He conquered Persia, Greece and Babylon and was revered and feared in East and West alike. At the age of 32, in the height of his power and glory, he was brought down… by a mosquito. According to recent speculations, Alexander the Great may have died as a result of infection by West Nile virus.
West Nile virus (WNV) infections may have been occurring in the Middle East for centuries. The virus has spread to new areas of the world and to new populations, causing infections that are characterized by signs and symptoms. Detected in North America for the first time in 1999 in the state of New York, it has since spread throughout the continent. Three years later, in 2002, the number of human cases of infection by this virus had increased dramatically. In the United States, 4156 cases and 284 deaths were confirmed. Moreover, 9862 confirmed cases (with 264 deaths) were reported in 2003 and 2470 (with 88 deaths) in 2004 (as of January 11th, 2005)].
The WNV was detected in mosquitoes, Corvidae (American Crows, Blue Jays and Common Ravens in our context) and humans for the first time in the province of Quebec, Canada, in early summer of 2002. A total of 20 confirmed human cases (including 3 deaths) were reported in 2002, 17 confirmed cases in 2003 (no deaths) and 3 cases in 2004 (1 death). This situation forced the Government of Quebec to adopt, in 2003, a public health protection plan against the virus  that remains in place to date. The main objectives of this plan are: to prevent complications and human deaths related to WNV infections, to ensure the early detection of the presence of the virus in a geographic area, to identify areas of potential transmission to humans for preventive action and monitoring, and to qualify the level of transmission to humans.
Adapted from: http://www.ij-healthgeographics.com/content/4/1/21
When the risk of a particular disease outcome is known to be increased in a particular industry, a strategy to determine the group experience with that outcome is called surveillance. When a particular outcome is not the focus of attention and the overall health experience of the individual or group is to be observed, the strategy is termed monitoring.
Monitoring may be applied to individual workers, as in the case of voluntary periodic health evaluations, or to groups, as in special research studies to determine the health risks of certain workplace exposures or certain jobs. The principal challenges facing physicians in individual voluntary monitoring programs are maintaining a high level of attention while examining many individuals, most of whom are healthy. Such programs are often performed by physicians in private practice who are the workers’ family physician. When the experience of a group of workers is followed over time, however, patterns of illness may appear which suggest either unusual characteristics of that working population or exposures requiring control.
An increasing number of employers in high-risk industries provide surveillance programs at company expense. Monitoring programs, however, are usually limited to key employees or executives as a means of protecting the employee and the company from unexpected losses due to illness. As such, they are primarily intended for individual health care and not for population monitoring, and seldom cover high-risk groups unless they are arranged as part of a research study.
Surveillance is applied when the diseases in question are associated with a particular industry, such as the asbestos-associated lung disorders which will be used as an example in this chapter. Monitoring lends itself more readily to an effort to discover previously unrecognised health problems in an individual to control absenteeism from non-occupational illnesses or to identify previously unsuspected health effects. In general, monitoring programs are more extensive and more costly to establish than surveillance programs. Surveillance is usually restricted to certain high-risk groups and is required by law to be provided at the employer’s expense to workers exposed to specific hazards covered by OSHA or other regulations, such as asbestos, noise or lead.
The design of monitoring and surveillance programs is advancing rapidly. Surveillance is a strategy to determine a group experience with a particular disease outcome, while monitoring focuses on the overall health experience of the individual or group. Screening tests for early detection of abnormalities related to exposure are selected based on sensitivity, specificity and the prevalence of the abnormality in a complicated relationship. Physicians involved in these programs must understand their rationale and legal framework to make sense of the results. Because the field of study is advancing so rapidly, certain terms have not yet become standardized. In reading other sources, one should determine exactly how an author is using such words as monitoring, surveillance and screening.
Principles of screening
The selection of a screening test is based on three variables: the sensitivity and specificity of the test and the prevalence of the disease in the community. These terms describe essential concepts that apply to the use of tests in detecting any disease.
Sensitivity refers to the proportion of diseases persons in the population who are identified by the test. The higher the sensitivity of the test, the more effectively it will identify the diseased individuals.
The specificity of a test refers to the proportion of non-diseased individuals in the population who will have a negative result. The higher the specificity of a test, the more reliably it will exclude non-diseased individuals. The ideal is a combination of high sensitivity and high specificity. Most tests in clinical use fall well short of the ideal.
A test with low sensitivity but high specificity will detect only a small fraction of diseased individuals, but a positive result will be a more reliable indication that disease is present in an individual. However, a negative test result will not reliably rule out the disease. A test with high sensitivity and low specificity will correctly identify most true cases but will also yield positive results for many individuals who do not, in fact, have the disease. In other words, an insensitive but specific test may yield many false-negative results, whereas a sensitive but nonspecific test may give many false-positives. If a disease is rare in the population, the false-positive results of a sensitive but nonspecific test may outnumber the true positives, requiring additional diagnostic tests to confirm the result. The diagnostic efficiency of a test is called its predictive value. Chapter 23 presents an example of the calculation of the predictive value of a test using these three factors to evaluate the efficacy of a drug screening program.
Studying the specificity and sensitivity of diagnostic tests and calculating their diagnostic yield in a population with a given prevalence of a disease, constitute one aspect of clinical epidemiology. When physicians order diagnostic tests for their patients, they are applying the same concepts but the diagnostic yield is higher because the predictive value of the test is much higher. The tests are used in a small population of persons who have a high prevalence of the disease because they were selected for testing because they have suggestive symptoms. When tests are used for surveillance or monitoring, however, the predictive value is low because most of those tested are normal.
For reasons of cost and reliability, occupational health surveillance programs tend to be simple, to utilise a few well-established tests and to be rigid in their requirements. For example, the surveillance program mandated by the Occupational Safety and Health Administration (OSHA) for workers exposed to high noise levels relies on an annual audiogram covering prescribes frequencies and performed by certified personnel with specified training, using well-known equipment with prescribed specifications.
Periodic health evaluation of persons at low risk
Most large and many small companies provide periodic evaluations for their key managers and their executives. Employers are concerned about the fitness of key personnel whose death or incapacity would deprive them of special skills of knowledge and would result in disruption of activities during the preparation of a replacement. These evaluations are usually in the form of an “annual physical,” a screening strategy considered obsolete by most authorities but sanctioned by years of tradition in industry.
Many employers send their employees for screening in groups during the off-season in business. Others conduct screening programs throughout the year and have employees screened on the anniversary of their employment or reassignment. The annual physical examination has been a traditional function of occupational medicine practice and is deeply entrenched among the services customarily provided. The annual physical examination enjoys the force of law in certain Occupational Safety and Health Administration standards, the acceptance of industry, and the tradition of an executive perquisite in the form of the “executive physical.” A consensus is emerging, however, that more individualized periodic evaluations are likely to yield more useful health information and are more likely to result in effective interventions. It appears likely that a trend will develop toward the substitution of the annual physical examination with another form of periodic health evaluation for the asymptomatic patient, as recommended by the American College of Physicians.
Alternatives to the annual physical examination include multiphasic health screening (the performance of a battery of standard tests at periodic intervals) and a lifetime health monitoring program (LHMP). Multiphasic health screening is often provided for nonexecutive personnel but its effectiveness is limited because in practice it is seldom integrated into the patient’s personal health care by the family physician reviewing the information.
The most sophisticated strategy is the lifetime health monitoring program (LHMP) developed by Breslow and Somers. The LHMP has not been offered on a large scale in industry. This form of health monitoring involves certain tests, examinations and health education interventions on a variable schedule according to the patient’s age and known risk factors. The LHMP offers a medically sound approach to periodic health evaluation that is unlikely to miss preventable diseases or conditions identifiable by the screening elements selected. Properly performed, the LHMP absolutely requires personal health and risk factor information with absolute guarantees of confidentiality. A personal health problem uncovered in a periodic evaluation must be communicated in confidence to the worker and where appropriate, the occupational physician should be prepared to advise the worker as a patient apart from the occupational context and to suggest follow-up by the worker’s own physician or referral.
The strategy is designed to create opportunities to provide educational and preventive services, to encourage extension of the LHMP into retirement years, and to articulate with a suitable health promotion program implemented at a younger age. This program is therefore intended to be a segment of a true lifetime program rather than a self-limited periodic health evaluation program covering only the working years. For optimal effect the LHMP should be combined with a program of health promotion designed to enhance fitness and to encourage self-help measures.
All periodic health evaluation formats are to some degree arbitrary in derivation and subjective in formulation, based as they are on interpretations of data from many sources
A health monitoring and surveillance plan comprises several areas of intervention and their performance criteria:
Analysis of the monitoring data makes it possible to target preventive interventions such that the appropriate personal, community, and environmental protection interventions can be considered; however, to be useful, the monitoring data must be available in real-time.
Using this system, the evaluation of the situation should be carried out by an expert group that may include representatives of the appropriate government departments, scientists, and regional health authorities. In the event of a major epidemic situation, a high-level Advisory Committee is recommended.