Chapter 1
Motivation

The prevention of infectious disease transmission from human exposure to contaminated food, water, soil, and air remains a major task of environmental and public health professionals. There are numerous microbial hazards, including exposure via food, water, air, and malicious release of pathogens that may arise. Indeed, some have argued that the property of virulence of human pathogens is one which is favored by evolutionary interactions between pathogens and host populations and therefore will always be of important concern [1]. To make rational decisions in preparing, responding, and recovering from exposures to such hazards, a quantitative framework is of high benefit.

The objective of this book is to comprehensively set forth the methods for assessment of risk from infectious agents transmitted via these routes in a framework that is compatible with the framework for other risk assessments (e.g., for chemical agents) as set forth in standard protocols [2, 3].

In this chapter, information on the occurrence of infectious disease in broad categories will be presented, along with a historical background on prior methods for assessment of microbial safety of food, water, and air. This will be followed by an overview of key issues covered in this book.

Prevalence of Infectious Disease

Outbreaks of infectious waterborne illness continue to occur, although it remains impossible to identify the infectious agent in all cases. For example, in 1991, a waterborne outbreak in Ireland resulting from sewage contamination of water supplies infected about 5000 persons. However, the infectious agent responsible for this outbreak could not be determined [4]. In the United States, it has been estimated that 38 million cases of foodborne infectious disease occur annually with unidentified agents [5].

In the United States, there have typically been three to five reported outbreaks per year in community drinking water systems involving infectious microorganisms, with perhaps up to 10,000 annual cases [6]. The 1994 Milwaukee Cryptosporidium outbreak with over 400,000 cases [7, 8] was a highly unusual event among these statistics. As shown in Figure 1.1, there has been an increasing ability to identify microorganisms responsible for waterborne diseases, and it is expected that with advances in molecular biology, this will increase.

Figure 1.1 Percentages of outbreaks associated with public water systems (n = 680) by time period 1971–2006 that had unknown etiologies based on data from Ref. [6].

There are substantially more outbreaks and cases of foodborne infectious diseases than are reported. Table 1.1 summarizes reports of U.S. cases of principal microbial infectious foodborne illnesses for two 5-year periods (1988–1992 and 2002–2006). There is a mix of causal agents, including bacteria, virus, and protozoa. It is noteworthy that (as in the case of waterborne outbreaks) the frequency of outbreaks of unknown etiology has dramatically decreased but the frequency of outbreaks associated with norovirus has dramatically increased. These changes are due in part to the ability to better identify causal agents (e.g., via molecular methods).

Table 1.1 Comparison of Five-Year Averages for Common Foodborne Reported Outbreaks

Source: From Refs. [9, 10].

a Include both Shiga toxigenic and enterotoxigenic.

It is generally recognized that reported outbreaks, either of water- or foodborne infectious disease, represent only a small fraction of the total population disease burden. However, particularly in the United States, voluntary reporting systems and the occurrence of mild cases (for which no medical attention is sought but nevertheless are frank cases of disease) have made it difficult to estimate the total caseload.

In the United Kingdom, comparisons between the number of confirmed cases in infectious disease outbreaks and total confirmed laboratory illnesses (occurring in England and Wales) have been made (Table 1.2). This suggests that the ratio of reported outbreak cases to total cases that may seek medical attention may be from 10 to 500:1, with some dependency on the particular agent.

Table 1.2 Comparison of Laboratory Isolations and Outbreak Cases in England and Wales, 1992–1994

Source: Modified from Ref. [11].

Colford et al. [12] developed estimates for the total disease burden associated with acute gastroenteritis from drinking water. This relies on combining the reported outbreak data with interventional epidemiologic studies. Based on their analysis, the total U.S. disease burden is estimated to be 4.26–11.69 million cases per year in the United States, which is substantially in excess of the reported outbreaks. In the case of foodborne illness, there are an estimated 14 million cases per year [13].

Drinking water and food are by no means the only potential routes of exposure to infectious agents in the environment. Recreation in water (either natural or artificial pools) containing pathogens can produce illness [14].

Indoor air transmission can be a vehicle of infection. Legionella transmitted through indoor environments has been a concern since the 1970s [15]. The multinational epidemic of severe acute respiratory syndrome (SARS), caused by a coronavirus, was abetted at least in one location in Hong Kong by indoor aerosol transmission between apartments of infected individuals and susceptible individuals [16]. A broad spectrum of other respiratory pathogens including influenza, rhinoviruses, and mycobacteria can be transmitted by this route [17].

The deliberate release of Bacillus anthracis spores in 2001 (the “Amerithrax” incidents) brought widespread awareness to the potential for indoor releases (as well as releases in other venues) of bioterrorist agents to cause risk [18]. Therefore, of necessity, microbial risk assessors may need to consider the impact of malicious activity in certain applications.

Prior Approaches

Concerns for microbial quality of food, water, and other environmental media have long existed. In the early twentieth century, the use of indicator microorganisms was developed for the control and assessment of the hygienic quality of such media and the adequacy of disinfection and sterilization processes. The coliform group of organisms was perhaps first employed for this purpose [19–21]. Indicator techniques have also found utility in the food industry, such as the total count for milk and other more recent proposals [22]. Other indicator groups for food, water, or environmental media have been examined, such as enterococci [23–25], acid-fast bacteria [26], bacteriophage [27–29], and Clostridia spores [29–31].

The use of indicator organisms was historically justified in because of difficulty in enumerating pathogens. However, with the increasing availability of modern microbial methods, for example, PCR, immunoassay, etc., for direct pathogen assessment, this justification has become less persuasive. In addition, in order to develop health-based standards from indicators, extensive epidemiologic surveillance is often necessary. The use of epidemiology has limitations with respect to detection limits (for an adverse effect) and is also quite expensive to conduct. Indicator...