The epidemiology of aids in the u.S.
Today AIDS has become a major cause of morbidity and mortality in the U.S. Indeed, it has become the leading cause of death in the country among people with hemophilia and users of illegal intravenous (IV) drugs. Moreover, nation-wide morbidity and mortality rates will increase in the next few years as some of the one to 1.5 million Americans who are already infected with the human immunodeficiency virus (HIV) develop AIDS. Most of those drug abusers, and a significant proportion of them will be blacks and Hispanics. Yet, given the fact that the virus is transmitted through sexual contact, through the traces of blood in needles and other drug paraphernalia and from mother to newborn infant, one can envision many possible chains of infection, which leave no segment of the U.S. population completely unaffected by the threat of AIDS.
The discovery of the epidemic, the enumeration of the varied manifestations of HIV infection and the analysis of the circumstances that made it possible for such an infection to spread have been missions assigned to epidemiology: the study of the occurrence and distribution of disease as well as its control in a given population. Epidemiologists monitor mortality and morbidity rates associated with HIV infection and AIDS; they also make predictions of likely changes in HIV infection rates in the course of time.
Most important, by carrying out studies to define the ways HIV is transmitted from person to person, epidemiologists can identify the population groups that are at greatest risk of acquiring AIDS and thereby develop strategies for the prevention and control of the disease – strategies that are independent of the development of an effective vaccine or therapy. Indeed, determining the risk factors for AIDS enabled the U.S. Public Health Service and other groups to issue recommendations for the prevention of AIDS as early as 1983, a full year before HIV was firmly identified and two years before laboratory tests to detect the presence of the virus became widely available.
To carry out all these tasks epidemiologists depend on surveillance: the gathering of high-quality, consistent and interpretable data on a disease or an infection. Surveillance data are routinely compiled from reports filed with state and local health departments that are then forwarded to the U.S. Centers for Disease Control (CDC).
Because the disease appeared to be transmitted through the exchange of blood or by sexual contact, most investigators were convinced by late 1982 that the cause of AIDS was an infectious agent (most likely a virus) and not the result of exposure to toxic substances or other environmental or genetic factors. The infection hypothesis was finally confirmed when HIV was isolated by Luc Montagnier and his colleagues at the Pasteur Institute in Paris and by Robert C. Gallo and his colleagues at the National Cancer Institute.
Soon after the discovery of the AIDS agent a laboratory test was developed to detect antibodies to HIV in the blood. A positive result in a test of a person’s blood sample was a reliable sign that the person was infected with the virus. Such a serological test made it possible to detect HIV infection in people who showed no clinical symptoms, and to confirm clinical diagnoses of AIDS and other HIV-related conditions. It also made it possible to measure directly the prevalence of HIV infection (the number of infected people in a given time) and its incidence (the number of new infections occurring within a defined period in a specific population). Most important, perhaps, was the fact that the national supply of donated blood could now be screened, so that additional cases of AIDS due to blood transfusions and contaminated blood products could be avoided.
RISK OF HIV INFECTION
The possibility that one can become infected with HIV if contaminated blood penetrates the skin or mucous membranes also represents a small but definite occupational risk for health-care workers. In a national collaborative study done by the CDC, four of 870 health-care workers who had accidentally punctured their skin with needles contaminated with the blood of HIV-infected people developed HIV infection, but none of the 104 workers whose mucous membranes or skin had been exposed to blood became infected. In another study of health-care workers at the National Institute of Health, no HIV infections occurred among 103 workers with needle-stick injuries, nor were there any HIV infections among 691 workers who had a total of more than 2000 reported skin and mucous-membrane exposures to blood or body fluids of AIDS patients. The studies are consistent with other data indicating that the occupational risk of acquiring HIV infection in health-care settings is low and is most often associated with percutaneous inoculation of blood from an infected patient.
HIV is also transmitted from an infected mother to her newborn child, but the extent of transmission that takes place respectively during pregnancy, at birth or soon afterward is as yet unknown. Detection of HIV in fetal tissues supports the hypothesis that infection occurs in utero, and case reports of women who became infected with HIV immediately after giving birth, and subsequently infected their infants, suggest that the virus may be transmitted through breast-feeding.
Studies of such perinatal transmission are greatly complicated by the lack of a reliable diagnostic test to determine HIV infection in newborns. As is the case with other infections, infants born to HIV-infected mothers have maternally derived HIV antibodies circulating in their blood – regardless of whether or not they have been infected. The maternal HIV antibodies may persist for as long as 12 months and cannot be distinguished from antibodies that may be present in an infant infected with HIV. Other tests are under development for identifying HIV infection in these newborns. Currently all infants born to infected mothers must be followed closely for at least 12 months to see whether there is any clinical or laboratory evidence of HIV infection or AIDS.
To evaluate the risk of HIV transmission through other casual contacts, several prospective studies (which are carried out over several years) have been done of the families of infected adults and children. In spite of tens of thousands of days of household contact with infected individuals, not one of more than 400 family members has been infected with HIV – except for sexual partners of the infected person and children born to infected mothers. In these studies the documented risk of household transmission was zero, and therefore the actual risk must be extremely low, even in crowded households. The risk of transmission in other social settings, such as schools and offices, is presumably even lower than in household settings.
Epidemiological studies in the U.S. and other countries throughout the world show no patterns of HIV infection consistent with transmission by insect vectors. If HIV were transmitted by insect vectors, additional cases of infection would be seen in people who share environments with infected individuals. Such evidence is lacking, in spite of extensive surveillance efforts.
Although HIV can survive for from several hours to several days in insects artificially fed blood with high concentrations of the virus, there is no evidence that HIV actually grows in insects. Such a biological event is important in most viral diseases transmitted by insects.
To be sure, the existence of other unrecognized modes of HIV transmission can never be entirely excluded, but if they do exist, they appear to be extremely rare.