Biosurveillance of water supplies has been a constant area of focus in terms of environmental, clinical, and public health in the United States. The U.S. population has approximately doubled over the past 50 years, imposing new requirements and increasing demands on the infrastructure, availability, and quality of our water supply (U.S. Census Bureau, 1999; National Resource Council, 2004). As population growth and land development continue to affect the environment, new water safety issues have arisen. For example, increased agricultural development has contributed to water supply contamination from pesticides, fertilizers, and animal waste. These issues, among others, have caused new problems (most notably in the area of exposure to disease-causing microorganisms and their toxins), complicating the ability to provide potable drinking water. To counter these new problems, the tracking of illness has become more refined as communication networks and databases are developed. Even so, the prevalence of illness owing to ingestion of pathogenic microorganisms is a growing problem (Desselberger, 2000; Pedley and Pond, 2003). Further complicating the matter, many biological species are evolving, leading to the development of new pathogenic strains (Colwell, 1996) as well as antiviral-resistant (Mansky and Bernard, 2000; Gallant, 2002) and antibiotic-resistant species (Boggs, 2003; Chopra et al., 2003). The emergence of infectious biological agents is not a new issue, but their increased frequency and their effect on public health constitute growing areas of concern (Desselberger, 2000; Pedley and Pond, 2003).

Safeguarding the water supply means dealing with challenges, including damage associated with natural disasters, accidental environmental pollution, and intentional contamination. Although natural disasters and pollution are problems that have been recognized for years, the threat of bioterrorism to our water supplies is a modern problem. Over the past few years, federal government organizations-including the Federal Bureau of Investigation (FBI), the Department of Homeland Security (DHS), Environmental Protection Agency (EPA), and the Centers for Disease Control and Prevention (CDC)-have issued advisories regarding the risks to our water supply

(CDC/EPA, 2003; Meinhardt, 2005). Any compromised water source, particularly one serving a large urban area, could have major public health and economic consequences. Further complicating the issue is the limited funding available for proper preparation, education, and instruction in dealing with the occurrence of water-borne disease (Meinhardt, 2005). Biosurveillance of our water supplies, as a counter-terrorism measure, is a growing necessity and obligation, especially after the U.S. Postal Service anthrax and ricin incidents (CDC, 2001, 2003a,b). Just as the postal network served as a viable means to distribute Bacillus anthracis (anthrax) and ricin toxin, so, too, can food and water supplies serve as a conduit for pathogen mass distribution.

There is a general consensus that the extent of water-related illnesses each year is vastly underreported, which is problematic considering that, according to World Health Organization estimates, from 1991 to 2000, healthcare providers and public health authorities could identify the causative agent in fewer than 60% of all drinking water outbreaks (Pedley and Pond, 2003). Over the past few decades, more than 175 species of infectious agents have exhibited an initial occurrence or increased incidence and, as such, are defined as emerging pathogens (not all of these pathogens represent threats to the water supply). Of those 175 emerging pathogens, more than 75% are zoonotic, or those normally associated with animals but contracted by humans. One such zoonotic parasite is linked to the worst outbreak of illness originating from water supplies in documented U.S. history. In 1993, a Cryptosporidium parvum outbreak occurred in Milwaukee, Wisconsin, causing more than 400,000 reported cases of illness and more than 100 deaths. The illness cost the community an estimated $96.2 million ($31.7 million in medical costs and $64.6 million in loss of productivity) (Corso et al., 2003). Analysis linked the outbreak to greater than normal parasite counts in the source water, associated with wet weather, as well as ineffective filtration and clarification processes at a municipal water treatment plant.

The New York State Department of Health in 1999 reported an Escherichia coli (E. coli) O157:H7 outbreak, which resulted in 71 hospitalizations, 14 cases of hemolytic uremic syndrome,

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and two deaths (Bopp et al., 2003). More than 750 suspected and confirmed cases of illness were associated with attendance at the Washington County Fair. Pulsed-field gel electrophoresis identified the water distribution system as the main source of the outbreak. The report suggested that the outbreak resulted from contamination of a local well.

During December 1989 and January 1990, extraordinarily cold weather saw two major pipes break and dozens of water meters fail in Cabool, Montana. The utility company failed to disinfect its water mains after the repairs, choosing instead to merely flush the distribution system with finished water. The result was an outbreak of E. coli O157H7, resulting in 240 cases of diarrhea and four deaths (EPA, 2002b). We know of other documented water supply contamination incidents, including more than 1000 cases of gastrointestinal illness traced back to the Norwalk virus found in contaminated well water (Lawson et al., 1991).

Any public exposure to a biological agent through the water system could have dire economic and medical repercussions. Substantial economic damage can result from a credible threat alone, whereas the medical consequences owing to an actual exposure would depend not only on the virulence and toxicity of the agent but also on the extent of its dissemination through the water system. The general consensus is that conventional treatment procedures—including clarification (a term that includes coagulation/flocculation, and sedimentation), filtration, and disinfection—would be able to neutralize or remove the great majority of biological threats that could appear, or be added purposefully, to source water. However, certain select agents, such as B. anthracis exist in forms that are small (i.e., spores of 1 nm in diameter) and more resistant to disinfectant concentrations typically used by drinking water suppliers (Burrows and Renner, 1999). Even a trace contamination of a viable biological agent with the ability to propagate inside the water distribution system could have devastating effects.

A more pronounced vulnerability to the safety of the water supply is the sheer size of the finished water supply delivery infrastructure, which includes numerous accessible points along the delivery network. These access points, often including thousands of service lines and hydrants, make it virtually impossible to definitively protect the entire water supply system. Moreover, these points are downstream of most treatment and monitoring sites, which the utilities establish on intakes and main distribution trunks; hence, the contamination would reach the public.

Researchers have used simulation exercises to achieve a better understanding of the impact that a biological release can have on the public. These exercises evaluated the overall public health response infrastructure and led to discussions of the challenges associated with such a situation (Kaufmann et al., 1997; Walden and Kaplan, 2004). The exercises have demonstrated that the healthcare system would be massively strained, and that substantial investments must be made in the public health infrastructure, vaccine and drug stockpiles, and research and development. These drills effectively modeled the benefit of rapid and effective disease control and surveillance in response to the intentional release of a biothreat-ening agent.

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