Prevention Health Article

PREVENTIVE INTERVENTIONS

Some preventive interventions are intended for application to high-risk or special-risk individuals. Other inventions are intended for the entire population, such as routine childhood immunizations, various educational programs in primary and secondary educational settings, and the pasteurization of milk for general distribution. Some are obviously only necessary for one gender, such as cervical cancer screening for women or prostate cancer screening for men, and some are intended only for those at risk for the unhealthy exposure of interest, such as antimalarial prophylaxis for those traveling to or residing in areas where such exposure is possible. Most authorities believe that screening infants and children for increased lead exposure, using blood levels, should be reserved for those at increased risk based on environmental exposure characteristics. Risk levels are generally defined by epidemiological studies, although in many instances the evidence necessary for precisely defining risk is often incomplete. For this and other reasons, the threshold for what constitutes "increased" risk for interventions aimed at persons with increased risk does not follow any rule as to how high the risk must be before invoking that intervention. The level chosen may be related to the risk assessment (measurement) methodology, the resources to be expended, and the amount of expected benefit. Other considerations may be of a policy nature, including decisions concerning the alternative public health or preventive uses for the intended resources.

Depending on the preventive intervention, it may be deemed that all persons in a given population are at "high" risk. One important example is coronary artery disease (heart attack, stroke, and related conditions). Here the level of risk is not only stratified within the population, but also contrasts are made with other populations. Within many Western countries, even those at lower risk according to within-population standards may be at much higher risk compared to those in some developing countries where coronary artery disease is much less common. Thus, it may be deemed that all persons in certain Western countries are at "high" risk, leading to universal and more aggressive interventions across a given population. It follows from this principle that some risk levels may be characteristic of populations and not only of individuals within those populations. Public health practice should always take that into account when providing prevention programs.

While the rhetoric defining risk status doesn't lend itself to easy quantification, as previously noted, every society has identified persons by modern public health and epidemiological methods who are at "very high" risk of certain conditions. Examples include persons in certain occupations who are exposed to high levels of environmental toxins or persons with certain genetic characteristics that define very high rates of disease onset. Such situations should be addressed by rigorous preventive interventions where possible, or in the case of genetic conditions, by at least optimally defining and minimizing risk to individuals and families. Other individuals may be at high risk of various conditions by virtue of patterns of risk-taking behaviors, and at least in some instances these can be identified and modified to some extent. Persons at very high risk of conditions, often justifiably, require levels of attention and surveillance far different from others at increased risk as well as other preventive interventions. Whether intensive attention is merited depends on the evidence that disease occurrence and human suffering can be limited.

EPIDEMIOLOGY

While clinical prevention is aimed at individual patients and patient groups, or at those with special risks, prevention in public health is generally aimed at entire, geographically defined populations. But from a global public health perspective, clinical and population prevention are intimately linked in many ways. Patient groups sustain all of the general exposures and risks of others in a defined community, and thus require the same preventive interventions as those who are not patients. In fact, many important disease-causing factors are characteristics of communities only, and not of individuals, such as ambient air pollution levels or the availability of high-quality fire protection and health education programs in secondary schools. Conversely, many elements of population health promotion and disease prevention, such as behavioral modification programs and immunizations, are performed within the health care system, which is obviously a critical component of population health. Since nearly all citizens of communities are also health care patients at some point in their lives, reconciling the population and patient domains is necessary.

Epidemiology is the science that provides the rationale and quantitative basis for preventive interventions in both patients and communities, and in turn evaluates the effects of those interventions. This discipline describes the health characteristics and status of groups and populations, as well as their trajectories and outcomes, and quantifies the impact of various environmental exposures and personal factors on the occurrence of important health conditions. Epidemiology is largely an observational science, in that it observes disease population occurrence and environmental exposures, deduces causal pathways and mechanisms, and suggests control procedures. But it includes a strong element of randomized trials and other experimental designs where possible, such as in evaluating the efficacy of a vaccine. To do its work, epidemiology draws heavily from many other sciences and disciplines that inform health status and outcomes, such as clinical medicine, demography, behavioral science, microbiology, toxicology, administrative science, genetics, and molecular biology. As noted, this is testimony to the multifactorial nature of disease causation and the need for multidisciplinary approaches to disease prevention and health promotion. An example of cross-disciplinary activity within epidemiological disease control programs is the application of social marketing. This is an approach to communicating and disseminating health information to the community for behavioral change, using the techniques of commercial marketing. Examples of public health campaigns and programs that have used these techniques are promoting condom use among sexually active teenagers and the use of the "designated driver" in an attempt to decrease alcohol-related auto crashes and injuries.

Epidemiology's tasks usually require the calculation of disease rates, which in turn requires both a numerator (accurate disease counts), and a denominator (the population at risk for health change). Critical to both is accuracy. The population denominator, whether whole communities, important demographic segments, or groups of patients in clinical settings, must be understood. For groups defined administratively, such as patients in a hospital or clinical system, record systems will usually furnish adequate counts. For geographically defined populations, an accurate census is critical, and may not always be available. Even within industrialized nations, high levels of population migration or undocumented persons and lack of cooperation with the census in general may lead to population undercounting, often most acute for groups comprising the most important public health constituency. In many instances, effective epidemiological and public health practice requires population counts and assessments more accurate than those obtained by conventional means, employing network and other sampling or estimation techniques.

In addition to accurately determining the size of geographically defined populations, demographic trends in the United States and other industrialized countries are instructive for their effects on disease occurrence and public health. Perhaps the two most important trends are the increase in total population, although not to the extent this is occurring in developing countries, and the "aging" of populations, where the proportion of older persons is increasing relative to other age groups. Increasing population size has clear implications for environmental quality; the availability of basic resources, such as energy, transportation, and water; the transmission of infectious agents; and the nature of social interactions, which can have both negative and positive health effects. An older population similarly will have complex health effects. In general, rates for felonies and sexually transmitted diseases will be lower, but the number of cases of the chronic illnesses of older persons, such as coronary disease and stroke, cancer, diabetes, arthritis, and dementia, will likely increase. Also, since there is a progressively lower proportion of working-age persons, this may put stresses on national and regional economies, and in turn on population health status.

These demographic shifts will change the nature of prevention programs. There may be more emphasis on the prevention and early detection of the chronic conditions of older persons, and somewhat less emphasis on the conditions of younger persons, although all are important. Lower birth rates may shift resources away from maternal and child health programs toward the prevention of disability. The trend toward higher population size, improved survivorship, lower fertility rates, and a higher proportion of elders, the so-called epidemiologic transition, is occurring among many developing countries and over time will most likely shift their disease rates and prevention priorities in a similar direction to that of developed countries.

Determining the numerators for groups and defined populations (counting the diseases, conditions, and other health states in those populations) goes under the general heading of "surveillance." This takes many forms, and knowledge on disease occurrence and health status in many populations is often, at best, incomplete. For example, if a condition is never diagnosed, or if a sick individual doesn't seek medical care, then a clinical event usually remains unknown. Surveillance may be considered in two general categories: active and passive. In the former, information is collected by actively searching for disease occurrence, such as through population surveys, medical record review, and disease marker determinations in population samples. In the latter, reports are accepted from routine reporting and other voluntary sources, irrespective of whether disease reporting is a community regulation or law.

Depending on the condition, different types of surveillance techniques become most important. Historically and currently, communities have designated a set of diseases and conditions that, when medically recognized, should be reported to public health or other medical authorities. Most of these are infectious and communicable conditions, but chronic illnesses are often represented, as can any condition that might be a threat to population health. Many conditions are detected only with appropriate serological or microbiological laboratory techniques, such as many infectious and communicable conditions. Without these techniques, the infections usually would not be precisely identified and control measures executed. Thus, the public health laboratory becomes an indispensable part of a surveillance system. Laboratory ("biomarker") surveillance for some communicable diseases may be routinely performed irrespective of human clinical illness, such as by routine monitoring of sylvatic animals or patients who present to emergency rooms with any relevant clinical syndrome. Some chronic illnesses all require laboratory confirmation, such as accurate histopathology for cancer cases.

Surveillance systems may operate with other techniques, such as clinical record review. While ethical and privacy issues may deter some disease detection, this can be a very important tool for early identification of conditions with public health and preventive import. For those conditions that are not brought to medical attention or for which a diagnosis is not made, the most common approach would be population sample surveys. Here, representative samples would be interviewed or otherwise studied in an attempt to determine the prevalence and incidence of conditions not otherwise detected, or among persons not availing themselves of medical care. Population surveys also afford the opportunity to determine rates of personal behaviors and exposures that are related to disease causation, and further target disease control programs. Special studies on these population samples may reveal physical, mental, dental, or other disorders and form the basis for targeted preventive interventions. The same surveillance systems that define populations at risk and the nature and extent of preventive programs can continue to determine population disease rates as preventive interventions are invoked and applied as part of public health program evaluation.

ROBERT B. WALLACE

(SEE ALSO: Demographic Transition; Disease Prevention; Epidemiologic Transition; Epidemiology; Notifiable Diseases; Prevention Research; Preventive Health Behavior; Preventive Medicine; Primary Prevention; Registries; Secondary Prevention; Surveillance; Tertiary Prevention)