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398
Am J Epidemiol
2004;159:398–405
American Journal of Epidemiology
Copyright © 2004 by the Johns Hopkins Bloomberg School of Public Health
All rights reserved
Vol. 159, No. 4
Printed in U.S.A.
DOI: 10.1093/aje/kwh050
Did a Severe Flood in the Midwest Cause an Increase in the Incidence of
Gastrointestinal Symptoms?
Timothy J. Wade
1
, Sukhminder K. Sandhu
2
, Deborah Levy
3
, Sherline Lee
3
, Mark W.
LeChevallier
4
, Louis Katz
5
, and John M. Colford, Jr.
2
1
Human Studies Division, National Health and Environmental Effects Research Laboratory, Epidemiology and Biomarkers
Branch, Environmental Protection Agency, Chapel Hill, NC.
2
School of Public Health, University of California, Berkeley, Berkeley, CA.
3
Division of Parasitic Diseases, Centers for Disease Control and Prevention, Atlanta, GA.
4
American Water, Voorhees, NJ.
5
Scott County Health Department, Davenport, IA.
Received for publication May 20, 2003; accepted for publication August 19, 2003.
Severe flooding occurred in the midwestern United States in 2001. Since November 2000, coincidentally, data
on gastrointestinal symptoms had been collected for a drinking water intervention study in a community along the
Mississippi River that was affected by the flood. After the flood had subsided, the authors asked these subjects
(
n
= 1,110) about their contact with floodwater. The objectives of this investigation were to determine whether
rates of gastrointestinal illness were elevated during the flood and whether contact with floodwater was
associated with increased risk of gastrointestinal illness. An increase in the incidence of gastrointestinal
symptoms during the flood was observed (incidence rate ratio = 1.29, 95% confidence interval: 1.06, 1.58), and
this effect was pronounced among persons with potential sensitivity to infectious gastrointestinal illness. Tap
water consumption was not related to gastrointestinal symptoms before, during, or after the flood. An association
between gastrointestinal symptoms and contact with floodwater was also observed, and this effect was
pronounced in children. This appears to be the first report of an increase in endemic gastrointestinal symptoms
in a longitudinal cohort prospectively observed during a flood. These findings suggest that severe climatic events
can result in an increase in the endemic incidence of gastrointestinal symptoms in the United States.
communicable diseases; diarrhea; gastrointestinal tract; natural disasters; water
Abbreviations: CI, confidence interval; IRR, incidence rate ratio; RDD, random digit dialing; WET, Water Evaluation Trial.
While there is clear potential for increased transmission of
infectious gastrointestinal illness following flooding (1–3),
there is little direct epidemiologic evidence of this associa-
tion. Attempts to study increases in the rates of illness in
communities affected by a flood have been limited by the
lack of accurate or comparable illness incidence data from
the period before the flood, making quantification of the
impact of the flood difficult or impossible.
In the United States, reports of outbreaks of or increases in
illness following a flood are rare. Epidemiologic investiga-
tions following massive flooding in the Midwest in 1993
found no evidence of outbreaks or increased levels of
gastrointestinal illness, and no mortality associated with
gastrointestinal symptoms was identified (4, 5). However, a
recent analysis of the occurrence of waterborne disease
outbreaks in the United States found that such outbreaks
were likely to be preceded by periods of extreme precipita-
tion (6). In these outbreaks, the route of exposure was inges-
tion of contaminated drinking water, and the rainfall or
flooding probably washed contamination into water sources.
Severe flooding occurred in the midwestern United States
during April and May of 2001. At the time of the flooding, a
randomized trial of in-home drinking water treatment (the
Water Evaluation Trial or “WET” Study) was being
conducted in a community along the Mississippi River. As
part of this study, household members completed daily
Correspondence to Dr. John M. Colford, Jr., School of Public Health, University of California, Berkeley, 140 Warren Hall, MC 7360, Berkeley,
CA 94720 (e-mail: jcolford@socrates.berkeley.edu).
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Flooding and Gastrointestinal Symptoms 399
Am J Epidemiol
2004;159:398–405
diaries detailing their incidence of gastrointestinal symp-
toms. This allowed us to evaluate the impact of flooding on
the incidence of gastrointestinal symptoms.
The principal objectives of this investigation were to
determine whether rates of gastrointestinal symptoms were
elevated during the flood and whether contact with flood-
water was associated with increased risk of gastrointestinal
symptoms.
MATERIALS AND METHODS
The WET Study
Rates of gastrointestinal symptoms were determined from
subjects enrolled in the WET cohort. Enrollment in the WET
Study began in fall 2000, and enrollment was completed in
May 2001. Follow-up of the WET cohort was completed on
June 29, 2002. A total of 456 households (1,296 persons)
were enrolled, and the follow-up period was 1 year. In brief,
each household was randomly assigned to receive either an
active water treatment device or an outwardly identical inac-
tive (placebo) device installed at the kitchen tap. After 6
months, the devices were removed, and after a 1-week
washout period, the devices were replaced with a device of
the alternate type. Subjects maintained a daily health diary
and recorded any occurrence of gastrointestinal symptoms
(diarrhea, vomiting, nausea, or abdominal cramps). The
WET cohort was restricted to persons without serious
immune-compromising conditions (i.e., human immunodefi-
ciency virus/acquired immunodeficiency syndrome) and
residents whose source of home tap water was municipal
water.
Immediately following the flood, subjects were sent a
survey asking them about the extent to which they had come
into contact with floodwater or flood-contaminated items.
Questions about contact included the extent to which
subjects had walked through, played in, touched, or been
directly exposed to floodwater, participated in flood control
activities (i.e., sandbagging), and cleaned up flood-contami-
nated items.
Water quality data were provided to investigators by the
local water utility. Additional water quality data were also
collected and provided to the investigators by American
Water as part of a related study. Source water was tested
weekly for Cryptosporidium parvum by both cell culture/
polymerase chain reaction (7) and filtration/microscopy
using Method 1623 (8). The raw-water occurrence of
Giardia subspecies was evaluated weekly by filtration/
microscopy using Method 1623 (8). The recovery efficiency
for the cell culture/polymerase chain reaction test averaged
52.2 percent (standard deviation, 39.2) (n = 7). The recovery
efficiencies for Giardia and Cryptosporidium using Method
1623 were 38.0 percent (standard deviation, 26.8) and 52.7
percent (standard deviation, 35.0), respectively (n = 9).
Male-specific coliphages were enumerated weekly using the
single agar overlay method, and culturable enteric viruses
were enumerated in raw and finished water monthly with the
buffalo green monkey cell line, using methods proposed by
the Environmental Protection Agency for water and waste-
water (9). Numbers of total and fecal coliforms in raw and
finished (treated) water were analyzed using standard
membrane filter methods (10). Full details on the water
quality monitoring program have been presented by
LeChevallier et al. (11).
Telephone survey
Concurrently with the WET Study, a random digit dialing
(RDD) telephone survey was conducted in the study area.
Survey data from 3,506 respondents were available for anal-
ysis. The goal of the survey was to obtain population-based
estimates of the use of various home water treatments, water
consumption, and the monthly occurrence of gastrointestinal
illnesses. Following the flood, questions were added for
assessment of the frequency with which respondents came
into contact with floodwater and flood-contaminated items.
The sampling frame was defined by the zip codes for the
communities participating in the WET Study. Once a house-
hold had been contacted, one subject from the household was
selected at random using a household roster. Parents or
guardians were asked to complete surveys for children aged
≤12 years. Results from the RDD survey were used to deter-
mine the population attributable risk percentage (12) and to
estimate the total number of cases of gastrointestinal symp-
toms in the community attributable to contact with flood-
water. Because the RDD survey did not collect detailed data
on daily incidence of symptoms, the results were used to
calculate population attributable risks but were not used to
determine the impact of flooding on symptom rates.
Outcome definition
Symptom rates were determined for the WET cohort by
season and for the flood period. The primary symptom
outcome, “highly credible gastrointestinal symptoms,” was
defined as the occurrence of any of the following during a
single 24-hour period: vomiting, liquid diarrhea, diarrhea
(not liquid) with cramps, or nausea with cramps. This defini-
tion is consistent with previously published work (13–15). A
secondary outcome was a more specific definition of diar-
rhea: three or more loose stools during a single 24-hour
period. For an instance of diarrhea to be considered a unique
episode, six consecutive symptom-free days were required.
Other secondary outcomes evaluated included number of
days hospitalized for gastrointestinal symptoms; visits to a
health care provider for diarrhea; severe diarrhea (six or
more loose stools in 24 hours); and number of days of work
or school missed because of gastrointestinal symptoms.
Data analysis
The flood period was defined as the period between April
14, 2001, and May 30, 2001. This period included the time
during which the Mississippi River was above the 15-foot
(4.6-m) flood stage and 1 additional week to account for the
incubation periods of some gastrointestinal pathogens.
Counts of episodes, symptoms, and person-time were
collapsed for each subject by season and flood period, so that
each subject with complete data had five observations (one
for each of the four seasons and one for the flood period).
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400 Wade et al.
Am J Epidemiol
2004;159:398–405
Note that seasonal data included combined observations for
two seasons for the follow-up period between fall 2000 and
spring 2002, with the exception of summer, for which data
were only available for 2001.
The outcome of interest was number of gastrointestinal
symptom episodes. A variable indicating the flood period
was the primary independent variable of interest. Poisson
extensions of generalized estimating equations (16) were
used to model the outcome and to account for the multiple-
correlation structure. Models were fitted with an exchange-
able (i.e., equal) correlation structure with robust specifica-
tion for the standard error. The household was the unit of
nonindependence (i.e., models were clustered on house-
hold). Since gastrointestinal illness has a distinct seasonal
component (17), indicator variables for season (spring,
summer, fall, winter) were included in all models. Other
covariates included in the model were indicators for cycle
(first or second 6-month cycle) and type of water treatment
device (active or placebo). Models were offset by the person-
time contribution of each individual. Incidence rate ratios
(IRRs), defined as the rate in the exposed divided by the rate
in the unexposed, were estimated directly from the models
by exponentiating the coefficients.
All analyses were conducted using Stata 7.0 (18). Graphs
were created in S-Plus, version 4.5 (19).
All protocols, consent forms, and study materials for the
randomized trial and the RDD survey were reviewed and
approved by the institutional review boards of the University
of California, Berkeley, the state of California, and the
Centers for Disease Control and Prevention. Subjects
enrolled in the randomized trial signed a written consent
form, and RDD respondents consented verbally.
RESULTS
Flood description
Late, rapid snowmelt combined with heavy rain caused
severe flooding throughout the Midwest in spring 2001.
Flooding occurred in Minnesota, Wisconsin, North Dakota,
Iowa, and Illinois, primarily along the Mississippi River and
its tributaries.
Flooding began in the study area when the Mississippi
River reached the 15-foot flood stage of the study commu-
nity on April 15. The river remained above flood stage
through May 23. The flood crested in the study area at 22.33
feet (6.85 m) on April 24, the third-highest level in recorded
history (20). The river receded below flood stage on May 23.
During part of this time, sewage treatment processes were
bypassed, and untreated sewage and wastewater were
discharged directly into the river at the study site as well as
upstream and downstream (21).
During the flood, there was marked deterioration in source
water quality, as illustrated by coliform levels in the source
water (figure 1). Source water concentrations of Giardia
cysts increased 330 percent (mean = 0.4 cysts/liter during the
flood), and concentrations of male-specific coliphages
increased 270 percent (mean = 11.6 plaque-forming units/
100 ml during the flood) during this period. However,
Cryptosporidium subspecies oocysts and culturable viruses
were not detected in the source water. Despite the source
water conditions during the flood, treated water continued to
meet all regulatory standards for quality. Additional
measures taken by the local water utility to ensure that
treated water was safe included the addition of extra chlorine
(hyperchlorination) and additional monitoring for water
quality indicators and pathogens. The individual filter
turbidities and combined filter effluent turbidities were
consistently less than 0.1 nephelometric turbidity units
during this period. The plant effluent was consistently free of
coliforms, C. parvum, and viruses. No “boil water” alerts
were issued for persons using municipal water (22).
Rates of gastrointestinal symptoms
Health data were available for 1,257 (from 445 house-
holds) of the 1,296 subjects enrolled in the WET cohort (the
remaining subjects enrolled in the WET cohort did not return
health diaries because they dropped out or were noncom-
pliant). These subjects contributed a total of 1,116 person-
years of observation. Of these, 1,204 subjects (contributing
130 person-years of observation) were enrolled in the WET
Study at the time of the flood. The remaining subjects who
were not enrolled during the flood were retained in the anal-
ysis, since they represented unexposed persons.
Rates of highly credible gastrointestinal symptoms and
diarrhea episodes among the WET participants were higher
in winter than in any of the other seasons (table 1). Crude
rates of both highly credible gastrointestinal symptoms and
diarrhea were higher during the flood than in any other
season, including winter. After adjustment for season, cycle,
and device type, the number of episodes of highly credible
gastrointestinal symptoms remained significantly elevated:
Rates of highly credible gastrointestinal symptom episodes
were 1.29 times higher during the flood than during the rest
of the WET cohort follow-up period (95 percent confidence
interval (CI): 1.06, 1.58). Numbers of diarrhea episodes were
also elevated, although the 95 percent confidence interval
included the no-effect value of 1 (IRR = 1.23, 95 percent CI:
0.94, 1.62).
Rates of the following conditions were not elevated during
the flood period: diarrhea that resulted in a doctor’s visit;
days of missed work or school due to gastrointestinal symp-
toms; and days of vomiting. Six subjects were hospitalized
for a total of 29 days during the flood for gastrointestinal
conditions, but specific reasons for the hospitalizations were
not available. Hospitalizations for gastrointestinal conditions
were elevated during the flood (IRR = 8.10, 95 percent CI:
0.77, 85.01), but the 95 percent confidence interval included
1, probably because of the low statistical power resulting
from the small number of hospitalizations.
Impact on sensitive individuals
To examine whether the impact of the flood was greater in
certain potentially sensitive groups, we stratified the data in
the models by age (≤12 years and ≥50 years), frequency of
gastrointestinal symptoms during the past year, and the pres-
ence of a chronic gastrointestinal condition. A chronic
gastrointestinal condition included any of the following: irri-
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2004;159:398–405
table bowel syndrome, lactose intolerance, diverticulitis,
ulcerative colitis, and Crohn’s disease. Table 2 shows IRRs
for gastrointestinal symptoms for the flood period compared
with the rest of the study period, stratified by these factors.
As the table illustrates, the IRR for the flood was greater
among persons who were potentially sensitive to gastrointes-
tinal illness than among those who were not. For cases of
severe diarrhea (six or more loose stools in 24 hours), the
effect was even more pronounced among persons with
chronic gastrointestinal conditions (IRR = 7.05, 95 percent
FIGURE 1. Total coliform counts in raw water from the Mississippi River, November 2000–November 2001. cfu, colony-forming units.
TABLE 1. Crude rates of gastrointestinal illness in the Water Evaluation Trial cohort, by season and during
the 2001 flood, Mississippi River, 2001
* “Highly credible gastrointestinal symptoms” included any of the following: liquid diarrhea, soft diarrhea and
cramps, nausea and cramps, or vomiting.
† CI, confidence interval; IRR, incidence rate ratio.
‡ Three or more occurrences in a 24-hour period.
§ April 14, 2001–May 30, 2001.
¶ Results were controlled for cycle, water treatment device, season, and household clustering effects.
No. of illness episodes per year
Highly credible
gastrointestinal
symptoms*
95% CI† Diarrhea‡ 95% CI
Flood period§ 2.82 2.54, 3.12 0.87 0.72, 1.05
Season
Spring (excluding flood period) 1.95 1.74, 2.18 0.63 0.51, 0.77
Summer 2.07 1.91, 2.24 0.60 0.52, 0.70
Fall 1.64 1.40, 1.91 0.61 0.47, 0.70
Winter 2.19 2.06, 2.33 0.68 0.60, 0.76
IRR† (flood period vs. other times)¶ 1.29 1.06, 1.58 1.23 0.94, 1.62
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402 Wade et al.
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CI: 0.80, 61.9). Severe diarrhea was also more frequent in
persons with good, fair, or poor health than in those with
excellent or very good health. IRRs were not elevated to a
greater extent in children; however, persons aged ≥50 years
had elevated rates of highly credible gastrointestinal symp-
toms (IRR = 1.47, 95 percent CI: 1.01, 2.13), and this was
more pronounced for severe diarrhea (IRR = 2.92, 95 percent
CI: 1.04, 8.25). The rate of highly credible gastrointestinal
symptoms was elevated still further among persons aged ≥60
years (IRR = 1.89, 95 percent CI: 1.06, 3.38), but there were
too few subjects to evaluate severe diarrhea in this age
group.
Association between gastrointestinal symptoms and
contact with floodwater
A total of 1,118 subjects in the WET cohort completed the
flood survey, of whom 1,110 provided health data. Of the
1,118 participants, 143 (13 percent) reported some type of
direct (e.g., touching or walking through floodwater) or indi-
rect (e.g., cleaning up items contaminated by floodwater)
contact with floodwater. Table 3 shows rates of highly cred-
ible gastrointestinal symptoms and IRRs related to the
various types of flood exposure. Among all subjects, having
the house or yard flooded was a strong risk factor for highly
credible gastrointestinal symptoms. None of the other types
of flood exposure were associated with symptoms among all
subjects. Although few episodes of diarrhea occurred during
the flood, the IRR estimate for those whose homes or yards
were flooded was similar to that for highly credible
gastrointestinal symptoms (IRR = 2.41, 95 percent CI: 0.84,
6.94).
Among children aged ≤12 years, all types of flood contact
were associated with elevated IRRs, with the exception of
having a septic tank on the home property (table 3). Living in
a home where the house or yard was flooded was signifi-
cantly associated with highly credible gastrointestinal symp-
toms among children aged ≤12 years (IRR = 2.42, 95 percent
CI: 1.22, 4.82), and the IRR for any flood contact was of
borderline significance (IRR = 1.90, 95 percent CI: 0.93,
3.85). The association between highly credible gastrointes-
tinal symptoms and any flood contact was even more
pronounced among children aged ≤5 years (eight exposed
children; IRR = 3.18, 95 percent CI: 1.79, 5.66). No episodes
of diarrhea were experienced by children aged ≤12 years
during the flood among those exposed to floodwater.
There were few data for persons aged ≥50 years, but
symptom rates were elevated among those who had had any
flood contact (23 persons exposed; IRR = 1.46, 95 percent
CI: 0.65, 3.27) and among those whose homes or yards had
been flooded (five persons exposed; IRR = 6.20, 95 percent
CI: 3.34, 11.51).
Among persons with potential sensitivity to gastrointes-
tinal illness, those in excellent or very good health who had
had any contact with floodwater had slightly lower relative
rates of gastrointestinal symptoms (IRR = 1.08, 95 percent
CI: 0.82, 1.43) compared with those in good, fair, or poor
health (IRR = 1.39, 95 percent CI: 0.94, 2.95). IRRs for any
contact with floodwater were similar among persons with
and without chronic gastrointestinal conditions and among
TABLE 2. Incidence rate ratios for interactions between chronic gastrointestinal conditions, health status,
and flooding in the Water Evaluation Trial cohort, Mississippi River, 2001
* “Highly credible gastrointestinal symptoms” included any of the following: liquid diarrhea, soft diarrhea and
cramps, nausea and cramps, or vomiting.
† Three or more loose stools in 24 hours.
‡ Six or more loose stools in 24 hours.
§ IRR, incidence rate ratio; CI, confidence interval.
¶ There were five missing values for this variable.
# Model estimates did not converge; one or more of the parameter estimates were infinite.
Highly credible
gastrointestinal
symptoms*
Diarrhea† Severe diarrhea‡
IRR§ 95% CI§ IRR 95% CI IRR 95% CI
Chronic gastrointestinal condition¶
Yes (
n
= 140) 1.47 1.00, 1.26 1.69 0.82, 3.49 7.05 0.80, 61.9
No (
n
= 1,112) 1.26 1.00, 1.57 1.15 0.84, 1.57 1.41 0.74, 2.69
Self-reported “frequent” gastrointestinal
symptoms during the past year¶
Yes (
n
= 70) 2.03 1.15, 3.57 2.78 1.05, 7.36 —#
No (
n
= 1,182) 1.22 0.99, 1.50 1.10 0.83, 1.48 1.41 0.73, 2.72
Self-reported health status¶
Excellent or very good (
n
= 934) 1.30 1.02, 1.66 1.18 0.82, 1.70 1.37 0.62, 3.03
Good, fair, or poor (
n
= 318) 1.25 0.91, 1.73 1.30 0.86, 1.97 2.57 1.03, 6.41
Age (years)
≤12 (
n
= 307) 1.20 0.78, 1.85 0.91 0.41, 1.99 —#
≥50 (
n
= 266) 1.47 1.01, 2.13 1.76 2.92 1.04, 8.25
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2004;159:398–405
persons who reported gastrointestinal symptoms at baseline.
Relevant covariates that potentially could have confounded
the association between floodwater contact and gastrointes-
tinal symptoms for which data were available included age,
sex, self-rating of health, and presence of a chronic
gastrointestinal condition. For highly credible gastrointes-
tinal symptoms, separate models including these covariates
were fitted for each of the types of flood exposure (small
numbers of illnesses prevented complete multivariate anal-
yses for diarrhea as the outcome and for children aged ≤12
years). Inclusion of covariates did not have an impact on the
estimated IRR for contact with floodwater.
Cases of illness attributable to flood contact
Using the population data collected from the RDD tele-
phone survey, we were able to estimate the actual numbers
of illnesses caused by the flooding. These calculations
assumed that the observed increase in symptoms was, in fact,
due to the flood and not a result of underlying differences
between the exposed and unexposed groups. Eleven percent
of the RDD survey respondents aged ≥12 years reported at
least some contact with floodwater or flood-contaminated
items. These children had rates of gastrointestinal symptoms
1.9 times greater than those of children whose homes or
yards were not flooded. From these figures, it can be esti-
mated that 9 percent of episodes of highly credible
gastrointestinal symptoms among children aged ≤12 years
during the flood were attributable to contact with floodwater
(population attributable risk percentage (12)). Using the inci-
dence rate in unexposed children aged ≤12 years (2.47
episodes per person-year) and 2000 US Census figures of a
total population of approximately 159,000 in the study
county, approximately 20 percent of whom are children aged
≤12 years (23), an estimated total of 6,546 highly credible
gastrointestinal symptom episodes occurred among children
aged ≤12 years during the 1-month flood period. Approxi-
mately 589 of these episodes (0.09 × 6,456) can be attributed
to exposure to floodwater and flood-contaminated items.
Two percent of all respondents in the RDD survey
reported having their house or yard flooded. Using the same
calculations as above, an estimated 202 excess episodes of
highly credible gastrointestinal symptoms (including both
children and adults) were specifically attributable to having
a home or yard flooded.
DISCUSSION
Based on the daily health diaries of 1,257 persons in 445
households collected over a 19-month period, an increased
rate of gastrointestinal symptoms was observed during a
severe midwestern flood in the spring of 2001. There was
evidence that the effect of the flood was more severe among
persons with potential sensitivity to gastrointestinal illness
(those with a chronic gastrointestinal illness; those in poor,
fair, or good health; those aged ≥50 years; and those with
frequent gastrointestinal symptoms), particularly for severe
illness.
Because of the longitudinal structure of the data, factors
related to gastrointestinal illness that remain relatively
constant within an individual over time (age, sex) were
unlikely to have biased the observed result. When factors
associated with gastrointestinal illness, such as age, sex, and
the presence of a chronic gastrointestinal condition, were
included in the model, the effect of the flood remained
unchanged. Factors that vary over time, such as travel and
number of weeks enrolled in the study, are more likely to have
biased the result. These factors were also evaluated as poten-
tial confounders, but including them in the model did not
affect the association between the flood period and symptoms.
Consumption of increased amounts of tap water was unre-
lated to gastrointestinal symptoms during the flood. For
persons in the top quartile of water consumption compared
with those in the bottom quartile, the IRR was not signifi-
cantly elevated (IRR = 1.14, 95 percent CI: 0.79, 1.66).
Furthermore, we observed no difference in the relation
between the effectiveness of the “active” water treatment
device and gastrointestinal symptoms during the flood. If
TABLE 3. Relation between contact with floodwater and flood-contaminated items and gastrointestinal illness in the Water
Evaluation Trial cohort, Mississippi River, 2001*
*Categories are not mutually exclusive.
† IRR, incidence rate ratio; CI, confidence interval.
‡ Includes having one’s home or yard flooded, touching an object that was in contact with floodwater, walking through floodwater, sandbagging,
touching floodwater with any body part, or cleaning up objects contaminated by floodwater.
Exposure
All subjects Children aged ≤12 years
No. exposed
(
n
= 1,110) % IRR† 95% CI† No. exposed
(
n
= 266) %IRR 95% CI
Floodwater touching any part of body 56 50.79 0.35, 1.75 11 41.59 0.51, 4.90
Walking through floodwater 37 31.13 0.66, 1.94 5 2 1.94 0.26, 14.4
Sandbagging (during flood) 89 80.83 0.53, 1.31 12 51.91 0.70, 5.18
Participating in flood cleanup 22 20.32 0.08, 1.27 2 1 1.40 1.07, 1.82
House or yard getting flooded 31 32.36 1.37, 4.07 13 52.42 1.22, 4.82
Any flood exposure‡ 153 14 1.14 0.80, 1.63 29 11 1.90 0.93, 3.85
Septic tank on the home property 50 5 1.17 0.64, 2.12 10 4 0.66 0.20, 2.13
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404 Wade et al.
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exposure to tap water were the primary cause of the increase
in symptoms, we would have expected to observe a strong
relation between water consumption and gastrointestinal
symptoms and would have expected to observe an enhanced
effect of the active water treatment device during the flood.
There is some evidence that the increase in gastrointestinal
symptoms during the flood may have been due to direct
exposure to floodwater. During the flood, persons whose
homes or yards were flooded were at increased risk of
gastrointestinal symptoms. This finding is consistent with a
recently published health survey conducted by the Centers
for Disease Control and Prevention in southeastern Texas
following severe flooding in 2001 (24). This door-to-door
survey of 420 households found that residents of flooded
households were significantly more likely to report diarrhea
within 1 week of the worst flooding than residents whose
households were not flooded (odds ratio = 4.7, 95 percent
CI: 1.8, 12.0).
Children aged ≤12 years were at increased risk of
gastrointestinal symptoms if their home had been flooded or
if they had cleaned up flood-contaminated material. In
general, children had higher relative risks of gastrointestinal
symptoms for most types of flood exposure, which is consis-
tent with the greater susceptibility of children to gastrointes-
tinal pathogens (25). This finding may also be related to
poorer hygiene among children following exposure to flood-
water and flood-contaminated items. The association
between highly credible gastrointestinal symptoms and any
flood contact was even more pronounced among children
under 5 years of age (IRR = 3.18, 95 percent CI: 1.79, 5.66).
Because no episodes of diarrhea or severe diarrhea occurred
among children who had contact with floodwater or flood-
contaminated items, it seems apparent that, in children at
least, these symptoms were rather mild.
Direct contact with floodwater did not entirely explain the
increase in incidence observed during the flood period, prob-
ably because few subjects reported coming into contact with
floodwater. In fact, the IRR for gastrointestinal symptoms
during the flood period remained elevated when persons who
had had any contact with floodwater (IRR = 1.45, 95 percent
CI: 1.16, 1.82) and persons whose homes had been flooded
(IRR = 1.36, 95 percent CI: 1.10, 1.67) were excluded from
the analysis.
An outbreak of gastrointestinal illness related to norovirus
(Norwalk-like virus) was reported during the flood.
However, it involved few cases and was restricted to a
nursing home. County health department and water officials
visited the site and collected and tested samples but found no
evidence of the virus or other evidence of substandard water
quality (L. Katz, personal communication, Scott County
Health Department, 2002).
Limitations
The results of this study were based entirely on self-
reported occurrences of gastrointestinal symptoms. Such
data may be susceptible to recall bias. While recall bias
could have resulted in the observed increases in highly cred-
ible gastrointestinal symptoms or diarrhea, it would be
unlikely to be associated with more severe outcomes, such as
hospitalization for gastrointestinal symptoms. Information
on some potential confounders, such as diet and socioeco-
nomic status, was not available for analysis, although it is
difficult to predict how these factors may have influenced
the results. The calculations of numbers of symptoms attrib-
utable to the flood (population attributable risk calculations)
assumed that the entire increased rate observed was, in fact,
caused by the flood. If part or all of this increase was due to
other factors, the numbers of illnesses calculated will have
been overestimates.
The results observed in this study are likely to have been
highly dependent on local factors related to community
transmission of gastrointestinal illness. Therefore, these
findings may be limited in their application and generaliz-
ability to floods experienced in other communities.
In conclusion, this study documented a measurable
increase in gastrointestinal symptoms during a severe flood
in the Midwest in 2001. This effect was greater among
persons who were susceptible to gastrointestinal illness.
During the flood, children who had direct contact with flood-
water were at increased risk of gastrointestinal symptoms, as
were adults whose homes or yards were flooded. There was
no evidence for transmission of gastrointestinal symptoms
through the public water supply during the flood. However,
the rate of gastrointestinal symptoms during the flood
remained elevated after persons who had contact with flood-
water were excluded. This suggests that people either
contracted illness through sources other than contact with
floodwater or flood-contaminated items or unknowingly
came into contact with floodwater or flood-contaminated
items. The increase may also have been attributable, in part,
to other transmission pathways, such as secondary transmis-
sion or consumption of food that had been in some way
affected or contaminated by floodwater.
ACKNOWLEDGMENTS
Support for this work was provided by the Centers for
Disease Control and Prevention (Cooperative Agreement
U50/CCU916961-01) and the American Water Works
Research Foundation (Project 2580).
The authors acknowledge the following people and institu-
tions for their vital support in the WET Study: Joel Mohr,
Brock Earnhardt, Dr. Alan Brookart, Dr. Alan Hubbard,
Susan Shaw, Dr. Rebecca Calderon, Susan Burns, Anne
Benker, the Survey Research Center (Berkeley, California),
Macro International (Burlington, Vermont), Jen Muncil,
James Dayton, Jeannette Apple, Donald Sorenson, and
Jeffrey Robinson.
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