytotoxic T cells in three HIV-exposed but uninfected women.25
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Citation: Hospital Pharmacist URI: 10974565