DISEASE PREVENTION

Disease prevention is the deferral or elimination of specific illnesses and conditions by one or more interventions of proven efficacy. While the term is generally applied to human health, the principles apply to other plant and animal species.

It is useful to distinguish among three levels of prevention—primary, secondary, and tertiary— although the boundaries between these levels are not always perfectly understood. "Primary prevention" refers to the prevention of diseases before their biological onset. For example, pasteurizing milk essentially eliminates bacterial pathogens that could cause illnesses, and measles immunization prevents clinical illness before it can get started. Another kind of primary prevention occurs when older persons with osteoporosis wear hip protectors, which absorb the shock of a fall and are capable of preventing hip fracture. Behavioral interventions such as smoking cessation, preventive dental care, and maintaining physical exercise are also examples of primary prevention, as are the provision of uncontaminated food and water. Routine searching for genetic abnormalities is usually a form of primary prevention, in that abnormal genes that are associated with various diseases can often be detected before any disease occurs. An example is newborn screening for phenylketonuria, a metabolic disease that can be subverted with an appropriate diet. There are many logistical and ethical difficulties in routine searching for genetic abnormalities, however, and appropriate approaches are still being developed.

A term sometimes used, related to primary prevention, is "primordial prevention," which refers to creating an environment where certain challenges to health are eliminated, and thus no other preventive interventions are necessary. Two examples of primordial prevention are the global elimination of smallpox, so that no immunization is necessary; and the potential for eliminating added salt from all foods, which would, if achieved, be quite effective in preventing hypertension.

"Secondary prevention" refers to the prevention of clinical illness through the early and asymptomatic detection and remediation of certain diseases and conditions that, if left undetected, would likely become clinically apparent and harmful. This is often referred to as "screening." There are many examples of secondary disease prevention, including routine bacteriological culturing for sexually transmitted organisms in asymptomatic persons; routine serological testing for preclinical infections such as syphilis; screening for high blood pressure, which may indicate clinical hypertension; screening for early breast cancer using mammography or for early cancers or precancerous lesions of the colon using sigmoidoscopy or colonoscopy. An example of a form of screening that is actually a primary prevention is to routinely examine the paint on walls of older homes, where lead contamination and its exposure to children may be a problem. One variation of secondary prevention is to screen for conditions that might be clinically overt but have gone undetected, such as clinical depression or other mental illnesses. This is also quite useful because such conditions are often quite treatable. Sometimes it may be possible to apply primary preventive interventions to diseases that are already developing: smoking cessation and increasing exercise may prevent the emergence of heart attack or stroke, even though some atherosclerotic lesions (hardening of the arteries) are already developing.

"Tertiary prevention" refers to the prevention of disease progression and additional disease complications after overt clinical diseases are manifest. This is generally the province of physicians and other health professionals, who manage acute and chronic conditions. While the distinction between disease treatment and tertiary prevention may be sometimes uncertain, many examples exist— lowering a high blood cholesterol level after a heart attack can prevent the occurrence of further heart attacks and related conditions such as stroke and angina pectoris (chest pain with activity). Similarly, treating high blood pressure after a stroke may decrease the risk of subsequent strokes. For persons with diabetes mellitus, eye examinations to detect diabetic retinal disease, and steps taken to prevent its progression, are routinely undertaken. Routine podiatric care can deter the effects of diabetic vascular disease on the feet. Among persons who are severely disabled, provision of special mattresses and other interventions can prevent some chronic skin ulceration. Providing handrails in the homes of persons at high likelihood of repeated falling can prevent fractures and other injuries. Tertiary prevention is perhaps the least well developed of the three domains, and is ripe for considerable prevention research.

The construct of primary, secondary, and tertiary prevention considers preventive intervention in the context of the onset and natural history of specific human diseases and their outcomes. Another way to view disease prevention is to consider where particular preventive interventions are carried out. Some of the most important disease prevention is carried out by environmental modification. This includes all sanitary services, such as the provision of safe food and water; adequate housing; and a general environment free of diseasecausing physiochemical and biological pollutants. Many work environments require substantial environmental protections, since they would otherwise be extremely hazardous. There are countless other environmental modifications that yield disease prevention, such as highway engineering to control speeds and dangerous road segments, the elimination of overhanging building cornices, and the removal of sharp edges or provision of shielding devices on consumer products.

Another general source of disease prevention is through appropriate individual and group behavior. Part of the disease-prevention burden lies with the individual, who must practice behaviors that minimize disease risk and occurrence, and maximize health states. Some obvious examples are maintaining an appropriate weight, never taking up or ceasing the use of tobacco products, avoiding exposures that may lead to unwanted pregnancy or passage of sexually transmitted diseases, avoiding carcinogenic sun exposure, maintaining active exercise habits appropriate for one's health status, appropriately using prescription drugs or other substances, refusing to drive a vehicle after consuming alcohol, and discouraging participation in social behaviors that may lead to disease or injury. Some persons are more prone to risk-taking behaviors in general, but there are currently not many interventions for these situations. It is clearly important that individuals have sufficient and accurate information in order to assist in initiating and maintaining disease-preventing behaviors. Thus, a related source of appropriate disease prevention is wherever health education and behavioral training takes place. This may include educational institutions, medical care sites, the Internet, and all other venues where health information and knowledge are offered, including the media and marketing activities.

Another important source of disease prevention lies largely with health-professional practice. In general, only health professionals can conduct and interpret many screening procedures, administer immunizations, or prescribe chemopreventive interventions and provide tertiary preventive services for persons with existing medical conditions. A substantial amount of health counseling and education is done by health professionals. Thus, it is important that health professionals provide clinically appropriate and comprehensive preventive services in the practice setting—as well as at other community locales, where the entire population can acquire access to them. In recent years, the extent of clinical preventive services provision has served as a key indicator of the general quality of health professional practice.

Underlying preventive-intervention delivery, no matter the source, is the need for political action. The citizens of every community or jurisdiction must provide the political impetus and the resources to assure that modern prevention is available, whether in regulating and policing the general or workplace environment, assuring high-quality sanitary procedures, furnishing effective educational programs and services, or providing fiscal and geographical access to clinical services. Provision of suitable research programs and prevention professionals is also critical. Prevention interventions may vary considerably in the evidence of their efficacy, the proportion of the population that will be positively affected by the intervention, and intervention delivery costs. Thus, some prioritization of the universe of potential interventions will often be necessary; methods to conduct such prioritization are often lacking, however, and more research is needed in this area.

Disease prevention may not be equally applied to all persons in the community. While many clinical and environmental interventions, such as routine childhood immunizations, air pollution control, and public health sanitary measures, are appropriately intended for all persons, individuals may differ dramatically in their risk of various diseases for genetic, behavioral, or environmental reasons. If groups with varying risks can be effectively discerned and efficiently identified, then some disease prevention activities may be differentially targeted for high-risk groups, both for reasons of efficacy and cost. For example, screening for blood-lead levels in children may only be useful for those who reside in older housing, where lead paint exposure is much more likely. Persons with a clear family history of some chronic conditions, such as coronary heart disease and cancer, may benefit from more intensive screening and intervention programs.

Applying disease-preventing interventions requires the same care and consideration as any clinical treatment, for several reasons. There should be evidence of efficacy—that the intervention has sufficient scientific basis to know that it works. Some interventions proffered under the guise of prevention have insufficient evidence of benefit, and more systems to monitor and provide evidence summaries are needed—existing systems include the United States Preventive Task Force, the Task Force on Community Preventive Services, and the Cochrane Collaboration. Also, it is important to note that most direct preventive interventions, even when appropriately applied, do not prevent disease in all persons. Thus, routine mammographic testing results in only a 20 to 30 percent reduction in breast cancer mortality, and physician counseling for smoking cessation has only a small effect on the smoking behavior of patients. The effect of these interventions will only be enhanced by new research and more efficient delivery programs. On the other hand, some preventive interventions are highly effective, such as many vaccines, food safety procedures, and other public health environmental activities. A related problem for some disease-prevention interventions is that long-term efficacy may be uncertain.

As in the case of clinical treatments, preventive interventions may have actual or potential adverse effects, some of which may be undetected. It is often pointed out that disease prevention has a greater moral burden to be free of adverse effects than do treatments, since they target individuals who are generally healthy. Some adverse effects are clearly and immediately identifiable, such as acute allergic reaction to a vaccine or a perforated colon during an endoscopic procedure. Others require longer term surveillance. For example, a long-term randomized trial of an early cholesterol-lowering drug revealed that the drug caused lower rates of nonfatal heart attacks but higher overall mortality rates. Similarly, a particular intervention may have an effect in a definitive randomized trail that was not predicted from epidemiological studies. For example, while consuming beta-carotene-containing vegetables has been associated with lower cancer incidence rates, randomized trials of beta-carotene in smokers have been associated in some studies with higher incident lung cancer rates. Also an intervention may have varied effects on different disease outcomes. A recent clinical trial for stroke prevention using a newer cholesterol-lowering agent found that overall strokes were prevented, but hemorrhagic strokes were significantly more frequent in the intervention group.

Behavioral and psychological adverse effects of the disease-prevention activities may be more subtle, but they are still important. An individual who has undergone a screening test may conceivably abandon health-promoting behaviors, incorrectly feeling that he or she is disease-free. Since most screening tests are not 100 percent sensitive in detecting the presence of the target disease, a false sense of security may occur. Similarly, an individual may not understand that many screening interventions must be done repeatedly to be effective, and thus may fail to participate at appropriate intervals. Some individuals may not be emotionally or educationally prepared for dealing with a disease that might be detected by screening, and this poses additional challenges for health professionals and health systems.

Robert B. Wallace

(see also: Behavior, Health-Related; Behavioral Change; Environmental Determinants of Health; Prevention; Preventive Medicine; Primary Prevention, Risk Assessment, Risk Management; Secondary Prevention; Social Determinants; Tertiary Prevention; and articles on specific diseases mentioned herein )

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