Molecular Epidemiology

Although the recognition of molecular epidemiology as an epidemiology subspecialty is relatively recent, laboratory methods have long been used to classify disease and determine exposure in epidemiologic studies. For example, the ability to detect and identify bacteria was essential to the success of studies illuminating the epidemiology of typhoid by Wade Hampton Frost and others, as was the ability to measure blood lipids in the identification of an association between cholesterol and heart disease risk in the Framingham study. The distinction implied in the term molecular epidemiology arises from the challenges and opportunities of applying the rapidly expanding array of modern molecular techniques to studies of health and disease in populations. Modern molecular techniques include the ability to directly study genes (genomics), gene expression (transcriptomics), proteins (proteomics), and metabolites left behind by cell processes (metabolomics). These techniques are applied in the rapidly expanding number of epidemiology specialties, most notably genetic, cancer, environmental, and infectious disease epidemiology.

The application of molecular techniques to epidemiology gives epidemiologists the tools to move beyond risk factor epidemiology and gain insight into the overall system of the disease. For infectious diseases, the system includes the transmission system, pathogenesis, and virulence of the agent. The inclusion of molecular tools has the potential to enhance diagnosis of outcome, and to detect low levels of exposure or markers of previous exposure, decreasing misclassification of outcome and exposure. Molecular epidemiologic studies can identify previously undetectable agents, enhance outbreak investigation, help describe disease transmission systems, and give insight into pathogen gene function and host-agent interaction. When applied in combination with appropriate epidemiologic methods, modern molecular techniques allow us to identify novel methods of disease prevention and control, markers of disease diagnosis and prognosis, and fertile research areas for potential new therapeutics and/or vaccines. However, the success of these studies depends not only on the molecular measure chosen, but also on whether the strengths and limitations of the chosen measure are considered in the design, conduct, and analysis, and interpretation of the study results.

Examples in this entry focus on applications in infectious disease epidemiology, which provide the additional challenges and opportunities of at least two genomes (sets of transcripts, proteins, and metabolites), that of the infectious agent and that of the host(s). However, the general principles described are applicable to all epidemiologic studies that incorporate molecular techniques.