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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Retroviruses, Human Immunodeficiency Virus, and Acquired Immunodeficiency Syndrome

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Epidemiology - Acquired Immuno Deficiency Syndrome ( AIDS )

The AIDS syndrome was first recognized in the United States in 1981, when it became apparent that an unusual number of rare skin cancers (Kaposi’s sarcoma) and opportunis-tic infections were occurring among male homosexuals.


The AIDS syndrome was first recognized in the United States in 1981, when it became apparent that an unusual number of rare skin cancers (Kaposi’s sarcoma) and opportunis-tic infections were occurring among male homosexuals. These patients were found to have a marked reduction in CD4 T lymphocytes and were subject to a wide range of opportunistic infections normally controlled by an intact immune system. The disease was found to progress relentlessly to a fatal outcome and was first identified in male ho-mosexuals, hemophiliacs who were receiving blood-derived coagulation factors, and in-jection drug users.


Retrospective serologic studies with material saved from patients in various studies indicate that the disease was already occurring in Africa in the 1950s and in the United States in the 1970s. In 1985, HIV-2 was found to be endemic in parts of West Africa and to cause AIDS. To date, this virus has been relatively restricted geographically, although HIV-2 infections have occurred in the western hemisphere.



The HIV virus is transmitted between humans in three ways: sexually, perinatally, and by exposure to contaminated blood or body fluids. The virus has been demonstrated in par-ticularly high titers in semen and cervical secretions, and the majority of cases result from sexual contact. Infection is facilitated by breaks in epithelial surfaces, which provide di-rect access to the underlying tissues or bloodstream. The relative fragility of the rectal mucosa, together with large numbers of sexual contacts, are probable contributing factors to the predominance of the disease among promiscuous male homosexuals. Transmission appears to be more efficient from men to women, but the reverse is clearly documented. The probability of HIV transmission per unprotected sexual act is estimated at 0.0003 to 0.0015. The risk of perinatal transmission from an infected mother to her child has been estimated to range from 15 to 40%.

Growth of the virus in cell culture and identification of its antigens allowed develop-ment of effective test procedures for detecting HIV infection. These almost eliminated the risk of transmission by blood transfusion; testing of donors and the use of recombinant or specially treated coagulation factors have now virtually eliminated these sources of infec-tion. Until serologic tests for the infection became available, in 1985, more than 10,000 cases of AIDS were probably acquired in the United States through blood transfusion, and about 80% of hemophiliacs treated with coagulation factors derived from pooled blood sources became infected. Transmission of infection by blood is now largely associ-ated with sharing of needles and syringes by injecting drug users, and this has been an in-creasing source of the disease. In some areas of the world, the seroprevalence of HIV positivity among injecting drug users has been as high as 70%. It became apparent that heterosexual transmission could occur and that the infection could be transmitted from mother to infant either by intrauterine spread or during the birth process. It was also found that the disease had its greatest prevalence in parts of Africa, where the spread was pre-dominantly heterosexual.

Transmission of infection to health care workers after accidental sticks with potentially contaminated needles is very rare (considerably less than 1% of occurrences), presumably because the amount of infectious virus in the blood of infected cases is small and larger volumes or repeated exposures are needed for a significant chance of infection. Neverthe-less, cases have occurred from both clinical and laboratory exposure, and extreme care in handling needles, sharps, and so on, is necessary. Transmission does not occur through day-to-day nonsexual contact with infected individuals or through insect vectors, because of the fragility of the virus and the need for direct mucosal or blood contact. It is of interest that the virus has been detected in saliva, tears, urine, and breast milk. With the possible exception of breast milk, these sources have not been shown to be infectious.



As of December 2001, there have been 816,000 cases of AIDS in the United States, with 468,000 deaths. The highest prevalence rates of HIV infections have been in homosexual and bisexual males, intravenous drug users, prostitutes, and sexual partners of HIV-infected per-sons. In some areas of the United States, 40 to 60% of homosexual males attending sexually transmitted disease clinics were found to be infected. The epidemiology of HIV infection is changing in the United States as the pandemic evolves and as the modes of transmission be-come more generally understood. The numbers and proportions of heterosexually transmit-ted, drug abuse – related, and neonatal cases are increasing, particularly among the poor and disadvantaged racial minorities. Antibody rates in prostitutes may be as high as 40%, de-pending partly on the degree of associated intravenous drug abuse. Prevalence rates in the heterosexual population, in general, are currently less than 1% but have been increasing. In 1985 in the United States, only 7% of AIDS cases were in women; by 2000 the percentage had risen to 25%. Approximately 2000 newborns per year are infected by HIV perinatally, but this number may be decreasing as more pregnant women receive antiretroviral therapy. The current distribution of AIDS cases is men who have sex with men (MSM) (40%), in-travenous drug users (30%), heterosexual (25%) persons, and others (5%). Black patients now account for 50% of cases, exceeding the percentages in non-Hispanic white men.

In contrast to the situation in the United States and Western Europe, heterosexual trans-mission is the primary route of transmission in Africa and Asia, where there is an approxi-mately equal distribution of infection and disease between the sexes. This may be due to a high frequency in these areas of ulcerative genital lesions caused by other sexually trans-mitted diseases. These lesions facilitate passage of virus into the tissues of others during intercourse. In central and eastern Europe, where there is an emerging epidemic, the most common risk factor is intravenous drug use.

AIDS has been reported in more than 150 countries. The disease continues to spread rapidly in Africa and South America. In sub-Saharan Africa alone, 25 million people are infected, and there are 4 million new cases per year. Until recently, the Far East had few cases, but now there is epidemic spread, especially in South and South East Asia (India, South China, Burma, Thailand, Cambodia, Viet Nam, and Malaysia). In China, there are more than 600,000 patients with AIDS, and the rate of new cases is increasing by more than 30% per year. HIV-2 infection is found primarily in West Africa and is spread by heterosexual transmission. Infection by this virus has, however, been reported in Europe in homosexual men, injection drug users, transfusion recipients, and hemophiliac men. For example, in Russia, there were 40,000 new cases of AIDS in 2000. In some countries in Africa, 25% of the population and up to 60% of women are HIV antibody – positive.

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