SYSTEMATIC REVIEW Extreme water-related weather events and waterborne disease
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Epidemiol. Infect. (2013), 141, 671–686. f Cambridge University Press 2012
doi:10.1017/S0950268812001653
SYSTEMATIC REVIEW
Extreme water-related weather events and waterborne disease
K. F. C A NN 1*, D. RH. T HO M A S 1, R. L. SA L MO N 1, A. P. W Y N - J O N E S 2 A N D D. KA Y 2
1
Communicable Disease Surveillance Centre, Public Health Wales, Temple of Peace and Health, Cathays Park,
Cardiff, UK
2
IGES, Aberystwyth University, Aberystwyth, UK
Received 12 October 2011; Final revision 26 June 2012; Accepted 11 July 2012;
first published online 9 August 2012
SUMMARY
Global climate change is expected to affect the frequency, intensity and duration of extreme
water-related weather events such as excessive precipitation, floods, and drought. We conducted a
systematic review to examine waterborne outbreaks following such events and explored their
distribution between the different types of extreme water-related weather events. Four medical
and meteorological databases (Medline, Embase, GeoRef, PubMed) and a global electronic
reporting system (ProMED) were searched, from 1910 to 2010. Eighty-seven waterborne
outbreaks involving extreme water-related weather events were identified and included, alongside
235 ProMED reports. Heavy rainfall and flooding were the most common events preceding
outbreaks associated with extreme weather and were reported in 55.2 % and 52.9% of accounts,
respectively. The most common pathogens reported in these outbreaks were Vibrio spp. (21.6 %)
and Leptospira spp. (12.7 %). Outbreaks following extreme water-related weather events were
often the result of contamination of the drinking-water supply (53.7 %). Differences in reporting
of outbreaks were seen between the scientific literature and ProMED. Extreme water-related
weather events represent a risk to public health in both developed and developing countries, but
impact will be disproportionate and likely to compound existing health disparities.
Key words: Climate – impact of, water-borne infections.
INTRODUCTION to acceleration of the water cycle caused by atmos-
pheric heating. Altered pressure and temperature
Global climate change is expected to affect the fre-
patterns, caused by global warming, may also shift the
quency, intensity and duration of extreme water-
distribution of when and where extreme water-related
related weather events such as excessive rainfall,
events usually occur [4]. .The frequency of heavy pre-
storm surges, floods, and drought [1–3]. Recent ex-
cipitation events is thought to have increased over
treme water-related weather events have included
many mid-latitude regions since 1950, even where
drought in Russia and flooding in Sri Lanka, the
there has been a reduction in the total precipitation.
Philippines, Pakistan, Australia and Brazil. Weather
The area affected by drought is thought to have in-
is expected to become more extreme and variable due
creased since the 1970s in many areas of the world [4].
There is also evidence to suggest that other ex-
* Author for correspondence: Miss K. F. Cann, CDSC, Temple of
Peace and Health, Cathays Park, Cardiff CF10 3NW, UK.
treme water-related weather events such as El Niño
(Email: Kimberley.cann@wales.nhs.uk) Southern Oscillation (ENSO), hurricanes, and
The online version of this article is published within an Open Access environment subject to the conditions of the Creative Commons
Attribution-NonCommercial-ShareAlike licence . The written permission of
Cambridge University Press must be obtained for commercial re-use.
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. https://doi.org/10.1017/S0950268812001653672 K. F. Cann and others
cyclones are becoming more frequent, intense and of Search strategy
greater duration [4, 5].
Four major medical and meteorological databases
Excessive or heavy rainfall events can mobilize
(Medline, EMBASE, GeoRef, PubMed) were
pathogens in the environment and increase run-off of
searched on 6 May 2010 to identify documented out-
water from fields, transporting them into rivers,
breaks of waterborne infectious disease in humans,
coastal waters and wells [1, 6]. Such events can there-
occurring since 1910, where an extreme water-related
fore increase raw water turbidity, which has been
weather event was believed to have been involved. An
found to be associated with gastrointestinal illness [7].
extreme water-related weather event was defined as a
Heavy rainfall can also lead to changes in the direc-
meteorological change in the conditions of a region,
tion of flow of water through channels that would not
involving a quantity of water more or less than is
normally occur [8]. During periods of heavy rainfall,
usually seen in the region. Key terms used for identi-
water treatment plants may be overwhelmed, there
fication of extreme water-related weather events were
may be cross-contamination between sewage and
identified from the National Climate Data Centre [13]
drinking-water pipes (particularly where water infra-
and included : flooding, drought, heavy rainfall, El
structure is old), sewage overflow, or bypass into local
Niño Southern Oscillation (ENSO), hurricane, cyc-
waterways [9]. Extreme precipitation events may
lone, other extreme storm, seawater inundation, ex-
also increase the risk of flooding in many areas, in-
treme water run-off, and extreme changes in water
creasing human exposure to waterborne pathogens
level or temperature (see Table 1). A list of known
[10]. Droughts or extended dry periods are known to
waterborne pathogens was compiled and used to
reduce the volume of river flow and potentially in-
generate key search terms for the identification of
crease the concentration of effluent-derived patho-
waterborne infectious disease. Search strategies which
gens, due to reduced dilution by stream-receiving
combined the key search terms and subject headings
waters [11].
for both waterborne infectious disease and extreme
Outbreaks caused by the contamination of com-
water-related weather events were used to interrogate
munity water systems have the potential to cause ex-
the online databases. The titles, key words and ab-
tensive disease [12], particularly where the public
stracts of articles included in the online databases
health infrastructure is less resilient. Waterborne dis-
were searched for these search terms. It was not
eases are expected to rise with increases in extreme
possible to search the full texts as not all citations had
rainfall and deterioration in water quality following
been indexed and scanned into the online databases
wider drought events [2]. It is important to establish
and relevant published articles without abstract or
the current impact of such events on public health to
key words may not have been picked up based on
allow future predictions, aid policy formulation, and
their titles alone. An example of the search strategies
improve adaptive capacity. The impact of recent
used can be found in the Supplementary online
events demonstrates that even high-income countries
material (Appendix 1). The grey literature was also
are not well prepared to cope with extreme weather
searched on 12 May 2010 using the Program for Moni-
events [2]. There is also limited information available
toring Emerging Diseases (ProMed-mail) [14] and an
on how different extreme water-related weather events
online search engine [15]. Both sources were searched
will impact different geographical areas and patho-
using combinations of the key search terms; a list of
gens. This is the first global systematic review of
these combinations can be found in Appendix 2 (online).
the impact of weather on waterborne disease. The
aim was to assess how the different categories of ex-
treme water-related weather events impact water- Selection criteria
borne disease, by geographical area, pathogen and
All waterborne pathogens resulting in infectious dis-
outcome.
ease were included, with the exception of those that
require an obligate intermediary host. All study de-
sign types were included. Non-English language stu-
METHODS
dies were included and translated. To check whether
A full protocol was written a priori specifying the key papers had been identified and to validate
search strategy, selection criteria, and data extraction the sensitivity of the search strategy, the following
and analysis strategies (available upon request). journals were manually searched : Journal of Water
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. https://doi.org/10.1017/S0950268812001653Extreme weather and waterborne disease 673
Table 1. Key search terms used to identify waterborne disease outbreaks involving an
extreme water-related weather event
Waterborne disease outbreak Extreme water-related weather event
Water ; waterborne Ocean ; sea ; seawater ; lake; river ;
rain ; rainfall ; water supply; water movement ;
WITH
weather ; storm ; climate ; precipitation
Infection ; infectious ; WITH
communicable ; disease ; illness; Extreme ; spate ; excessive ; surge; disaster
enteric ; pathogen ; organism;
OR
agent; WBDO ; gastroenteritis ;
AG; Giardia ; Cryptosporidium; Floodwater ; drought ; water scarcity;
E. coli ; Shigella ; Legionella; heavy rainfall ; flood; heavy precipitation ;
Salmonella; Naegleria fowleri ; el Niño; la Niña
Plesiomonas shigelloides ; Campylobacter;
Amoebiasis ; Dracunculus ; Hymenolepis ;
Ascaris; Enterobius ; Mycobacterium marinum ;
dysentery; Leptospira ; Vibrio ; Enterovirus ;
Norovirus; Norwalk ; Hepatitis A ; HAV ;
Hepatitis E; Adenovirus ; Astrovirus ; Calicivirus ;
Coronavirus; Poliomyelitis ; Poliovirus ;
Picornaviridae ; Coxsackievirus ; Echovirus ;
Rotavirus ; Reovirus
WITH
Outbreak; epidemic ; occurrence
WITH
Human
and Climate Change ; Journal of Water and Health ; Population : human.
Water ; Water Policy ; and Water Research. The Event : extreme water-related weather event.
reference lists of included publications were also Outcome measure : waterborne disease outbreak
checked for further eligible articles. Duplicate articles, (described as waterborne by the author or attribu-
identified from their titles, were included only once. ted to a pathogen which is transmitted solely
Citations identified from the literature were through water).
screened for inclusion criteria by two independent
Exclusion criteria were as follows :
reviewers (R.S., D.T.), first using the titles and ab-
stracts, then using the full papers (where available). Literature type : news articles.
Publications identified from the online search engine Population : non-human ; displaced populations.
were first screened for inclusion criteria by one re- Event : extreme climatic weather events not in-
viewer (K.C.) using either the abstract or first page. volving water.
Relevant documents were downloaded in full and Outcome measure : non-waterborne disease out-
screened by two independent reviewers (R.S., D.T.) breaks; outbreaks due to pathogens with obliga-
alongside the articles published in peer-reviewed tory, intermediary hosts.
journals. The ProMED reports were screened by one
Algorithms and notes for citation review were devel-
reviewer (K.C.) and inclusion criteria applied.
oped to reduce variation between reviewers and
Relevant ProMED reports were analysed separately
to clarify which pathogens and events should be
to allow comparison with the published literature.
included (see Fig. 1). Reviewer agreement on the in-
clusion/exclusion of abstracts and full papers was
Inclusion criteria were as follows :
80.4% and 75.4 %, respectively. Any disagreement
Literature type : published articles ; official pub- over inclusion of a publication was resolved by a third
lications. reviewer (K.C.).
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. https://doi.org/10.1017/S0950268812001653674 K. F. Cann and others
environmental events rarely provide sufficient detail.
Citation inspection Articles were rated according to their applicability as
either ‘ direct ’ to signify strong causality and direct
applicability of its content ; ‘moderate ’, if they pro-
NO vided strong circumstantial evidence but data taken
Is an extreme water* event Exclude:
mentioned? No extreme event from these articles required careful interpretation be-
fore it could be used ; or ‘indirect ’, if they did not
YES / UNCLEAR support causal inference and if the content could only
Is an outbreak/epidemic/increase
NO be used as background information.
Exclude:
in cases of infectious disease
No outbreak
Large differences in study designs and in the popu-
mentioned? lations involved precluded the pooling of data from
YES / UNCLEAR
different outbreaks and a traditional meta-analysis.
Frequency distributions of the type of publication,
Is the outbreak/epidemic/increase NO
Exclude: type of extreme water-related weather event, country
in cases due to waterbore
pathogens†?
Not waterborne affected, and pathogens involved were listed. Those
publications which provided information on the
YES / UNCLEAR numbers of cases, deaths or attack rates were included
in the quantitative synthesis which calculated geo-
Include metric means (due to the highly skewed nature of the
data) by causal pathogen and by type of weather
Fig. 1. Algorithm used by reviewers when screening ab- event. Numbers of cases were stratified by case defi-
stracts and grey literature to determine if inclusion criteria nition type : laboratory-confirmed, clinical diagnosis,
were met. * A list of examples of extreme water-related
or self-reported.
events to be included was provided. # A list of examples of
waterborne pathogens to be included was provided
Data extraction RESULTS
Data from included publications were extracted into a Evidence from the scientific literature
database using a custom-made form (K.C.). The pri-
A total of 83 identified papers were included in the
mary outcome was the number of cases of waterborne
analysis, four of which were not in English (Spanish,
disease as defined by the author. Where the number of
Portuguese, French, Czech) (see Fig. 2). Full details
cases was stated in the publication, these data were
of included publications are available in Appendix 4
extracted regardless of whether these were laboratory
(online). Four relevant papers were identified by hand
confirmed, self-reported, or diagnosed on clinical
searching of relevant journals; all of which had
grounds. Where the number of cases was stated to be
been identified by the search strategy and already in-
an estimate this was recorded. Secondary outcome
cluded.
measures were the attack rate and the number of
Of the 83 papers identified, 35 (42.2 %) were out-
deaths. Other data extracted included : citation de-
break investigations, 28 (33.7 %) were other quanti-
tails, type of study, applicability, details of the ex-
tative studies, 13 (15.7%) were reviews, three (3.6%)
treme water-related weather events, water quality,
were qualitative studies, two (2.4%) were mixed
details of the outbreak, and details of how the weather
methods studies, one (1.2%) was a case study, and
event was thought to have led to the outbreak. Full
one (1.2%) was an official report. Eight (9.6%)
details of extraction fields used are available in
of these publications were classed as having direct
Appendix 3 (online). Where more than one pub-
applicability, 48 (57.8 %) had moderate applicability,
lication referred to the same study, any further rel-
and 27 (32.5%) had indirect applicability.
evant data available on the event found in subsequent
The papers included 93 accounts of 87 different
accounts were added to the initial data identified.
waterborne outbreaks involving extreme water-
related weather events from 29 different countries
Data analysis
(eight reviews documented two or three outbreaks).
No pre-defined tool for the assessment of the qua- The majority of the reported outbreaks, where ex-
lity of evidence was used as publications about treme water-related weather events were involved,
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Records identified through Additional records identified
database searching through online search
(n = 281) (n = 18)
Records after duplicates removed
(n = 196)
Records screened Records excluded
(n = 196) (n = 133)
Additional records Full-text articles assessed Full-text articles
identified from for eligibility excluded
reference lists (n = 88) (n = 5)
(n = 25)
Studies included
in qualitative synthesis
(n = 83)
Studies included in
quantitative synthesis
(n = 65)
Fig. 2. Flow chart detailing the passage of scientific and grey literature through the systematic review process.
were in North America, followed by Asia and Europe. events reported more than one event to have been in-
The pathogens involved were specified in 74 (85.1 %) volved. Heavy rainfall and flooding were the most
of the outbreaks following extreme weather events common combination of events preceding outbreaks
and are listed in Table 2. The most commonly re- (27.6 % of all accounts). Heavy rainfall was also
ported were Vibrio spp. (28.4 %) and Leptospira spp. linked with heavy water run-off and hurricane, and
(17.6 %). Most reported outbreaks of Vibrio spp. fol- flooding was also linked with hurricane. Fifty-four
lowing extreme water-related weather events occurred (58.0 %) reports of outbreaks following extreme
in Asia, followed by Africa and South America, while water-related weather events gave details of how the
most reported outbreaks due to Leptospira spp. were event was thought to have led to the outbreak. In just
in North America or Asia. More than one pathogen over half (53.7%) of reports providing this infor-
was identified on 16/74 (21.6 %) occasions, mostly in mation, the extreme water-related weather event
North America or Asia (37.5 % and 31.3 %, respect- caused the outbreak through contamination of the
ively), followed by Europe (18.8%). Of the 74 out- water supply, usually through increased run-off of
breaks in which the causal pathogen was identified, 31 water from the surrounding area (22.2%) or by in-
(41.9 %) reported testing the water supply. Of these, undation (20.4 %). Exposure to contaminated water
the causal pathogen was stated to have been identified by physical activity occurred in 16.7% of the out-
in just over half (54.8 %), confirming the water supply break reports associated with extreme water-related
as the source. weather events, but this was recreational activity in
Out of all outbreaks associated with extreme water- only 3.7 %. More commonly contact with floodwater
related weather events, heavy rainfall and flooding occurred while wading or during the cleaning up
were by far the most commonly reported antecedents process (9.3 %). A change in the survival rates of
(in 55.2% and 52.9 % of published accounts, re- pathogens due to changing environmental conditions
spectively) (see Fig. 3). Forty-five (51.7 %) of the (such as water temperature or stagnation) was
outbreaks following extreme water-related weather thought to be the cause in 9.3 % of outbreak reports
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. https://doi.org/10.1017/S0950268812001653676 K. F. Cann and others
Table 2. Waterborne pathogens implicated in outbreaks following extreme water-related weather events identified
from the scientific literature (6 May 2010) and ProMED reports (12 May 2010)
No. (%)* of times reported
Waterborne pathogen Scientific literature ProMED reports
All viruses 19 (25.7) 5 (2.4)
Hepatitis virus 7 (9.5) 3 (1.4)
Hepatitis A virus 4 (5.4) 2 (0.9)
Hepatitis E virus 2 (2.7) 1 (0.5)
Hepatitis virus : type unknown 1 (1.4) —
Norovirus 6 (8.1) 1 (0.5)
Rotavirus 3 (4.1) 1 (0.5)
Adenovirus 2 (2.7) —
Enterovirus 1 (1.4) —
All bacteria 66 (89.1) 198 (93.8)
Vibrio spp. 21 (28.4) 145 (68.7)
Vibrio cholerae 20 (27.0) 137 (64.9)
Other Vibrio spp. 2 (2.7) 8 (3.8)
Leptospira spp. 13 (17.6) 36 (17.1)
Leptospira interrogans 4 (5.4) —
Leptospira sp. not known 9 (12.2) 36 (17.1)
Campylobacter spp. 10 (13.5) 3 (1.4)
Campylobacter jejuni 6 (8.1) —
Campylobacter sp. not known 4 (5.4) 3 (1.4)
Escherichia coli 9 (12.2) 9 (4.3)
Shigella spp. 4 (5.4) —
Shigella flexneri 2 (2.7) —
Shigella boydii 2 (2.7) —
Salmonella spp. 3 (4.1) 5 (2.4)
Salmonella typhi 1 (1.4) 4 (1.9)
Salmonella sp. unknown 2 (2.7) 1 (0.5)
Burkholderia pseudomallei 3 (4.1) 9 (4.3)
Yersinia enterocolitica 2 (2.7) —
Aeromonas spp. 1 (1.4) —
All protozoa 16 (21.6) 12 (5.7)
Cryptosporidium spp. 9 (12.2) 3 (1.4)
Cryptosporidium parvum 2 (2.7) —
Cryptosporidium sp. not known 7 (9.5) 3 (1.4)
Giardia lambia 5 (6.8) —
Acanthamoeba spp. 1 (1.4) —
Cyclospora spp. 1 (1.4) —
* Percentage of either 74 outbreak accounts or 211 ProMED reports reporting the pathogens involved.
in which an extreme water-related weather event was infection was through the mains water supply for
implicated. Failure to cope by the water treatment 66.9 %, through other treated water for 37.0 %, and
plant were blamed in 7.4 % of papers providing in- through a well supply for 29.6 %. By comparison,
formation on the role of the extreme water- 22.2 % were infected through environmental exposure
related event in the outbreak. Failure or inability and 11.1 % of the outbreaks involved both environ-
to cope by sewage systems, resulting in contamination mental exposure and the water supply. Campylobacter
of the water supply were also blamed in 7.4 % of spp. and Cryptosporidium spp. were common causal
reports. pathogens in outbreaks associated with extreme
Of 27 accounts of waterborne outbreaks following water-related weather events originating from treated
extreme water-related weather events from developed mains water. Waterborne pathogens originating from
countries which reported the information, the route of environmental exposure following extreme weather
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. https://doi.org/10.1017/S0950268812001653Extreme weather and waterborne disease 677
Table 3. Attack rates reported during outbreaks of infectious disease due to waterborne pathogens, where extreme
water-related weather events are involved identified from the scientific literature (6 May 2010), by pathogen
No. reporting Mean attack Median attack Lowest attack Highest attack
Pathogen attack rate* rate (%)# rate (%) rate reported (%) rate reported (%)
Viruses 10 7.8 27.6 0.2 79.0
Rotavirus 1 79.0 79.0 79.0 79.0
Norovirus 3 44.0 36.2 30.6 77.0
Hepatitis A 3 3.1 5.5 0.2 24.5
Hepatitis E 2 3.4 29.2 0.2 58.2
Enterovirus 1 0.4 0.4 0.4 0.4
Bacteria 16 0.8 0.5678 K. F. Cann and others
(a) (b)
120 (4) 70000
(2)
Mean number of cases
60000
Mean number of cases
100
50000
80
40000
60
(1) 30000
40 (1)
20000
20 (2) (8)
(3) (1) 10000
(1) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (2) (0) (7) (2)
(0) (0)
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(2) 16000
Mean number of cases
Mean number of cases
2000 14000
12000
1500 10000
8000
1000 (4) 6000
4000 (1)
500 (1) (3) (3)
(1) (1) 2000 (0) (0) (1) (0) (0) (0) (0) (0) (1) (1) (1)
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Fig. 4. Mean numbers of cases reported in accounts of waterborne disease outbreaks where extreme weather events have
been implicated, by pathogen and case definition type (scientific literature) : (a) using a laboratory-confirmed diagnosis,
(b) using a self-reported diagnosis, (c) using a clinical diagnosis, (d) where type of diagnosis is not reported. Figures in
parentheses indicate number of accounts reporting this information.
reported laboratory-confirmed cases, 13 (20.6%) re- following a severe storm, as shown in Figure 5. Of
lied on self-reporting of cases, 13 (20.6%) relied on a those using a clinical diagnosis or where the type
clinical diagnosis, and 16 (25.4 %) did not report this of case definition was not given, the highest number
information. of cases was seen following a cyclone and seawater
Where a laboratory-confirmed or self-reported case inundation.
definition was used, the highest mean number of cases
resulted from Cryptosporidium outbreaks, as shown
Evidence from ProMED reports
in Figure 4. However, most outbreaks using a self-
reported case definition identified multiple pathogens There were 235 eligible ProMED reports of water-
(7/11). Where a clinical diagnosis was used or the type borne outbreaks following extreme water-related
of case definition was not given, V. cholerae outbreaks weather events, involving 304 events from 66 different
following extreme water-related weather events re- countries. The majority of these reports were in
sulted in the highest reported mean number of cases. Africa, followed by Asia and North America. Far
Outbreaks in which the type of case definition was not more of the outbreaks following extreme water-re-
given reported the highest mean number of cases for lated weather events were based in Africa than in the
any pathogen (n=2726), followed by self-reported scientific literature (42.6 % vs. 6.9 %, respectively) and
cases (n=994), clinical diagnoses (n=309), and lab- less were based in either Europe or North America
oratory-confirmed (n=19). Of those accounts re- (4.4% vs. 18.4% and 12.0 % vs. 33.3 %, respectively).
porting laboratory-confirmed cases or self-reported The pathogen was given in 211 (89.8 %) reports, of
cases, the highest mean number of cases was seen which V. cholera was by far the commonest (64.9 %),
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. https://doi.org/10.1017/S0950268812001653Extreme weather and waterborne disease 679
(a) (b)
180 450000
(2) (2) (1)
160
Mean number of cases
Mean number of cases
400 000
140 350000
120 300000
100 250000
80 200000
60 150 000
40 100 000
(12) (13) (2) (3) 50000 (1) (0)
20 (0) (0) (0) (0) (0) (0) (7) (7) (0) (0) (4) (0) (0) (0) (0)
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Mean number of cases
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Fig. 5. Mean numbers of cases reported in accounts of waterborne disease outbreaks where extreme weather events have
been implicated, by event and case definition type (scientific literature) : (a) using a laboratory-confirmed diagnosis, (b) using
a self-reported diagnosis, (c) using a clinical diagnosis, (d) where type of diagnosis is not reported. Figures in parentheses
indicate number of accounts reporting this information.
of which 65.4 % occurred in Africa and 20.6 % oc- The number of cases involved was reported in 174
curred in Asia. Out of all ProMED waterborne out- (74.0 %) and the number of deaths in 145 (61.7 %)
break reports which were associated with an extreme of the ProMED accounts of waterborne outbreaks
water-related weather event, the most common events following extreme water-related weather events. We
were again flooding and heavy rainfall (see Fig. 3), were unable to stratify ProMED reports by type of
with 15.7 % of reports involving both. Sixty-four case definition due to the limited detail available in the
(27.2 %) of the reports implicated more than one type brief reports.
of extreme water-related weather event. These also
linked heavy rainfall with El Niño, drought and cyc-
DISCUSSION
lone, while flooding was also linked with extreme
storm, cyclone, hurricane, drought and tidal surge. This review has a number of limitations which can be
Ninety-eight (41.7 %) of the reports of waterborne considered in two groups ; systematic review limita-
outbreaks following extreme water-related weather tions and primary literature reporting limitations. The
events gave details of the likely cause; the most com- identification of so many suitable new references
mon were contamination of water (32.9%), shortage through the reference lists of publications identified
of clean drinking water (18.9%), and poor sanitation by the search engines, i.e. 23.1% of all references in-
and hygiene following the event (14.7 %). cluded, suggests that the search strategies may have
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. https://doi.org/10.1017/S0950268812001653. https://doi.org/10.1017/S0950268812001653
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680
K. F. Cann and others
Table 4. Number of known deaths during outbreaks of infectious disease due to waterborne pathogens, where extreme water-related weather events are involved
identified from the scientific literature (6 May 2010) and ProMED (12 May 2010), by event
Scientific literature ProMED reports
No. reporting Mean Median Lowest Highest No. reporting Mean Median Lowest Highest
Extreme event* no. deaths no. deaths# no. deaths no. deaths no. deaths no. deaths no. deaths# no. deaths no. deaths no. deaths
Heavy rainfall 8 10 11 1 253 54 19 19 0 1156
Flooding 14 29 42 1 500 80 19 18 0 70 000
Heavy rainfall and flooding 6 11 15 0 53 23 13 12 1 352
Cyclone 2 64 66 51 81 8 37 18 1 70 000
Drought 1 2 2 2 2 19 15 12 1 274
Extreme water run-off — — — — —
Severe storm 1 15 15 15 15 5 49 89 5 167
Extreme water temperature — — — — — — — — — —
change
Hurricane — — — — — 8 3 4 0 5
ENSO — — — — — 15 13 5 1 2231
Seawater inundation 2 64 66 51 81 1 28 28 28 28
Extreme change in water level — — — — — — — — — —
Other — — — — — — — — — —
All 16 20 28 1 500 190 221 263 42 144 018
* Where more than one extreme event was involved, the figures were imputed into each type of event.
# Geometric mean (accounts reporting 0 not included).. https://doi.org/10.1017/S0950268812001653
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Table 5. Number of known deaths during outbreaks of infectious disease due to waterborne pathogens, where extreme water-related weather events are involved
identified from the scientific literature (6 May 2010) and ProMED reports (12 May 2010), by pathogen
Scientific literature ProMED reports
No. reporting Mean no. Median no. Lowest no. Highest no. No. reporting Mean no. Median no. Lowest Highest
Pathogen no. deaths# deaths* deaths deaths deaths no. deaths# deaths* deaths no. deaths no. deaths
Viruses 1 1 1 1 1 1 127 127 127 127
Enterovirus 1 1 1 1 1 0 — — — —
Hepatitis E virus 0 — — — — 1 127 127 127 127
Bacteria 11 24 41 1 276 130 18 14 0 70 000
Vibrio cholera 6 70 52 41 276 90 24 19 1 70 000
Non-choleragic 0 — — — — 7 3 4 0 5
Vibrio spp.
Leptospira 4 4 4 1 15 23 17 17 0 182
Salmonella typhi 0 — — — — 1 2 2 2 2
Extreme weather and waterborne disease
Shigella spp. 1 51 51 51 51 0 — — — —
E. coli 0 — — — — 4 3 4 1 6
B. pseudomallei 0 — — — — 5 6 7 1 15
Protozoa 0 — — — — 0 — — — —
Multiple pathogens 4 26 44 2 500 0 — — — —
Not stated 1 42 42 42 42 14 35 54 2 1350
All 16 20 28 1 500 145 186 234 134 71 687
* Geometric mean.
# Includes only those accounts which report this information.
681682 K. F. Cann and others
been too specific. It is difficult to achieve the optimal evidence but in need of careful interpretation ’ and few
balance between sensitivity and specificity, as time of the publications described a comparison group.
and resource constraints limited the number of ab- The limited reporting of many of the included studies
stracts which could be screened for inclusion and a also prevented many in-depth conclusions being
number of known papers were not identified by this drawn.
approach. This was due to relying on authors men- These study reporting limitations highlight key
tioning the extreme water-related weather event in areas which future outbreak reports should seek to
either the title, abstract or key words to enable them address. Future research in this area should aim to
to be identified by the search strategy. Papers without measure and report clearly population, weather
abstracts or key words may not have been identified and water parameter details when investigating
on the basis of their title alone, for example, the article waterborne disease outbreaks where an extreme
by Walzer et al. on the balantidiasis outbreak in Truk water-related weather event is thought to be involved.
following a typhoon [16]. Papers where the event was Where an outbreak is reported, some effort should
only mentioned as a detail in the full text and not in also be made to classify the probable route (or routes)
the title, key words or abstract may also have been of transmission. This would allow a greater pro-
missed. We therefore suggest that future reviews take portion of waterborne outbreaks to be identified and
a more comprehensive approach. It was also very included in analyses such as these. Where the number
difficult to make comparisons between outbreaks fol- of cases involved in an outbreak is reported, the case
lowing different types of extreme events as there were definition used should be clearly stated. If an extreme
vast differences in key characteristics of the popula- water-related climatic event is thought to be im-
tions affected. Where reported, there was also sub- plicated in an outbreak, details of how it may have led
stantial variation in case definitions between to contamination of the water should be reported. It is
outbreaks identified ; accounts based on self-reported also important to raise awareness of the potential role
cases had a substantially higher mean number of cases of such events in waterborne outbreaks, to encourage
than those requiring a clinical diagnosis or laboratory authors to question explicitly whether such an event
confirmation and those that did not report the type of occurred prior to the outbreak and if so, to detail it in
case definition used at all had, on average, the largest a structured way.
number of cases. Calculation of attack rate is also The global distribution of waterborne outbreaks
likely to have varied by study. following extreme water-related weather events as re-
The review also suffered from a lack of reporting of ported in the scientific literature is also likely to be
detail. For example, it was difficult to assess the evi- prone to considerable publication bias. A greater
dence supporting the classification of the outbreaks as proportion of those identified through ProMED were
waterborne or the degree of association between in Africa and South America compared to those re-
water and disease given the limited amount of infor- ported in peer-reviewed journals, with less from
mation often provided, particularly in the ProMED North America or Europe. Outbreaks following
reports. For example, where testing of the water sup- drought or an ENSO event, both more common in
ply was reported, the causal pathogen was identified developing countries, were more often reported via
in just over half of the outbreaks. A quarter of ProMED than in the scientific literature but out-
those which provided the number of cases seen breaks following hurricanes were more often reported
did not report the type of case definition used, i.e. in the latter, with 40.8 % from the USA alone. Risk of
whether they were laboratory-confirmed, clinically diarrhoeal disease outbreaks following natural dis-
diagnosed, or self-reported cases. Details of the asters has previously been found to be higher in de-
extreme water-related weather events thought to be veloping countries than in industrialized countries
involved in the outbreaks, such as the amount of [17]. Flooding events, for example, in high-income
precipitation seen or parameters such as water tem- countries are rarely thought to result in epidemics of
perature, pH and level of turbidity, were rarely given. infectious disease or, where they do occur, they are
This severely limits the suitability of the results for thought to often be easily controlled and not wide-
extrapolation to different circumstances and geo- spread due to the rapid implementation of prevent-
graphical locations. Such were the quality of the ative measures [18, 19]. The scientific literature is also
literature identified, the majority of the data were likely to be dominated by accounts of waterborne
classed only as providing ‘strong circumstantial disease from higher-income countries, with greater
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. https://doi.org/10.1017/S0950268812001653Extreme weather and waterborne disease 683
academic and surveillance capacity. The amount and the water source through run-off or inundation.
type of literature published on an extreme weather While in developing countries this was usually un-
event, therefore, may not be proportionate to the size treated water, in developed countries, in the majority
and impact of the event. Risk of publication bias was of cases, this was contamination of a treated water
addressed, to some extent, by searching grey literature source. This suggests that even in developed countries
and by comparisons with the ProMED reports. the water supply system is not immune to the effects of
However, ProMED is a passive reporting tool and such events. Although well-managed public water
there may also be discrepancies in reporting practices. supply systems are expected to be able to cope with
Griffith et al. [20] found, for example, that outbreaks weather extremes, such extremes can cause both
of cholera in Africa were more likely to be reported in physical and managerial stresses which may impact
areas of international interest and where there were water quality [21]. For example, rainstorm events and
fewer commercial consequences. following increases in natural organic matter have
Nevertheless, this review suggests that outbreaks of been shown to significantly impair turbidity removal
waterborne infectious disease do occur following ex- at water treatment works in England [22]. Similarly,
treme water-related weather events in both developed the dry summer of 2003 and resulting low river flows
and developing countries. This already constitutes a were shown to cause deterioration in water quality
significant burden on public health and as the fre- in The Netherlands [23]. Public health practitioners
quency of such events increases, so too will associated and water companies should be aware of the risks
outbreaks of disease. The outbreaks identified in this of waterborne disease outbreaks following these
review are also likely to be underestimates of the true events. Addressing the infectious waterborne disease
prevalence of outbreaks resulting from extreme water- consequences of climate change is likely to require
related weather events due to under-reporting of specific engineering solutions to protect potable
waterborne outbreaks and the difficulties in identify- water.
ing papers which implicate such events (see above). The reported outbreaks associated with extreme
Future research into what proportion of all water- water-related weather events were primarily due to
borne outbreaks involve an extreme weather event heavy rainfall with or without flooding. Flooding may
and what proportion of all extreme weather events have resulted from heavy rainfall in many cases and
result in waterborne outbreaks would help to estimate many of the other extreme water-related weather
the true burden to public health. events may have involved heavy rainfall even if it was
The impacts of extreme water-related weather not specified in the paper or report. Authors may have
events on waterborne disease will disproportionately focused only on the causal event and not its climatic
affect certain populations and will likely compound effects. Whereas much has been published on the
existing health disparities. Less developed countries health impacts of flooding [24–26], there is less on the
may be at greater risk due to both higher sensitivity impact of heavy rainfall which did not result in
and lower adaptive capacity [21]. Non-climate stres- flooding, even though the latter may still result in
sors such as poverty or conflict can increase vulner- pressure on the water supply. Studies have linked
ability by reducing resilience and adaptive capacity waterborne disease with heavy rainfall in several
due to competing resource needs [17]. Less developed countries [27–29]. Curriero et al. [27] reviewed 548
countries are therefore thought to be more vulnerable disease outbreaks between 1948 and 1994 in the USA
and less able to recover rapidly or effectively. and found a significant association between rainfall
Climate change is predicted to increase the burden of and illness ; 68% of the events were found to be pre-
diarrhoeal disease in low-income regions by about ceded by precipitation events above the 80th percen-
2–5 % by 2020, while countries with an annual GDP tile. While Thomas et al. [30] found that from 1975 to
per capita of oUS$6000 are thought to be at no 2000 in Canada rainfall events over the 93rd percen-
additional risk [21]. However, responses to recent ex- tile increased the risk of a waterborne disease out-
treme weather events suggest higher levels of vulner- break by a factor of 2.3. In light of the expected
ability in both developing and developed countries increases in frequency of heavy rainfall events in
than thought previously [2]. This review found that in many regions, it is important to assess the individual
both developing and developed countries the most impact of such events in local and regional areas
common cause of outbreaks following extreme and incorporate these into health and infrastructure
water-related weather events was contamination of policy.
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. https://doi.org/10.1017/S0950268812001653684 K. F. Cann and others
The scientific literature suggests that, of those and the elderly, who are more likely to be based in and
extreme water-related weather events linked to around the home, are more likely to be affected by
waterborne outbreaks, cyclones, other severe storms, flooding events with a rapid onset [35]. Indigenous
ENSO events, or seawater inundation result in the populations are also likely to be greatly impacted due
highest numbers of cases of waterborne illness. Risk to their occupation of economically and politically
of seawater inundation is expected to increase over marginal areas and fragile ecosystems [36]. Such
the next few decades, due to predicted sea level rise variability needs to be incorporated into the develop-
and increasing frequency of extreme weather events, ment of any policies or interventions to improve
with the risk zone predicted to spread further inland adaptive capacity. There is evidence to suggest that
and higher [31]. ENSO is known to be linked with individual weather-related extreme events can facili-
extreme water temperature change. Two of the four tate adaptations such as policy and regulatory
accounts of outbreaks (both of which were cholera) change, as immediately afterwards the policy climate
following an ENSO event in the scientific literature may be more conducive to change [37]. Yet pressure
were also linked to an extreme water temperature for a quick recovery and short-term risk reduction can
change. Studies have repeatedly linked ENSO events actually result in greater vulnerability to future events
and extreme water temperature change to large-scale [38, 39]. There is also a lack of information on avail-
V. cholerae outbreaks [32, 33] and V. cholerae is able and successful extreme water-related weather-
known to show an increased growth rate at increased event adaptation strategies for waterborne disease
temperatures, with increasing global temperatures outbreaks. Research into the impact of waterborne
also expected to increase prevalence both geographi- outbreaks following extreme weather events on dif-
cally and temporally [34]. V. cholerae was by far the ferent sub-populations which may be particularly
most common pathogen implicated in outbreaks fol- vulnerable and the effectiveness of different adap-
lowing extreme water-related weather events, from tation strategies should be undertaken.
both the scientific literature and ProMED, which
may in part reflect the predominance of outbreaks
CONCLUSIONS
following heavy rainfall and flooding. In a review of
ProMED cholera outbreak reports from 1995 to Waterborne diseases are one of the major con-
2005, Griffith et al. [20] found that rainfall and tributors to global disease burden and mortality [40].
flooding were the most common risk factors globally Improving the understanding of the impact that the
(constituting 25% of all risk factors), alongside water different extreme water-related weather events have
source contamination (29 %) and refugee settings on waterborne disease is an important step towards
(13 %). The number of cholera outbreaks reported finding ways to mitigate the risks. At a time when cli-
following extreme water-related weather events may mate change is predicted to increase both the frequency
also be attributable to a number of other factors such and intensity of extreme water-related weather events
as the severity of the disease or reporting bias. When in many regions, understanding and reducing the im-
assessing the risk to public health of such outbreaks it pact of these events is vital to the health of many.
is important to incorporate information not only
from the most likely extreme water-related weather
S U P P LE M E N T A R Y M A T E R I A L
events (such as heavy rainfall and flooding), but also
those from lower-probability but higher consequence For supplementary material accompanying this paper
events (such as ENSO or seawater inundation) [21]. visit http://dx.doi.org/10.1017/S0950268812001653.
The ability of a population to adapt and limit
the effects of such events is likely to depend on
ACKNOWLEDGEMENTS
socioeconomic and environmental circumstances and
the availability of information and technology [21]. The authors thank the Library and Knowledge
There is also evidence to show that human and social Management Team at Public Health Wales for their
capital are key determinants of adaptive capacity at help with the collation of supporting materials. We
all scales [21]. Adaptive capacity is uneven both be- also thank Dr Meirion Evans and the Reviewers at
tween and within societies. For example, following Epidemiology and Infection for their helpful re-
weather-related disasters there is a differential impact commendations on the layout and content of this
on deaths and well-being by gender, while children paper.
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. https://doi.org/10.1017/S0950268812001653Extreme weather and waterborne disease 685
D E C L A R A T I O N O F IN T E R E S T mail (http://www.promedmail.org/). Accessed 2 June
2011.
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