Epidemiology of HIV Infection

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There are three principal modes of transmission of HIV, as worked out com-mendably quickly within a couple of years of the recognition of the first cases of AIDS in Los Angeles- sexual intercourse, exchange of blood, and perinatal transmission.

Sexual intercourse now accounts for some 80% of all infections The risk of transmission is higher to the passive (receptive) partner, generally the female, and higher following anal intercourse (estimated at 1% per episode) than vaginal intercourse (estimated about 0.1% per episode). Another concurrent sexually transmitted disease enhances the risk by an order of magnitude, especially if genital ulcers are present, as in syphilis or chancroid. Obviously the risk is also greatly enhanced the greater the number of different sexual partners.

Perinatal infection currently accounts for about 10% of all HIV infections. The risk of infection from an infected mother to her baby has been estimated as 12-40% in various studies. The precise routes are not clearly understood but probably include prenatal transmission across the placenta, intrapartum transmission via blood and/or genital secretions during parturition, and postnatal transmission via breast milk. Risk factors have been shown to include the stage and severity of the maternal infection (maternal virus titers being highest early and late in the mother's infection), low maternal titers of neutralizing antibody, prematurity at delivery, and breast-feeding; obstetric procedures may also be significant.

Blood transfusion and the administration of blood products such as factor VIII to hemophiliacs account for only 3-5% of all past and present HIV infections and declined rapidly following the introduction in 1985 of routine screening procedures in blood banks. The efficiency of transmission of HIV infection by transfusion of infected blood is greater than 90%. Another 5-10% of all infections result from sharing of needles by injecting drug users (IDUs) The risk per episode will vary with the precise circumstances but may be comparable with that of needle-stick injuries to medical or laboratory workers receiving small volumes of infected blood (about 0.5%).

Although trace amounts of virus can be detected from time to time in a range of bodily secretions including saliva, there is no evidence that infection can be transmitted from person to person by kissing or any form of casual contact other than sexual intercourse or the exchange of blood.

Worldwide, three different epidemiologic patterns were observed during the 1980s. Pattern I, seen in North America, Western Europe, and Australasia, where AIDS was first reported in 1981, implying that HIV infection had probably been present since the 1970s, was characterized by spread principally via anal intercourse among male homosexuals, and to a lesser extent by needle sharing among IDUs. In recent years HIV has moved increasingly into the female population, and heterosexual transmission is occurring, especially among teenagers. The annual incidence of AIDS is projected to peak in the mid-1990s.

Pattern II, seen in sub-Saharan Africa, where AIDS has been present at least since the 1970s, is characterized by heterosexual transmission, with infection being equally common in males and females, and a correspondingly high incidence of perinatal transmission to infants. Massive economic problems including inflation and unemployment have led to urban immigration, breakdown of traditional tribal values, and sexual promiscuity especially involving casual liaisons between female prostitutes and males separated from their families. Prevalence of HIV infection in Central and East Africa has escalated alarmingly during the past decade, and the situation appears certain to deteriorate further.

Pattern III applies to Asia, Eastern Europe, North Africa, and the Middle East. The virus was not introduced into these areas until the early to mid-1980s. However, an alarming change became apparent in 1988 when an outbreak caused by H1V-1 genotype B (resembling the United States strains) occurred in IDUs in the Golden Triangle (at the junction of China, India, Myanmar, and Thailand). Then in 1989 it became clear that female prostitutes in India and Thailand had become infected with genotype A (resembling African strains), and this genotype has since spread rapidly by heterosexual intercourse. It is predicted that by the turn of the century the number of cases in Asia will exceed the number in Africa.

These three official patterns are not static, and the distinctions are no longer clear-cut. For instance, the situation in Central and South America was originally comparable to that in the United States, but spread to the heterosexual population has occurred much more quickly. Whereas the classification was invaluable for some years, it is now clear that AIDS everywhere will sooner or later become a heterosexually transmitted disease, with intravenous drug use also making a significant contribution. The principal difference between different geopolitical regions is likely to be socioeconomic: AIDS will become increasingly a disease of the poor and underprivileged, as has been their lot with regard to other devastating infectious diseases throughout history. In addition to recognizing the reality of global health interdependence, the wealthier nations will be called on to display a high degree of altruism if the pandemic is to be controlled.

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