Communicated by David M. Whitacre.
Kelly A. Reynolds (
The University of Arizona, Mel and Enid Zuckerman College of Public Health, 1295 N. Martin
Ave., Tucson, AZ 85724, USA
University of Texas-Houston 1100 N. Stanton, Suite 110, School of Public Health, El Paso, TX
Department of Soil, Water and Environmental Science, University of Arizona, 429 Shantz Bldg
#38 Tucson, AZ 84721, USA
Risk of Waterborne Illness Via Drinking Water in
the United States
Kelly A. Reynolds, Kristina D. Mena, and Charles P. Gerba
II. Population Impacts of Waterborne Pathogens ......................................... 118
III. Agents of Waterborne Disease ................................................................... 121
A. Viruses ...................................................................................................... 122
B. Bacteria ..................................................................................................... 123
C. Protozoa .................................................................................................... 124
IV. Drinking Water Outbreaks .......................................................................... 124
V. Sources of Microbial Contamination .......................................................... 126
A. Source Water ............................................................................................ 126
B. Treatment ................................................................................................. 128
C. Distribution System ................................................................................. 128
VI. Geographical Distribution of Reported Violations .................................. 133
VII. Evidence of Groundwater Vulnerability .................................................... 134
VIII. Estimating Waterborne Disease risk in the United States ...................... 136
A. Estimates of Gastroenteritis from Epidemiological Studies ............. 136
B. Estimates of Gastroenteritis from Exposure ....................................... 141
C. Estimates of Waterborne Disease from Exposure ............................. 143
IX. Water Treatment at the Point-of-Use ........................................................ 146
A. Signifi cance of Regrowth in POU/POE Water Treatment
Devices ...................................................................................................... 147
B. Health Benefi ts of POU/POE Water Treatment Devices ................. 148
X. Conclusions ..................................................................................................... 149
Summary ......................................................................................................... 150
References ...................................................................................................... 151
I. Introduction .................................................................................................... 118
Rev Environ Contam Toxicol 192:117–158 © Springer 2008
118K.A. Reynolds et al.
The quality of drinking water in the United States is among the best in the
world; however, waterborne disease outbreaks continue to occur, and many
more cases of endemic illness are estimated. Documented waterborne
disease outbreaks are primarily the result of technological failures or failure
to treat the water (Craun et al. 2006). Current federal regulations require
that all surface waters used for a drinking water supply be treated to reduce
the level of pathogens so as to reduce the risk of infection to 1 : 10,000 per
year (Regli et al. 1991). To achieve this goal, water treatment must, at a
minimum, reduce infectious viruses by 99.99% and protozoan parasites by
99.9% (Regli et al. 2003). If Cryptosporidium concentrations exceed a
certain level in the source water, additional reductions are required. This
degree of treatment is usually achieved by a combination of physical pro-
cesses (coagulation, sedimentation, and fi ltration) and disinfection (chlori-
nation, ozonation). Filtration is essential for the removal of protozoan
parasites due to their resistance to chlorination and ozonation at doses
normally used in drinking water treatment (Barbeau et al. 2000; Korich et
al. 1990; Rennecker et al. 1999). A variance from fi ltration is allowed in
some cases if the watershed is protected and carefully monitored for pro-
Before fi nalization of the U.S. Environmental Protection Agency
(USEPA) Ground Water Rule in November 2006, disinfection was not
required for drinking water from groundwater sources if coliform bacteria
were not detected, as long as the source water was not directly under
the infl uence of surface water. The Groundwater Rule, however, requires
all municipal groundwater sources to be disinfected, unless they meet
certain monitoring and sanitary survey requirements by December 1, 2009
Despite the increase in source water treatment requirements in the U.S.,
with current and newly promulgated regulations, questions remain as to
how much illness is caused by microorganisms in drinking water in the U.S.
and what additional approaches may be used to further reduce this risk,
especially to sensitive subpopulations who may be at greater risk of infec-
tion and more serious adverse health outcomes. The objective of this review
is to assess current threats to the water supply in the U.S., provide estimates
of total drinking water illness, and suggest approaches for risk reduction.
II. Population Impacts of Waterborne Pathogens
A pathogen is a microorganism capable of causing disease in a host. Rela-
tive to the microbial population on earth, only a small number are capable
of causing disease in humans. Waterborne pathogens are excreted in the
feces of humans and transmitted via ingestion. In contrast, water-based
pathogens occur naturally in water and are usually not transmitted from
person to person (e.g., Legionella spp.). This review primarily deals with
Waterborne Illness 119
waterborne pathogens. Currently, more than 140 known microorganisms
are recognized as waterborne pathogens.
Waterborne pathogens have emerged in importance for a number of
reasons, including (1) an increase in the size of sensitive subpopulations; (2)
recognition of the importance of additional health effects, including chronic
sequelae; (3) an increase in the importation of foods from developing coun-
tries, where poor water quality plays a role in foodborne illness; (4) natural
evolution of microbes with increased virulence; and (5) the use of molecular
source tracking to improve methods for identifi cation of outbreaks and
In recent decades there has been a steady growth in the number of sensi-
tive populations, now thought to comprise 20%–25% of the total U.S. popu-
lation. Sensitive populations include the elderly, the very young, the
chronically ill, recipients of immunosuppressive therapies, and pregnant
women (Table 1). Studies show that these subpopulations are more likely
to be infected and experience increased morbidity and mortality following
exposure to microbial pathogens than the general population (Gerba et al.
1996a; Nwachuku and Gerba 2006). Total diarrheal deaths in aged popula-
tions (>74 yr) are about 50% compared to less than 5% in those between
the ages of 5 and 24 (Lew et al. 1991). Adenovirus infections have proved
problematic for immunocompromised populations, in which a 60% and
53% case-fatality rate is prevalent in bone marrow transplant and cancer
patients, respectively (Table 2). In addition, Cryptosporidium is identifi ed
in 2.2% of all diarrhea cases in developed countries compared to a 7% rate
in children and 14% (range, of 6%–70%) in acquired immunodefi ciency
syndrome (AIDS) patients (Chen et al. 2002).
Table 1. Sensitive Subpopulations in the United States.
Persons with diabetesc
aVelkoff and DeBarros (2005).
bUS Census Bureau (2005).
dJemal et al. (2005).
eAmerican Pregnancy Association (2006).
fGlynn and Rhodes (2005).
gUS Department of Health and Human Services (2005).
hFive-year total recipients, 2000–2004.
120K.A. Reynolds et al.
Although diarrhea is the major symptom associated with waterborne
pathogens, other chronic sequelae are possible (Parkin 2000). Chronic
sequelae are diseases that develop in the days, weeks, or years after initial
infection. Chronic sequelae, such as diabetes, heart disease, autoimmune
disease, and cancer, can have a signifi cant impact on the individual’s quality
of life and are sometimes related to infectious disease agents. In addition,
exposure to some pathogens can lead to adverse effects on the endocrine
system (i.e., Giardia lamblia is linked to hypothyroidism and coxsackievirus
is linked to orchitis) (Lindsay 1997).
Globalization of commerce and travel contributes to the spread of water-
borne disease, as does the introduction of change in drinking water treat-
ment technology or food supply production. For example, efforts to address
concerns over protozoan contamination of water and chlorine resistance
have led to an increase in the use of ultraviolet light for compliance with
water quality standards. An increased reliance on UV light treatment has
subsequently raised concern over virus resistance, in particular, adenovirus
(Nwachuku et al. 2005; Thurston-Enriquez et al. 2003). Therefore, changes
in current applications must be carefully evaluated. Expansion of our food
supply sources to include regions with deteriorated irrigation water quality
and poor hygiene has been linked to foodborne outbreaks where water
played a role in the transmission of disease. An example of this is the pro-
tozoan parasite Cyclospora caryentensis, which was imported into the U.S.
on produce from developing countries (Mansfi eld and Gajadhar 2004).
Microbes are highly adaptable to environmental pressures and continue
to evolve. The evolution of microbes can lead to a genetic reassortment that
may increase the virulence of, or expand the host base for, that organism. A
recent example of this is the severe acute respiratory syndrome (SARS) virus,
which moves from the bat population to other animals and humans (Bennett
2006). The development of molecular methods for pathogen detection and
source tracking has aided in the monitoring of water supplies and identifying
causative agents in outbreak situations. Cultural methods are often necessary
for the determination of pathogen viability in water, but for some of the most
Table 2. Case-fatality Rates among Immunocompromised Patients with Adenovirus
Patient group Overall % fatality Mean age of patients (yrs)
Bone marrow transplant
Source: Modifi ed from Hierholzer (1992).
Waterborne Illness 121
prevalent pathogenic microbes, laboratory methods for cultural detection
have not been developed or standardized (i.e., noroviruses).
III. Agents of Waterborne Disease
Pathogens capable of causing waterborne or water-based illnesses include
viruses, bacteria, and protozoa (Table 3). Also of concern in some geo-
graphical regions are helminths and blue-green algae. Since 1971, the
Table 3. Agents of Waterborne or Water-based Disease.
Toxigenic Escherichia coli
122K.A. Reynolds et al.
Centers for Disease Control (CDC), USEPA, and other agencies have been
collecting data regarding waterborne disease outbreaks in the U.S. From
1971 to 2002 there have been 764 documented waterborne outbreaks asso-
ciated with drinking water, with 12% caused by chemicals, 14% by bacteria,
19% by protozoa, and 8% by viral pathogens (Fig. 1). Nearly half of all
documented waterborne outbreaks since 1971 were caused by an undeter-
mined etiology, i.e., acute gastrointestinal illness (AGI). The characteristics
of these outbreaks of unknown AGI are often consistent with a viral etiol-
ogy, some of which are known to be nonculturable. Outbreaks during
1971–2002 are known to have resulted in 575,457 cases of illness and 79
deaths; however, the true impact of waterborne disease is estimated to be
much higher. For example, Morris and Levin (1995) estimate that 7 million
people become ill and more than 1,000 die each year as a result waterborne
Viruses range from 0.01 to 0.1 µm in size, are obligate, intracellular parasites,
and are capable of long-term survival in the water environment (weeks to
months). Viruses of greatest concern in water, and their associative illnesses,
include enteroviruses (diarrhea, meningitis, myocarditis, fever, respiratory
disease, nervous system disorders, birth defects), hepatitis A virus (hepatitis,
liver damage), noroviruses (diarrhea), astrovirus (diarrhea), adenovirus
(diarrhea, respiratory disease, eye infections, heart disease), and rotavirus
Viruses have the greatest infectivity, requiring the fewest number to
cause infection, of all waterborne microorganisms, are excreted in the feces
Fig. 1. Drinking water outbreaks by etiological agent, 1971–2002 (n = 764). AGI,
acute gastrointestinal illness. (From Blackburn et al. 2004; Calderon 2004.)
Waterborne Illness 123
in the largest numbers (up to 1011/g), and generally have the longest survival
in the environment; most only infect humans. They are not effi ciently
removed by conventional fi ltration and are more resistant to disinfectants
than bacteria. Because of their small size and ease of transport in the sub-
surface, viruses are of primary concern in groundwater. Viruses are known
to be the causative agent in 8% of drinking water outbreaks reported in
recent years (Fig. 2).
Bacteria are prokaryotic, single-celled organisms surrounded by a mem-
brane and cell wall, ranging in size from 0.1 to 10 µm. Enteric bacteria are
able to colonize the human intestinal and gastrointestinal tract. Generally,
enteric bacteria do not survive long in the environment, although some
have resistant spores or can form dormant stages that aid in their survival.
Waterborne outbreaks caused by enteric bacteria primarily occur because
of failed or absent treatment processes. Examples of waterborne enteric
bacteria include Salmonella (typhoid, diarrhea), Shigella (diarrhea), Cam-
pylobacter (diarrhea, nervous system disorders), Vibrio cholerae (diarrhea),
and Escherichia coli (certain strains: diarrhea, hemorrhagic colitis). Legio-
nella (pneumonia, respiratory infections) is an important water-based bac-
teria, and reports of Legionella outbreaks have only recently been added
to the CDC surveillance summaries; however, six water-associated out-
breaks were recorded in 2001–2002 (Blackburn et al. 2004). Non-Legionella
bacteria are known to have caused 17% of the waterborne outbreaks docu-
mented from 1991 to 2002 (see Fig. 2).
Fig. 2. Waterborne outbreaks associated with drinking water by etiological agent,
1991–2002 (n = 183). AGI, acute gastrointestinal illness. (From Barwick et al. 2000;
Blackburn et al. 2004; CDC 1993; Kramer et al. 1996; Lee et al. 2002; Levy et al.
124K.A. Reynolds et al.
Helicobacter pylori is a bacterium that has recently been recognized as
the primary cause of duodenal (90%) and gastric ulcers (80%) (CDC 2001).
It is considered a class A carcinogen, meaning that infections can lead to
gastric cancer, the second most common cancer worldwide. Although the
disease contribution related to the waterborne route of exposure is uncer-
tain, studies have found 10%–60% of individual groundwater wells con-
taminated with H. pylori (Park et al. 2001).
Protozoan parasites are single-celled animals that live in the gastrointesti-
nal tract of infected individuals. They range in size from 1 to 100 µm and
produce an environmentally stable cyst or oocyst stage. The thick cyst or
oocyst walls are highly resistant to disinfectants used in conventional water
treatment. Crytosporidium and Giardia lamblia, both causing diarrhea,
are the primary protozoa of concern with regard to water quality in the
U.S. Cyclospora caryentensis is another parasite that has been linked to
a possible waterborne outbreak in the U.S. (Mansfi eld and Gajadhar 2004).
Naegleria fowleri is a water-based pathogen of primary concern because of
a high fatality rate in diagnosed cases. Two deaths occurred in an outbreak
of Naegleria in 2002 (Blackburn et al. 2004). Overall, protozoa caused 21%
of drinking water outbreaks from 1991 to 2002 (see Fig. 2).
IV. Drinking Water Outbreaks
During the most recent 12-yr survey of waterborne disease (1991–2002),
there were 183 documented outbreaks associated with drinking water. Most
(76%) were from a groundwater source, with 18% linked to surface water
systems (Fig. 3).
Public noncommunity systems, including nontransient noncommunity
water systems (NTNCWS) serving water to at least 25 of the same people
at least 6 mon/yr, but not year round (i.e., schools, hospitals, and offi ces with
their own water systems), and transient noncommunity water systems
(TNCWS), serving persons who do not remain for long time periods (i.e.,
campgrounds, gas stations, etc.), collectively caused 39% of drinking water-
associated outbreaks from 1991 to 2002, followed by public community
water systems (CWS) serving the same population year round (36%) and
individual systems (25%) (Fig. 4). Approximately 264 million people in the
U.S. are served by a CWS, with 19.8 million served by a noncommunity
water source (12.9 million by a TNCWS and 6.9 million by a NTNCWS) in
Although a drinking water outbreak was more likely to occur in a non-
community supply utilizing a groundwater source, outbreaks involving the
greatest number of individuals exposed occurred in CWS from a surface
water source (Table 4).
Waterborne Illness 125
Fig. 3. Documented disease outbreaks associated with drinking water by source,
1991–2002 (n = 183). (From Barwick et al. 2000; Blackburn et al. 2004; CDC 1993;
Kramer et al. 1996; Lee et al. 2002; Levy et al. 1998.)
Fig. 4. Documented disease outbreaks associated with drinking water by system
type, 1991–2002 (n = 183). (From Barwick et al. 2000; Blackburn et al. 2004; CDC
1993; Kramer et al. 1996; Lee et al. 2002; Levy et al. 1998.)
126K.A. Reynolds et al.
In the most recently published survey period (2001–2002), 23 of 25 (92%)
outbreaks associated with drinking water were from a groundwater source,
and 9 (39%) of these were associated with individual homeowner systems
not regulated by the USEPA (Blackburn et al. 2004).
V. Sources of Microbial Contamination
Regarding pathogen exposure, contamination is not evenly distributed but
rather affected by the number of pathogens in the source water, the age of
the distribution system, the quality of the delivered water, and climatic
events that can tax the treatment plant operations. Because it is not practi-
cal to monitor water supplies in real time and at the point-of-use for all
groups of pathogens, episodic contamination events are diffi cult to predict
or identify. From 1991 to 2002, the majority of outbreaks occurred because
of a lack of treatment (primarily groundwater) or a treatment failure
(Fig. 5). Efforts to control microbial contamination in drinking water are
focused at four primary sites: (1) the source water, (2) treatment plant,
(3) distribution system, and (4) point-of-use. Source water protection is the
fi rst step in control of the water quality.
A. Source Water
All surface waters, no matter how pristine, contain waterborne pathogens,
because most of the signifi cant waterborne pathogens are zoonoses, meaning
they can be transmitted to humans from animals. Birds are a signifi cant
source of Campylobacter, as cattle are of Cryptosporidium. The more animal
husbandry taking place near a watershed, the greater the concentration of
zoonotic waterborne agents that can be expected in the water (Cox et al.
2005). Sewage discharges can also be a source of pathogens, even though
they may be disinfected. Although chlorination is effective in reducing
Table 4. Illness Cases Associated with Drinking Water Outbreaks, 1991–2002.
Source type Community Individual Noncommunity Total
Source: Barwick et al. (2000); Blackburn et al. (2004); CDC (1993); Kramer et al. (1996);
Lee et al. (2002); Levy et al. (1998).
Waterborne Illness 127
the number of bacterial pathogens, it has little effect on protozoan para-
sites and limited effectiveness on viral pathogens, as normally practiced
(Fallacara et al. 2004). Giardia is more abundant in sewage discharges
than Cryptosporidium (Smith and Grimason 2003). With the exception of
hepatitis E virus, enteric viruses are not zoonotic and only originate from
sewage sources, i.e., sewage treatment plants, combined sewer overfl ows,
and septic tanks.
The occurrence of enteric pathogens in surface waters is highly variable,
depending heavily on rainfall events. The elevated concentrations of patho-
gens after such events can pose a major challenge to water treatment plants.
Extreme rainfall events and waterborne disease outbreaks from drinking
water have been positively correlated in both the U.S. and Canada
(Curriero et al. 2001; Thomas et al. 2006).
In contrast to surface waters, groundwater supplies were historically
thought to be free of pathogenic microbes for reasons of the natural fi lter-
ing ability of the subsurface environment and distance a microbe would
have to travel to reach the groundwater source. Microbial contaminants
that fi nd their way into groundwater may originate as a result of lack of
wastewater treatment or improper management of wastewater disposal,
septic tank contamination, underground storage tank or landfi ll leaks, mis-
management of animal waste disposal, shallow wells, etc. Improved surveil-
lance using molecular and cultural detection methods has led to increased
evidence of human enteric viruses and other potentially harmful microbes
in groundwater. Private groundwater wells are a concern because they are
rarely, if ever, monitored and treated.
Fig. 5. Documented disease outbreaks associated with drinking water by defi ciency,
1991–2002 (n = 183). (From Barwick et al. 2000; Blackburn et al. 2004; CDC 1993;
Kramer et al. 1996; Lee et al. 2002; Levy et al. 1998.)
128K.A. Reynolds et al.
Much attention has been focused on enhancing current treatment processes
to eliminate pathogens that are resistant to conventional water treatment,
i.e., fi ltration for Cryptosporidium, and to expand treatment recommenda-
tions to source waters that were previously considered protected from
harmful microbes, i.e., protected surface waters and groundwater. Popula-
tions are still at risk of pathogen exposure partly because of lack of treat-
ment, i.e., no fi ltration of large municipal water supplies, such as New York
and Boston, and currently no disinfection of municipal groundwater sup-
plies. In addition, individual homeowners with private wells are at risk
where contamination events would go largely unnoticed due to a lack of
monitoring and reporting.
Even in the event of administering multibarrier treatment processes, it
is not possible to remove 100% of the pathogens from the source water
100% of the time (Haas and Trussell 1998). Table 5 shows the documented
frequency of various contamination events in a surface drinking water treat-
ment system in Sweden, including treatment failures and distribution system
contamination. Quantitative microbial risk assessment suggests that,
depending on the original raw water quality, such events could cause serious
health consequences (Westrell et al. 2003).
C. Distribution System
Even water that is adequately protected and treated is subject to pathogens
entering the distribution system. From 1971 to 2002 there were 133 (17%
of all outbreaks) documented waterborne outbreaks in the U.S. linked to
distribution system contamination (Barwick et al. 2000; Blackburn et al.
2004; CDC 1993; Kramer et al. 1996; Lee et al. 2002; Levy et al. 1998). Pre-
liminary data from the 2003–2004 survey period indicate that 38% of the
Table 5. Frequency and Duration of Pathogen Contamination Events in a Surface
Drinking Water System.
Type of incident Frequencya Duration (hr/incident)
Wrong coagulant dosage
aPer 1,000,000 persons/y.
Source: Westrell et al. (2003).
Waterborne Illness 129
reported outbreaks associated with drinking water systems were also asso-
ciated with distribution systems (NRC 2006; Liang et al. 2006).
Municipal Distribution Systems
During the most recently published survey period (2001–2002), 5 of 25
(20%) of the documented waterborne outbreaks were associated with
drinking water distribution system defi ciencies and, of the 7 outbreaks
reported involving community water systems, 4 (57.1%) were linked to
distribution system problems (Blackburn et al. 2004). Although the overall
number of reported outbreaks associated with community water systems
has decreased in the last decade, the proportion of outbreaks associated
with distribution systems has increased (Fig. 6). The reduction in total
waterborne outbreaks is largely attributed to the promulgation of numer-
ous regulations by the USEPA, including the surface water treatment rule,
primarily aimed at reducing the risks of waterborne protozoa and improv-
ing water treatment (Pierson et al. 2001; Blackburn et al. 2004), but the
current regulatory requirements do not appear to reduce the proportion of
outbreaks associated with distribution systems.
The distribution system includes both the pumping, piping, and storage
networks that deliver fi nished water to end users. There are approximately
1 million miles of distribution system networks in the U.S. and an estimated
Number of outbreaks in CWS
% due to distribution system deficiencies
Fig. 6. Waterborne disease outbreaks in community water systems (CWS) associ-
ated with distribution system defi ciencies. (Modifi ed from NRC 2006.)
130 K.A. Reynolds et al.
154,000 fi nished water storage facilities, with more than 13,000 miles of new
pipes installed each year (AWWA 2003; Grigg 2005; Kirmeyer et al. 1994).
In a 2005 report on the nation’s infrastructure, the USEPA acknowledged
the need for signifi cant investment in installing, upgrading, or replacing
infrastructure for delivering and storing drinking water at an estimated
20-yr cost of $208.4 billion (USEPA 2001, 2004). In the U.S. there is a wide
range of distribution pipe age and materials with varying life expectancies.
Pipes in the U.S. are replaced at an average rate of once every 200 yr (Grigg
2005); however, the life expectancies range from 75 to 120 yr (AWWA 2001;
AWWSC 2002). Approximately 26% of the distribution pipes in the U.S.
are in poor condition, and the annual number of documented main breaks
has signifi cantly increased from about 250 in 1970 to 2,200 in 1989 (AWWSC
2002). It is estimated that even well-run water distribution systems experi-
ence about 25–30 breaks per 100 miles of piping/yr (Deb et al. 1995). Using
a value of 27 main breaks/100 miles/yr, Kirmeyer et al. (1994) estimated
237,000 main breaks/yr in the U.S.; however, variation between utilities is
considerable. Haas (1999) reported results from a survey of water systems
that showed a range of average main breaks of 488/yr for systems serving
more than 500,000 people, to 1.33/yr for systems serving fewer than 500
people. The public health signifi cance of these breaks in the distribution
system is not currently known.
Maintaining the hydraulic integrity (positive pressure) of water distribu-
tion is important given that insuffi cient pressure has led to disease epidem-
ics worldwide (reviewed in Lee and Schwab 2005). Negative hydraulic
pressure creates a backfl ow of nonpotable water into the potable water
supply via back-siphonage, where signifi cant pressure drops siphon con-
taminants into the system at cross-connections or leakage points, or back-
pressure from pressures in the system that exceed the supply pressure
(Herrick 1997). Even minor pressure fl uctuations create back-siphonage
where intrusion rates are estimated at >1 gpm (LeChevallier et al. 2003a).
During power outages, up to 90% of nodes have been shown to draw a
negative pressure (LeChevallier et al. 2003b).
A survey of 26 water utilities in the U.S. found that the percent of leakage
(unaccounted-for water) ranged from <10% to as high as 32% (Kirmeyer
et al. 2001). Water systems commonly lose >10% of the total water produced
through leaks in the pipelines (AWWA and AWWARF 1992). At least 20%
of distribution mains are reported to be below the water table, but it is
assumed that all systems have some pipe below the water table for some
time throughout the year, thus providing an opportunity for intrusion of
exterior water under low or negative pressure conditions (LeChevallier
et al. 2003b). In addition, pipes buried in soil are subject to contamination
with fecal indicators and pathogens from the surrounding environment
(Karim et al. 2003; Kirmeyer et al. 2001). A survey of water utilities in North
America found that 28.8% of cross-connections resulted in bacterial
contamination (Lee et al. 2003). Negative hydraulic pressure can draw
pathogens from the surrounding environment into the water supply where
Waterborne Illness 131
residual disinfection effi cacy is uncertain and variable, depending on the
magnitude of such events (Gadgil 1998; Haas et al. 1998; Trussell 1999).
Little is known about the extensiveness of distribution system inadequacies
and whether they are sporadic or continuously occurring (Lee and Schwab
2005), but outbreaks have been documented following external contamina-
tion in the distribution system despite the presence or requirement of
residual disinfectant (Craun and Calderon 2001; Levy et al. 1998).
Decline in residual disinfectant is related to many factors, including the
distance traveled, water fl ow velocity, residence time, age and material of
pipes, and water pressure (Egorov et al. 2002). Although residual chlorine
is present in the distribution system of treated water, the levels do not
provide signifi cant inactivation of pathogens in intrusion events (Payment
1999; Snead et al. 1980). More recent modeling studies have evaluated intru-
sion events at specifi c locations, with consideration to mixing, contact
time, and other distribution system variables, before consumption. Under
these realistic exposure scenarios, monochloramine disinfectants performed
poorly against Giardia and Escherichia coli. Typical concentrations of chlo-
rine residual (0.5 mg/L) inactivated E. coli in simulated sewage intrusion
events but were again ineffective for Giardia (Baribeau et al. 2005; Propato
and Uber 2004). Intentional contamination events in the distribution system
are also a concern where public water supplies are potentially vulnerable
to bioterrorism threats.
According to a Centers for Disease Control (CDC) survey, cross-
connections and back-siphonage caused the majority (51%) of outbreaks
linked to the distribution system from 1971 to 2000, followed by water main
contamination (a collective 33%) and contamination of storage facilities
(16%) (Fig. 7). Data compiled by the USEPA indicate that only a small
percentage of contamination from cross-connections and back-siphonage
are actually reported and that the CDC data underreports known instances
of illnesses caused by backfl ow contamination events. For example, from
1981 to 1998, only 97 of 309 (31%) documented incidents were reported to
public health authorities (USEPA 2002). Of the 97 reported incidences,
75 (77%) reported illnesses (4,416 estimated cases); however, only 26 (27%)
appear in the CDC summaries of waterborne disease outbreaks.
Water quality may degrade in treated water storage facilities because of
loss of disinfectant residual, increased temperature, and external contami-
nation from birds, insects, animals, wind, rain, algae, etc. Storage tanks are
particularly vulnerable to contamination in the absence, or failure, of a
protective cover or barrier, open hatches, and vents; however, birds have
been known to contaminate even covered public water supply distribution
storage tanks (AWWA and EES 2002; Clark et al. 1996).
Home Distribution Systems
Bacterial colonization of pipes, connections, and faucets positioned
along the channels of drinking water distribution, including the utility’s
132K.A. Reynolds et al.
distribution system, the homeowner’s premise plumbing, and fi xtures in the
home is well documented. Pepper et al. (2004) found that the bacteriologi-
cal quality of water signifi cantly deteriorates in the home plumbing relative
to the distribution system, as evidenced by survey of heterotrophic plate
count (HPC) bacteria (Table 6). Stagnant water in premise plumbing pro-
vides an environment where bacteria can grow to values several orders of
magnitude higher than in the municipal distribution system (Edwards et al.
2005). Although HPC bacteria in drinking water is not considered a direct
health risk (WHO/NSF 2003), opportunistic pathogens such as Legionella
and Mycobacterium are associated with human disease and have been
found in premise plumbing biofi lms (Flannery et al. 2006; Pryor et al. 2004;
Thomas et al. 2006; Tobin-D’Angelo et al. 2004; Vacrewijck et al. 2005).
Contamination of mains during
construction, repair or flushing
Water main and sewer in same trench
or inadaquately separated
Cross-connection or back-siphonage
Broken or leaking water mains
Contamination of service lines or
Contamination during storage
Table 6. Tracking Deteriorating Water Quality to the
Sample site HPC (cfu/mL)
Source: Pepper et al. (2004).
Fig. 7. Waterborne outbreaks caused by distribution system defi ciencies, 1971–2000
(n = 120). (From Calderon 2004.)
Waterborne Illness 133
Since 2001, Legionella outbreaks have been documented in the CDC
surveillance summaries of waterborne disease and comprise a signifi cant
portion of drinking water outbreaks (19% in 2001–2002). All six of the
documented Legionella outbreaks in 2001–2002 were related to regrowth
of Legionella in the distribution systems of large buildings or institutions
(Blackburn et al. 2004).
VI. Geographical Distribution of Reported Violations
Predicting the most at-risk populations based on geographical distribution
is diffi cult for reasons of the relative signifi cance of source water type and
quality, treatment plant reliability, climatic events, distribution system integ-
rity, reporting bias, and other factors. The Safe Drinking Water Information
System provides data on CWS reporting health based violations of the
National Primary Drinking Water Regulations (NPDWRs). The NPDWRs
are legally enforceable standards that apply to public water systems subject
to inorganic, organic, radionuclide, microbial, or other health-effecting con-
taminants. These primary standards set maximum contaminant levels
(MCLs), the maximum permissible level of a contaminant in water deliv-
ered to any user of a public water system. In fi scal year 2003, 3,986 CWS
(8% of total systems reporting) serving 24.4 million people (9% of the
population) delivered drinking water in violation of at least one of the
health-based standards (Fig. 8). Although about half of these violations
0-6% of systems
6-11% of systems
11+% of systems
Fig. 8. Reported community water systems violating maximum contaminant levels
or treatment standards in FY 2002. (From USEPA 2004.)
134 K.A. Reynolds et al.
were the result of monitoring and reporting errors, the top two reported
violations were under the category of the total coliform rule (9,056 reported
violations) and the surface water treatment rule (1,747 reported violations).
U.S. commonwealths and territories (i.e., American Samoa, Puerto Rico,
U.S. Virgin Islands) were documented with an average of 44% of CWS
reporting health-based violations, potentially impacting 71% of the popula-
tion (USEPA 2004). Most of the U.S. population receives water from
a CWS. Although there are 54,064 community water systems, serving
a total of 263.9 million people, just 7% serve 81% of the population
VII. Evidence of Groundwater Vulnerability
Community water systems have more groundwater than surface water
sources, but more people drink from a surface water system. A reported
11,403 systems, serving 178.1 million people, relied on surface water com-
pared to 42,661 systems, serving 85.9 million people, reliant upon ground-
water sources (USEPA 2006b). States with groundwater sources serving the
greatest number of individuals are diagramed in Fig. 9. Before the newly
promulgated Ground Water Rule (USEPA 2006), utilities with a ground-
water source were not required to disinfect the water supply and many
small communities and individual homeowners continue to consume
untreated groundwater. Several national surveys have documented evi-
dence of viruses in groundwater (Table 7). The newly promulgated Ground
Water Rule applies to more than 147,000 public water systems and more
Fig. 9. States with the highest populations served by groundwater. (From USEPA
Waterborne Illness 135
than 100 million consumers, utilizing municipal groundwater sources. The
rule requires that sanitary surveys be conducted by December 31, 2012, for
most CWS and by 2014 for CWS with outstanding performance and for all
noncommunity water systems, to help identify defi ciencies that may lead to
impaired water quality. Source water monitoring for indicator microbes,
corrective actions for systems with signifi cant defi ciencies or source water
fecal contamination, and compliance monitoring are further required. The
USEPA estimates that the Ground Water Rule will reduce waterborne viral
illnesses by approximately 42,000 cases each year, a 23% reduction from
the current baseline estimate.
One survey of 448 utility wells in 35 states, using molecular methods
of detection (reverse transcriptase-polymerase chain reaction, RT-PCR),
found evidence of enteric viruses, including enterovirus, rotavirus, and
hepatitis A RNA, in approximately 32% of groundwater supplies
(Abbaszadegan et al. 2003). Molecular methods for virus detection do not
determine viability, and thus the public health signifi cance of these results
is not known, but the presence of viral RNA in groundwater suggests a
potential for exposure and adverse health risks.
An additional survey of 321 samples from 29 U.S. utility wells, collected
over 1 yr, detected human enteric viruses including enterovirus, reovirus,
norovirus, and hepatitis A virus in 72% of the sites and 16% of the samples
using RT-PCR (Fout et al. 2003). Similarly, 50% of samples from 48 midwest
utility wells tested positive for human viruses (Borchardt et al. 2004). In the
latter study, three samples were found positive for culturable hepatitis
A virus. Another study of 50 private homeowner wells found enteric viruses
in 8% of the samples collected (Borchardt et al. 2003a). In addition to human
viruses, protozoan parasites have been documented in groundwater. Of 199
groundwater samples surveyed, 5% of vertical wells, 20% of springs, 50%
of infi ltration galleries, and 45% of horizontal wells tested positive for
Cryptosporidium oocysts, calling for a reevaluation of the notion that ground-
water is inherently free of protozoan parasites (Hancock et al. 1998).
Helicobacter pylori has been found in biofi lms of water distribution
systems (Park et al. 2001) and individual groundwater wells. Epidemiological
Table 7. Evidence of Enteric Virus Contamination in U.S. Groundwater Wells.
Sample description Virus positive Source
448 utility wells, 35 states
50 homeowner wells
29 utility wells
48 midwest utility wells
32% enteric virus
8% enteric virus
16% enteric virus
6% norovirus group 1
Abbaszadegan et al. (2003)
Borchardt et al. (2003a)
Fout et al. (2003)
Borchardt et al. (2004)
211 Californian utility wells Yates (unpublished, 2004)
136K.A. Reynolds et al.
studies in Germany have linked infection in children with drinking untreated
well water serving individual homes (Herbarth et al. 2001), as did a study in
West Virginia linking contaminated homeowner wells (Elitsur et al. 1998).
Studies of groundwater quality have implicated an association with septic
systems and disease. Borchardt et al. (2003b) found that viral diarrhea in
children from 14 contiguous zip codes in Wisconsin positively correlated
with septic tank density. Water holding tanks and bacterial diarrhea were
also positively correlated. Raina et al. (1999) showed E. coli in well water
was correlated to diarrhea in rural families. The closer the septic system
was in proximity to the drinking water well, the greater the incidence of
disease. Overall, 46% of wells were contaminated if the septic system was
within 20 m.
VIII. Estimating Waterborne Disease risk in the United States
Variable approaches have been used to estimate gastrointestinal illness
from waterborne pathogens including epidemiological studies and expo-
sure analysis. Information is lacking, however, regarding risk estimates con-
sidering gastroenteritis and other illnesses related to microbial contaminants
in drinking water.
A. Estimates of Gastroenteritis from Epidemiological Studies
Estimating the incidence of endemic acute gastrointestinal illness attribut-
able to drinking water has been approached using information obtained
from household intervention trials (Colford et al. 2002, 2005; Hellard et al.
2001; Payment et al. 1991, 1997). Determining the illness attributable to
drinking water involves estimating the baseline of gastrointestinal illness
within communities, and such information can be useful when conducting
quantitative microbial risk assessments of drinking water quality. The
household intervention trials conducted in the research investigations cited
above are types of epidemiological studies that involve randomly designat-
ing one group of households as the “intervention group” where household
members utilize drinking water obtained via an in-home treatment system
and then having another group of households use water directly from their
tap or through a fake device that provides no additional water treatment.
In the latter situation, the study is blinded, meaning that neither group
knows during the study whether their in-home device is actually providing
Such household intervention trials have been conducted in the U.S.,
Canada, and Australia. Table 8 lists and highlights various aspects of these
studies. For all these trials, the human health outcome of interest was gas-
trointestinal illness, with some variation regarding the specifi c symptoms in
defi ning that outcome. All participants were immunocompetent individuals
who kept health diaries throughout the study to record symptoms related
Table 8. Household Drinking Water Intervention Trials Addressing Gastrointestinal Illness.
Payment et al. (1991)
Payment et al. (1997)
Hellard et al. (2001)
Colford et al. (2002)
Colford et al. (2005)
Iowa, United States
Sample size (individuals)
Ultraviolet and 1-µm
1-µm fi lter
1-µm fi lter
Bottled plant water
and bottled purifi ed
Means of assessing illness
Surface water (river)
Surface water (river)
Surface water from
Surface water (river)
fl occulation, settling,
fi ltration, ozonation,
fi ltration, ozonation,
Finished water quality
No total coliforms
Waterborne Illness 137
138K.A. Reynolds et al.
Table 8. (cont.)
Payment et al. (1991)
Payment et al. (1997)
Hellard et al. (2001)
Colford et al. (2002)
Colford et al. (2005)
No report of fecal
Total coliforms and
detected in some
0.08 for tap water
0.12 for tap water
0.02 for bottled
Cases attributable to
12% for tap water
17% for tap water
w/purge valve 3% for bottled plant
Source: Modifi ed from Colford et al. (2006).
Waterborne Illness 139
to gastrointestinal illnesses. The source (surface) waters were reported to
have varying levels of microbial contamination.
Payment et al. (1991) were fi rst to conduct a household intervention trial
addressing (gastrointestinal) illness attributable to drinking tap water. The
tap water met both Canadian and U.S. regulations, but the source water was
subject to contamination from sewage. The limitation of this study is that it
was not blinded, so those persons drinking tap water (therefore no treat-
ment device) may have been more inclined to report poorer health symp-
toms. An overall conclusion from this study is that an estimated 35% of
the gastrointestinal illnesses occurring within the tap water group may be
attributable to their drinking water.
Payment et al. (1997) conducted a follow-up study to address the results
from their previous study (Payment et al. 1991). A goal of this investigation
was to evaluate the role of distribution system water quality in gastrointes-
tinal incidence, which resulted in a study design involving four groups of
participants: a tap water group and a bottled purifi ed water group (to
address those exposed and unexposed, respectively), and a plant bottled
water group and a tap water group using a purge valve (to address distribu-
tion system water quality). The attributable risk percent ranged from 3%
for the bottled plant water group (rate of illness for this group is the same
as for those drinking the bottled purifi ed water), to 12% for the tap water
group, to 17% for those in the tap water group with a purge valve. The
investigators concluded that the excess number of gastrointestinal illnesses
observed in the fi rst study may not have been associated with surface water
contamination but rather was associated with contamination within the
distribution system because the rate of illness of the bottled plant water
group was similar to those drinking bottled purifi ed water. A limitation,
however, is that about half the participants in the bottled plant water group
dropped out during the course of the study. In addition, as with their previ-
ous study (Payment et al. 1991), this study was also unblinded.
Hellard et al. (2001) designed the fi rst blinded household intervention
study, which was conducted in Australia. Some participants used a water
treatment device that involved an ultraviolet application and fi ltration
while others were given a fake (no treatment) device. As with the Payment
et al. studies (1991, 1997), participants recorded gastrointestinal illness
symptoms in diaries, although this study used a slightly more strict defi ni-
tion of gastrointestinal illness. Participants were followed for more than 1 yr,
another strength of the study besides the blinding of participants, and the
investigators observed similar rates of illness of the participants using the
fake device as those using the water treatment device.
Colford et al. (2002, 2005) conducted two household intervention trials
in the U.S. Their fi rst study was designed as a pilot to obtain information
regarding the practicality of a study design to include blinding of partici-
pants. The investigators utilized a blinding index (James et al. 1996),
which led to the conclusion that the study they designed could incorporate
140K.A. Reynolds et al.
effective participant blinding. The investigators observed an attributable
risk of 0.85 and concluded that 24% of the gastrointestinal illnesses could
be attributable to tap water.
The goal of the Colford et al. (2005) follow-up study was to determine
if water treatment at the tap could reduce the number of gastrointestinal
illnesses. Again, the investigators used a blinding approach, and participants
recorded gastrointestinal health symptoms in diaries. The investigators
observed no difference in the rate of illness between those participants
using the fake device and those using the treatment device. The authors
offered the explanation that perhaps their study owed this conclusion to
successful water treatment practices and a well-maintained water distri-
bution system. In addition, it was recognized that water consumption by
the participants outside the home may have had some effect on the
Colford et al. (2006) reviewed the household intervention trials described
above as well as presenting an approach for estimating the occurrence of
acute gastrointestinal illness in the U.S. that can be attributable to drinking
water. Their proposed approach considers the following: (a) the estimated
incidence of acute gastrointestinal illness in the U.S. of 0.65 episodes per
person-year based on data collected from the Foodborne Diseases Active
Surveillance Network (FoodNet) (Hawkins et al. 2002; Jones et al., 2007);
(b) the attributable risks determined from the household intervention trials
(median attributable risk of 0.08 and a median attributable risk percent of
12%); (c) the proportion of risks of acute gastrointestinal illness associated
with source water and/or water treatment quality; (d) the number of people
in the U.S. served by community water systems and consuming drinking
water from surface water sources and groundwater sources; and (e) the
number of people in the U.S. served by community water systems that are
known to have either poor quality source water or poor water treatment.
Based on considerations just listed and various assumptions, Colford
et al. (2006) estimated that as many as 11.69 million cases of acute gastro-
intestinal illness, occurring each year, may be attributable to drinking tap
water in the U.S. Assumptions include the applicability of the attributable
risk percent estimates from the household intervention trials to the entire
U.S. population. In addition, the authors created scenarios assuming differ-
ent risk levels associated with either poor source water quality/poor water
treatment or problems with quality within a distribution system. The latter
resulted in the 11.69 million cases of acute gastrointestinal illnesses/yr esti-
mation and a lower estimate of 4.26 million cases/yr associated with poor
source water quality/poor water treatment (Colford et al. 2006). The authors
emphasize that the primary purpose of their estimation of acute gastroin-
testinal illness incidence attributable to drinking tap water in the U.S. is to
demonstrate a methodology that can be improved upon with more data.
Besides household intervention trials, community intervention studies
have also been conducted to address waterborne gastrointestinal disease
Waterborne Illness 141
risks (Calderon 2001; Frost et al. 2006; Goh et al. 2005; Hellard et al. 2002;
Kunde et al. 2006; McConnell et al. 2001). These types of studies offer some
advantages over household intervention trials including that they may be
simpler and less costly to conduct (Calderon and Craun 2006): they have
included cohort, case-control, and ecological types of designs. Two of these
(Calderon 2001; Goh et al. 2005) concluded that a reduction in gastrointes-
tinal illnesses was observed as a result of additional water treatment. A
preliminary report from the Kunde et al. (2006) study also indicates
a decrease in diarrheal illness risk in participants over age 35 following
the intervention. Conversely, preliminary data analysis from the Frost
study (2006) does not indicate a signifi cant difference; however, analysis is
reported to be ongoing for both of these aforementioned studies (reviewed
in Calderon and Craun 2006).
B. Estimates of Gastroenteritis from Exposure
Messner et al. (2006) described an approach for estimating the incidence
of gastrointestinal disease in the U.S. from drinking water. These investiga-
tors assume that for each population served by a community water system,
a distribution of incidence rates of acute gastroenteritis can be estimated
that can then be used to derive an overall national estimate of this disease
attributable to drinking water in the U.S. They emphasize the need for
addressing “mixtures” of pathogens as opposed to considering health risks
from exposure to an individual pathogen as one of the premises for the
approach described in this paper. The authors speculate that the mean
incidence of acute gastrointestinal illness attributable to drinking water
among community water systems ranges widely because of variations in
source water quality, water treatment effi ciencies, water quality within a
distribution system, and water quality management practices.
Messner et al. (2006) propose the development of a “risk matrix” to cat-
egorize community water systems (CWS) based on relative microbial risk
levels. The authors suggest connecting the information obtained from epi-
demiological studies regarding the incidence rate of acute gastrointestinal
illness to risk factors identifi ed in the epidemiological studies that have
been conducted and to other CWS. Identifi cation of these risk factors will
allow for risk-based categorizing of other CWS that have similar character-
istics, therefore assuming that generalizations can be made regarding all
U.S. CWS and the populations they serve. Completing this process involves
overcoming several challenges from the lack of data related to both patho-
gen occurrence and variation in survivability and infectivity, as well as
knowing the actual effi ciency of water treatment applications, as opposed
to theoretical information. Messner et al. (2006) utilize in their approach
specifi c information obtained during the Payment et al. studies (1991, 1997)
regarding factors associated with source water/water treatment quality and
factors related to distribution system defi ciencies, and also therefore utilize
142 K.A. Reynolds et al.
the specifi c defi nition of gastrointestinal illness as defi ned in these studies,
for highly credible gastrointestinal illness.
To estimate the incidence of acute gastrointestinal illness in the U.S.
caused by drinking water, Messner et al. (2006) selected 2004 as their refer-
ence year and assumed that a certain number of cases are the result of
source water/water treatment quality and a certain number are caused by
distribution system defi ciencies. In addition, the investigators assume a
lognormal distribution to address the variability of relative microbial risk
related to both, leading to an estimation of a statistical distribution of acute
gastrointestinal illness among CWS within the U.S.. The authors consider
that there is an approximate 5-log range regarding mean pathogen concen-
trations in source waters and a 2- to 6-log range regarding mean pathogen
reduction. They used Monte Carlo simulations to ultimately compute this
estimate of the distribution of acute gastrointestinal illness.
Based on the described assumptions, Messner et al. (2006) estimate that
the mean national estimate of gastrointestinal illness, using the Payment
et al. (1991, 1997) defi nition of highly credible gastrointestinal illness, attrib-
utable to drinking water is 0.11 cases/person/yr (with a 95% credible bound
of 0.03–0.22) (Table 9). The investigators relate their 0.11 cases/person/yr
estimate to the reported rate of diarrheal “episodes” of 1.3/person/yr
(Imhoff et al. 2004) and estimate that the percentage of “episodes” attribut-
able to drinking water is 8.5%. If this same percentage is assumed and
applied to the Imhoff et al. (2004) reported incidence, due to all causes, for
acute gastrointestinal illness of 0.72 cases/person/yr, the investigators esti-
mate that the incidence of acute gastrointestinal illness attributable to
drinking water is 0.06 cases/person/yr (95% credible interval of 0.02–0.12).
When applying the 0.72 cases of acute gastrointestinal illness/person/yr
Table 9. Estimates of Gastrointestinal Illness Attributable to Drinking Water.
Health outcome and drinking
Mean illness incidence
(cases/person-yr) 95% credible bounds
Total highly credible
Due to water source/
Due to distribution system
Total acute gastrointestinal
Due to water source/
0.03 0.006, 0.05
aEstimated using data from Payment et al. (1991, 1997) studies.
Source: Modifi ed from Messner et al. (2006).
Waterborne Illness 143
from all causes reported in Imhoff et al. (2004) to the 272.5 million people
served by CWS (based on data in USEPA 2006b), this results in an estimate
of approximately 196 million cases/yr of total acute gastrointestinal illness.
Assuming, based on estimates provided above, that 8.5% of the cases are
attributable to drinking water, this translates into approximately 16 million
cases of acute gastrointestinal illness/yr, which is a little higher than the
upper end estimate computed by Colford et al. (2006) (11.69 million
C. Estimates of Waterborne Disease from Exposure
The following section represents our estimates of waterborne infection and
illness risks in the U.S. categorized by source water type (Fig. 10). These
estimates are based on the total number of water systems in the U.S. and
total populations exposed. Illness risk estimates represent all possible ill-
nesses associated with the microbial infection, not only gastroenteritis.
Groundwater risk estimates are based on predicted number of viral
infections and illnesses associated with viruses in groundwater. For both the
community system and noncommunity system groundwater risks, it was
assumed that 10% of wells are positive for infectious viruses, assuming one
infectious virus for every positive well. Dose–response data for rotavirus
(Gerba et al. 1996b) were used, an exposure volume of 1.4 L/person/d was
assumed (Covello and Merkhofer 1993) and the yearly risk was calculated
based on a 350-d exposure (Aboytes et al. 2004).
When considering the number of people served by groundwater supplies,
the number of viral infections estimated is 10.7 million/yr and 2.2 million/yr
for community and noncommunity systems, respectively. Assuming that half
of all infections lead to illness (Haas et al. 1993), this results in 5.4 million
cases/yr associated with community groundwater systems and 1.1 million
cases/yr associated with noncommunity groundwater systems. These infec-
tion and illness estimates may offer higher estimates of risk due to the high
infectivity associated with rotavirus exposure. In addition, this exercise
represents a methodology for estimating risks associated with exposure to
any waterborne virus and infections that may lead to a wide range of clinical
outcomes, not only gastroenteritis.
The estimated number of infections and illnesses associated with exposure
to pathogens in municipal surface waters were also determined. This exercise
utilized the same assumptions described for the groundwater risks as well as
dose–response data for Cryptosporidium (exponential model; Messner et al.
2001) and Campylobacter (beta-Poisson model; Medema et al. 1996).
Assuming a 1% frequency of contamination among the more than 11,000
CWS in the U.S. using surface water, and combining risk estimates associated
with Cryptosporidium, Campylobacter, and rotavirus, 26 million infections/yr
are estimated. When assuming that 50% of these infections will result in some
type of illness, 13 million illnesses/yr are predicted.
144K.A. Reynolds et al.
Risk calculation 2: Estimated number of viral infections and illnesses
associated with noncommunity groundwater supplies
Of 111,036 groundwater systems serving 18 million people
(USEPA 2006b), 10% contain infectious virus
One infectious virus per liter (100-L samples analyzed)
1.4 L water consumed per day (Covello and Merkhofer 1993)
50% infections result in illness (Haas et al. 1993)
• Annual risk of infection = 0.12
• 2.2 million infections per year
• 1.1 million illnesses per year
Risk calculation 1: Estimated number of viral infections and illnesses
associated with community groundwater systems
Of 42,661 groundwater systems serving 86 million people
(USEPA 2006b), 10% contain infectious virus
One infectious virus per liter (100-L samples analyzed)
1.4 L water consumed per day (Covello and Merkhofer 1993)
50% infections result in illness (Haas et al. 1993)
• Annual risk of infection = 0.12
• 10.7 million infections per year
• 5.4 million illnesses per year
Risk calculation 3: Estimated number of infections and illnesses associated
with municipal surface water supplies
• 178,000,000 persons supplied by surface water supplies in the U.S.
• Combined risk of Cryptosporidium, Campylobacter, and rotavirus infections
• Based on 1% frequency of contamination events
• Assuming 50% infections result in illness (Haas et al. 1993)
• 26.0 million infections per year
• 13.0 million illnesses per year
Fig. 10. Estimates of waterborne infection and illness risks in the U.S. categorized
by source water type.
Waterborne Illness 145
Total estimated number of waterborne illnesses
per year in the U.S.
Groundwater (municipal) = 5,400,000
Groundwater (noncommunity) = 1,100,000
Surface water supplies = 13,000,000
Total estimate = 19,500,000
Risk of viral infection associated with exposure to contaminated groundwater
using rotavirus dose–response data
• beta-Poisson model for rotavirus (Gerba et al. 1996b):
Probability of infection (Pi) = 1 – (1 + N/β)-α
where α = 0.26 and β = 0.42
• Probability of infection for 1 virus (n = 1): 0.27
• Concentration of viruses in groundwater:
- Assuming 4,266 positives/total volume analyzed (100 L per sample)
- Assuming one virus per positive sample and 10% are positive = 0.001
• Daily risk:
[Concentration (0.001 viruses/L)][Pi for 1 virus (0.27)][1.4 L/d ingestion]
= 0.000378 infections
• Annual risk:
1 – (1 – daily risk)350 = 0.12 infections per year
Fig. 10. (cont.)
When combining illness estimations from all water sources addressed in
this exercise, the total is 19.5 million cases/yr in the U.S. associated with
drinking water. This estimation is higher than both the Colford et al. (2006)
illness estimate, upper estimate of almost 12 million cases/yr, and the
Messner et al. (2006) illness estimate, 16 million cases/yr, yet our estimate
146K.A. Reynolds et al.
potentially refl ects all health outcomes associated with exposure to patho-
gens in drinking water rather than just gastrointestinal illness.
IX. Water Treatment at the Point-of-Use
Water treatment technologies at the point-of-use can provide an additional
barrier of protection from waterborne contaminants, particularly those
entering the distribution system and present in premise plumbing. Point-
of-use (POU) water treatment devices may be installed at the end of the
faucet, plumbed in-line, or stand-alone pitchers, or they may be point-of-
entry (POE) systems installed where water from the distribution system
enters the premise plumbing. Many POU/POE systems are designed for
aesthetic (i.e., taste, odor, hardness) improvements only, while others employ
technologies to remove organic and inorganic chemicals, pathogens, bacte-
ria, and radionuclides. According to a survey by the Water Quality Associa-
tion (2001), 41% of homes in the U.S. report having a water treatment
device in place at the point-of-use or the point-of-entry and 39% drink
bottled water (Fig. 11). Most report the use of a tabletop pitcher, which are
currently not designed or marketed for eliminating microbial pathogens
from drinking water. Generally, systems designed to eliminate a wide variety
of physical, chemical, and biological contaminants are costly and require
routine professional maintenance. Membranes used for fi ltration in POU/
POE devices must be changed at regular intervals, and systems with ultra-
violet light disinfection must be routinely inspected for buildup on the
lamps that could prevent effective light emission. Improper maintenance of
Fig. 11. Use of point-of-use (POU) water treatment devices in the U.S., 1997–2001
Waterborne Illness 147
POU/POE treatment systems could result in exposure to a greater concen-
tration of pathogens. In addition, heterotrophic plate count (HPC) bacteria
often increase by several orders of magnitude in POU/POE water treat-
A. Signifi cance of Regrowth in POU/POE Water Treatment Devices
Several factors are related to bacterial regrowth in water, including, fi ltra-
tion, temperature, disinfectant type and residual, assimilable organic carbon
level, corrosion control, and distribution pipe material. HPC bacteria are
able to persist and grow in and on point-of-use/entry treatment device
media, membranes, fi lters, and other surfaces to concentrations 10 fold or
more higher in effl uent waters. Granulated activated carbon (GAC) is a
common medium used in POU/POE treatment devices known to support
growth of HPC bacteria. Among other contaminants, GAC removes chlorine
disinfectant residuals from tap water. Although this is desirable to improve
the taste and odor of drinking water, the lack of disinfectant residual and the
collection of bacterial growth substrates provides a suitable environment for
HPC bacteria to attach to the media and grow, especially following periods
of non-use and stagnation. HPC bacteria may also grow in water storage
vessels, distribution pipes, pressure tanks, and hot water heaters.
Although many studies have documented the presence of large numbers
of HPC bacteria in POU-treated water, there has been no correlation to
increased disease (Allen et al. 2004; Calderon 1991; Colford et al. 2002;
Edberg and Allen 2004; WHO/NSF 2003; Payment et al. 1991, 1997). Rela-
tive to HPC levels in common foods, water plays a minor role as a source
of ingested bacteria (Stine et al. 2005). Certain HPC bacteria, such as Pseu-
domonas,Klebsiella, and Aeromonas, are opportunistic pathogens, meaning
they are capable of causing disease in an immunocompromised host.
Although these organisms can be isolated from treated water systems
(Chiadez and Gerba 2004), ingestion is not their route of disease transmis-
sion. There is insuffi cient evidence linking these opportunistic pathogens to
disease transmission via drinking water (Allen et al. 2004).
Several studies have shown that HPC bacteria in POU/POE treatement
devices can outcompete human pathogens and may offer a protective effect
to consumers (Camper et al. 1985; Gerba 2003; Rollinger and Dott 1987).
Gerba (2003) found commercially available POU carbon fi lter devices
placed on home faucets and used for 3–6 wk established a background
culture of HPC bacteria within the systems. Salmonella typhimurium,
E. coli, poliovirus, and hepatitis A virus, all known human enteric pathogens,
were added to sterile tap water, regular tap water, and POU-treated tap
water that was high in HPC organisms. HPC bacteria in the POU-treated
water were clearly antagonistic to the pathogenic bacteria used in this study,
reducing their counts >10 fold in 1 d and >10,000 fold in 2 d. A similar, but
less dramatic, trend was seen with the pathogenic viruses.
148K.A. Reynolds et al.
Other studies supporting the antagonistic effect of HPC bacteria on
pathogens have been conducted (Camper et al. 1985). Three enteric bacte-
rial pathogens, Yersinia enterocolitica,Salmonella typhimurium, and entero-
toxigenic E. coli, readily grew on sterile GAC; however, in the presence of
water containing populations of HPC organisms, the pathogen counts grad-
ually decreased. The most dramatic results were seen when bacterial popu-
lations from river water were previously established on GAC and a mixture
of HPC and pathogenic bacteria were added to the media. Pathogens not
only decreased at a more rapid rate but were prevented from initial attach-
ment compared to sterile GAC fi lters. These studies suggest an antagonistic
effect on pathogenic bacteria caused by the presence of HPC bacteria on
the fi lters, possibly because pathogenic bacteria do not compete well in the
presence of high HPC bacteria.
B. Health Benefi ts of POU/POE Water Treatment Devices
Few studies have directly targeted the benefi ts of POU water treatment
systems for reduction of waterborne disease. Most of the available data are
from epidemiological studies with a few incidental pieces of information
from outbreak events. For example, survey data showed that no one who
died in the waterborne Cryptosporidium outbreak in Milwaukee was using
any type of fi ne fi ltration device for water treatment in the home (WQA
2002). In the same outbreak, persons who did have a point-of-use fi ltration
device in place reported signifi cantly lower incidences of diarrhea com-
pared to those without (Addiss et al. 1996).
Epidemiological studies, in Canada, by Payment et al. (1991, 1997) suggest
that 35% of all gastrointestinal illnesses could be waterborne when source
water quality was degraded (see previous discussion). The 1997 study also
found that children gain the most by having a POU water treatment system
in place. In 2- to 5-yr-old children, drinking tap water resulted in an excess
of 40% of gastrointestinal illness compared to those drinking tap water fi l-
tered at the point-of-use, and an excess of 17% was seen with children
drinking bottled tap water versus POU-treated water.
Two epidemiological studies using randomized, blinded, controlled trials
to evaluate risks related to tap water consumption determined that the risks
were equal among groups supplied with POU-treated (1-µm fi ltration and
UV disinfection chamber) water compared to untreated tap water (Colford
et al. 2005; Hellard et al. 2001). Some of the uncertainties of these studies
are that only a single water system was evaluated and, in the Hellard study,
a pristine water source. The Colford study evaluated the Iowa-American
Water Company, reported to be one of the best in the country, utilizing
conventional fi ltration and a combination of chlorine and chloramine dis-
infectants. The study included intensive monitoring of the distribution
system water quality and pressures and indicated high-quality delivery of
the fi nished product.
Waterborne Illness 149
More studies are needed to assess the impact of POU fi ltration systems
for waterborne disease reduction. Future studies should evaluate multiple
water systems over a wide geographical area to determine the effi cacy of
POU water fi ltration systems over varying fi nished water qualities. Direct
monitoring of POU fi lters, placed in residential or commercial applications
in regions where source water quality or distribution system integrity is
questionable, would provide much-needed pathogen occurrence and expo-
Although current protocols in municipal treatment requirements are effec-
tive at eliminating pathogens from water if properly applied, inadequate,
interrupted, or intermittent treatment has repeatedly been associated with
waterborne disease outbreaks. Factors to consider with regard to pathogen
exposure is that contamination is not evenly distributed but rather affected
by the number of pathogens in the source water, the age of the distribution
system, the quality of the delivered water, and climatic events that can tax
the treatment plant operations.
Weather events are diffi cult to predict but are known to infl uence expo-
sures to microbial pathogens by their increased transport and dissemination
via rainfall and runoff and the survival and/or growth through temperature
changes (CGER 2001). Effects of increased rainfall on watershed pro tection,
infrastructure, and storm drainage systems affected by increased rainfall may
lead to increased risk of contamination events. Extreme precipitation events
preceded 51% of outbreaks from 1948 to 1994 (Curriero et al. 2001).
Current regulatory standards and monitoring requirements do not guar-
antee the absence of human pathogens in tap water. For example, the Total
Coliform Rule, mandating the use of bacterial indicators of water quality,
does not predict vulnerability to an outbreak (Craun et al. 2002). In fact,
few community and noncommunity water systems that reported an out-
break from the survey period 1991–1998 had violated the coliform standard
in the 12-mon period before the outbreak.
In 2002, the USEPA reported that 94% of the U.S. population served by
community water systems received drinking water that met all applicable
health-based drinking water standards through treatment and source water
protection. An internal audit indicated that the fi gure was believed to be
closer to 81% (USEPA 2004). Furthermore, little is known about exposures
to waterborne pathogens in populations not served by public water systems
where there is a general lack of monitoring.
Finally, of particular concern are sensitive populations in the U.S. that
are susceptible to higher rates of infections and to more serious health
outcomes from waterborne pathogens. These subpopulations include not
only individuals experiencing adverse health status, but also those experi-
encing “normal” life stages, e.g., pregnancy, or those very young or old.
150K.A. Reynolds et al.
Individuals in any of these situations may want to consider additional
strategies to prevent waterborne illness attributable to drinking water,
such as the utilization of a point-of-use water treatment device. Better
communication between water quality professionals and healthcare provid-
ers is needed to develop and distribute materials to inform the public of
mitigation options beyond the current multibarrier approach of municipal
Outbreaks of disease attributable to drinking water are not common in the
U.S., but they do still occur and can lead to serious acute, chronic, or some-
times fatal health consequences, particularly in sensitive and immunocom-
promised populations. From 1971 to 2002, there were 764 documented
waterborne outbreaks associated with drinking water, resulting in 575,457
cases of illness and 79 deaths (Blackburn et al. 2004; Calderon 2004);
however, the true impact of disease is estimated to be much higher. If prop-
erly applied, current protocols in municipal water treatment are effective
at eliminating pathogens from water. However, inadequate, interrupted, or
intermittent treatment has repeatedly been associated with waterborne
disease outbreaks. Contamination is not evenly distributed but rather
affected by the number of pathogens in the source water, the age of the
distribution system, the quality of the delivered water, and climatic events
that can tax treatment plant operations. Private water supplies are not regu-
lated by the USEPA and are generally not treated or monitored, although
very few of the municipal systems involved in documented outbreaks
exceeded the USEPA’s total coliform standard in the preceding 12 mon
(Craun et al. 2002).
We provide here estimates of waterborne infection and illness risks in
the U.S. based on the total number of water systems, source water type, and
total populations exposed. Furthermore, we evaluated all possible illnesses
associated with the microbial infection and not just gastroenteritis. Our
results indicate that 10.7 M infections/yr and 5.4 M illnesses/yr occur in
populations served by community groundwater systems; 2.2 M infections/yr
and 1.1 M illnesses/yr occur in noncommunity groundwater systems; and
26.0 M infections/yr and 13.0 M illnesses/yr occur in municipal surface water
systems. The total estimated number of waterborne illnesses/yr in the U.S.
is therefore estimated to be 19.5 M/yr. Others have recently estimated
waterborne illness rates of 12 M cases/yr (Colford et al. 2006) and 16 M
cases/yr (Messner et al. 2006), yet our estimate considers all health out-
comes associated with exposure to pathogens in drinking water rather than
only gastrointestinal illness.
Drinking water outbreaks exemplify known breaches in municipal water
treatment and distribution processes and the failure of regulatory require-
ments to ensure water that is free of human pathogens. Water purifi cation
Waterborne Illness 151
technologies applied at the point-of-use (POU) can be effective for limiting
the effects of source water contamination, treatment plant inadequacies,
minor intrusions in the distribution system, or deliberate posttreatment acts
(i.e., bioterrorism). Epidemiological studies are confl icting on the benefi ts
of POU water treatment. One prospective intervention study found that
consumers of reverse-osmosis (POU) fi ltered water had 20%–35% less
gastrointestinal illnesses than those consuming regular tap water, with an
excess of 14% of illness due to contaminants introduced in the distribution
system (Payment 1991, 1997). Two other studies using randomized, blinded,
controlled trials determined that the risks were equal among groups sup-
plied with POU-treated water compared to untreated tap water (Hellard
et al. 2001; Colford et al. 2003). For immunocompromised populations, POU
water treatment devices are recommended by the CDC and USEPA as one
treatment option for reducing risks of Cryptosporidium and other types of
infectious agents transmitted by drinking water. Other populations, includ-
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Manuscript received January 9; accepted January 12, 2007.