U.S. Waterborne Disease Statistics 1991-2000
New International Partnership Announced at 3rd World Water Forum

This article is excerpted from "Drinking Water Chlorination: A Review of Disinfection Practices and Issues," a white paper published by the Chlorine Chemistry Division of the American Chemistry Council in April 2003. The white paper is available in its entirety at:
http://www.c3.org/chlorine_issues/disinfection/c3white2003.html.

The United States has one of the safest water supplies in the world due to the coordinated efforts of various federal, state and local agencies. Given the high quality of their drinking water, most Americans will never contract waterborne disease from their household taps. Nevertheless, when contaminants find their way into drinking water sources and are not eliminated by treatment processes, water can and does make people sick.

In the United States, detection and investigation of waterborne disease outbreaks is the primary responsibility of local, state and territorial public health departments, with voluntary reporting to the U.S. Centers for Disease Control and Prevention (CDC). The CDC and the U. S. Environmental Protection Agency (EPA) collaborate to track waterborne disease outbreaks of both microbial and chemical origins. Data on drinking water and recreational water outbreaks and contamination events have been collected and summarized since 1971.

The CDC recently released U.S. waterborne disease outbreak surveillance data for the time period 1999-2000. The Chlorine Chemistry Division of the American Chemistry Council aggregated these data with similar CDC data for previous reporting periods and produced the following graphical analysis.

 

Figure 1: The number of reported drinking water outbreaks rose in 1999-2000, reversing a previously declining trend.

Figure 2: Although the number of drinking water outbreaks increased in 1999-2000, reported cases of waterborne illness due to these outbreaks remained relatively static. (Note: An outbreak of Cryptosporidium outbreak occurred in Milwaukee in 1993.)

Figure 3: The causes of almost 40% of waterborne disease outbreaks are undetermined. Among waterborne pathogens, parasitic protozoa are reported to cause the greatest percentage of disease outbreaks, followed by bacteria and viruses.

Figure 4: The percentage of reported drinking water outbreaks for which causation was determined declined in the 1999-2000 time period, reversing the previous increasing trend. (1971-1998 data are from Craun, Nwachuku, Calderon and Craun, 2002; 1999-2000 data are from CDC.) Untimely investigation, a lack of specimen collection, a lack of testing, or incomplete testing are all obstacles to a more complete understanding of the causes of waterborne outbreaks (Craun et al., 2002).

From 1991 to 2000, there were 155 outbreaks and 431,846 cases of illness in public and individual U.S. water systems. Table 1 lists reported outbreaks, their causes, the numbers of cases of associated illness reported, and the types of water systems affected. By far, the largest outbreak of this period occurred in 1993 with the emerging pathogen Cryptosporidium in Milwaukee.

Table 1: Causes of Waterborne Outbreaks, by Type of Water System, 1991-2000

1Data in Table 1 are compiled from CDC Morbidity and Mortality Weekly Report Surveillance Summaries for 1991-1992, 1993-1994, 1995-1996, 1997-1998 and 1999-2000. Figures include adjustments to numbers of outbreaks and illness cases originally reported, based on more recent CDC data. 2Community water systems are those that serve communities of an average of at least 25 year-round residents and have at least 15 service connections. 3Non-community water systems are those that serve an average of at least 25 residents and have at least 15 service connections and are used at least 60 days per year. 4Individual water systems are those serving less than 25 residents and have less than 15 service connections. *There were 403,000 cases of illness reported in Milwaukee in 1993.

While useful, statistics derived from surveillance systems do not reflect the true incidence of waterborne disease outbreaks because many people who fall ill from such diseases do not consult medical professionals. For those who do seek medical attention, attending physicians and laboratory and hospital personnel are required to report diagnosed cases of waterborne illness to state health departments. Further reporting of these illness cases by state health departments to the CDC is voluntary, and statistically more likely to occur for large outbreaks than small ones.

Their limitations notwithstanding, surveillance data do yield information on the types of water systems and their deficiencies, their water quality, and the respective disease agents associated with outbreaks. These data may be used to evaluate the relative degrees of risk associated with different types of source water and systems, problems in current technologies and operating conditions, and the adequacy of current regulations. (Craun et al., 2002)

Drinking Water Pathogens

Drinking water pathogens may be divided into three general categories: bacteria, viruses and parasitic protozoa. Bacteria and viruses contaminate both surface and groundwater, whereas parasitic protozoa appear predominantly in surface water. The purpose of disinfection is to kill or inactivate microorganisms so that they cannot reproduce and infect human hosts. Bacteria and viruses are well-controlled by normal chlorination, in contrast to parasitic protozoa, which demand more sophisticated control measures. For that reason, parasitic protozoan infections may be more common than bacterial or viral infections in areas where some degree of disinfection is achieved.

Bacteria

Bacteria are microorganisms often composed of single cells shaped like rods, spheres or spiral structures. Prior to widespread chlorination of drinking water, bacteria like Vibrio cholerae, Salmonella typhii and several species of Shigella routinely inflicted serious diseases such as cholera, typhoid fever and bacillary dysentery, respectively. As recently as 2000, a drinking water outbreak of E. coli in Walkerton, Ontario sickened 2,300 residents and killed seven when operators failed to properly disinfect the municipal water supply. While developed nations have largely conquered waterborne bacterial pathogens through the use of chlorine and other disinfectants, the developing world still grapples with these public health enemies.

Viruses

Viruses are infectious agents that can reproduce only within living host cells. Shaped like rods, spheres or filaments, viruses are so small that they pass through filters that retain bacteria. Enteric viruses, such as hepatitis A, Norwalk virus and rotavirus are excreted in the feces of infected individuals and may contaminate water intended for drinking. Enteric viruses infect the gastrointestinal or respiratory tracts, and are capable of causing a wide range of illness, including diarrhea, fever, hepatitis, paralysis, meningitis and heart disease (American Water Works Association, 1999).

Protozoan Parasites

Protozoan parasites are single-celled microorganisms that feed on bacteria found in multicellular organisms, such as animals and humans. Several species of protozoan parasites are transmitted through water in dormant, resistant forms, known as cysts and oocysts. According to the World Health Organization, Cryptosporidium parvum oocysts and Giardia lamblia cysts are introduced to waters all over the world by fecal pollution. The same durable form that permits them to persist in surface waters makes these microorganisms resistant to normal drinking water chlorination (WHO, 2002). Water systems that filter raw water may successfully remove protozoan parasites.

References:

American Water Works Association (1999). Manual of water supply practices: Waterborne pathogens (1st ed.). Denver: AWWA.

Craun, G.F., Nwachuku, N., Calderon, R.L., and Craun, M.F. (2002). Outbreaks in drinking-water systems, 1991-1998. Journal of Environmental Health, 65, 16-25.

U.S. Centers for Disease Control and Prevention, (November 22, 2002). Morbidity and Mortality Weekly Report, CDC Surveillance summaries: Surveillance for waterborne disease outbreaks-United States, 1999-2000.

U.S Centers for Disease Control and Prevention (May 26, 2000). Morbidity and Mortality Weekly Report, CDC Surveillance summaries: Surveillance for waterborne disease outbreaks-- United States, 1997-1998.

U.S Centers for Disease Control and Prevention (December 11, 1998). Morbidity and Mortality Weekly Report, CDC Surveillance summaries: Surveillance for waterborne disease outbreaks-- United States, 1995-1996.

U.S Centers for Disease Control and Prevention (April 12, 1996). Morbidity and Mortality Weekly Report, CDC Surveillance summaries: Surveillance for waterborne disease outbreaks-- United States, 1993-1994.

U.S Centers for Disease Control and Prevention (November 19, 1993). Morbidity and Mortality Weekly Report, CDC Surveillance summaries: Surveillance for waterborne disease outbreaks-- United States, 1991-1992.

World Health Organization (2002). Water and Sanitation: Guidelines for drinking water quality. [On-Line]. Available: http://www.who.int/water_sanitation_health/GDWQ/Microbiology/Microbioladd/microadd5.htm (accessed 12-5-02).