Susceptibility in Microbial Risk Assessment: Definitions and Research Needs

Article excerpt

Under the 1996 Amendments to the Safe Drinking Water Act (1), the U.S. Environmental Protection Agency (U.S. EPA) must consider susceptible subpopulations in its health risk assessments. The amendments mention specific groups, including young children, the elderly, pregnant women, and people who are immunocompromised by disease or treatment for diseases. The concept of susceptibility to adverse health outcomes from environmental exposures can be extended to other groups as well. For chemical exposures, a great deal of current research is the analysis of how differences in metabolic phenotypes modify the effects of toxic exposures. For microbial exposures, concepts of susceptibility have generally been limited to the permanent or transient protection from infection afforded by previous exposure. Little consideration has been given to the degree to which individuals may differ in the completeness of protection offered by their immune systems. In addition to genetic factors, nutritional status, systemic diseases, and toxic insult may alter the ability to mount an effective immune response. Social factors, such as access to health care, may also modify the course of infection with microbial pathogens.

Risk assessment is an inherently multidisciplinary process, and yet the disciplines needed to address technical issues in microbial risk assessment (e.g., secondary spread, virulence) are not applicable to chemical risk assessment. Addressing this issue in a fresh, scientifically rigorous manner requires the interaction of scientists who have experience in conducting risk assessments with experts in recognizing, studying, and treating infectious diseases. On 30 November and 1 December 1999, the George Washington University's Center for Risk Science and Public Health (Washington, DC) convened a workshop titled "Incorporating Susceptibility into Microbial Risk Assessment." The goal of this workshop was to produce a consensus document with multidisciplinary input that defined susceptibility to microbial pathogens for the purposes of risk assessment and which provided a framework for incorporating data on susceptibility into microbial risk assessments. The specific objectives were to a) create a group process that effectively involved multiple disciplines, including those not traditionally involved in risk assessment, such as pathology and immunology; b) clarify the conceptual elements of susceptibility for the purposes of microbial risk assessment; c) identify the data sources for the elements of susceptibility in microbial risk assessment; and d) list the elements of susceptibility that need to be incorporated for different applications of microbial risk assessments.

Workshop Design

Individuals from a variety of disciplines were invited to the 2-day workshop. Represented disciplines and areas of expertise included infectious disease epidemiology, clinical infectious disease, molecular genetics, microbiology, laboratory practice, statistical modeling, toxicologic risk assessment, immunology, pathology, and environmental health of underserved populations. Because of the complexity of the subject, the workshop structure used the parallel efforts of three interdisciplinary breakout groups to address identical issues.

The workshop conveners devoted the morning of the first day to introductions and background presentations on approaches to susceptibility and microbial risk assessment. The first day's breakout session aimed to stimulate active exchange among disciplines in the context of specific scenarios of waterborne diseases. The second breakout session addressed the definition of susceptibility, and the third breakout session addressed data sources and research needs. The exact questions posed are shown in the appendix.

Background Presentations

The workshop conveners began by summarizing the definitions of susceptibility found in major dictionaries, relevant texts, interdisciplinary group reports, agency guidelines, and regulatory documents (2). …