QMRA summer school 2010, TU Delft
25.6.2010
Group 4 Boil Water Advisory (D)
Expert Panel
-
1
Tarja Pitkänen
Miriam Agullo
Monique van Veen
Ihab Habib
Louise Krojgaard
Vo Thi Yen Phi
INTRODUCTION
Faecal contamination of drinking water and subsequent waterborne gastrointestinal infection
outbreaks are a major global public health concern (WHO, 2004). Human and animal faeces
may contain pathogenic microbes (bacteria, viruses, protozoans and helminths). Preventing
fecal contaminated water from gaining access to treated waters of surface and ground
waterworks is essential in order to maintain water quality. It is essential that immediate
actions are taken following in cases when fecal contamination of drinking water is suspected
or evident. Full emergency level actions need to be initiated in those cases, including a water
avoidance or a boil water advisory (Hunter et al., 2003).
When tap water might be contaminated or is known to be contaminated with fecal pathogens,
water utilities or public health authorities commonly issue the public to boil water before
consumption (Mayon-White and Frankenberg, 1989). Some water utilities may give a boil
water advisory proactive, as the risk for water contamination may increase during repair
works. The major part of the boil water advisories is however issued after water quality
complaints or when water quality analysis utilizing Escherichia coli indicate the presence of
fecal contamination.
The boil water advisory is given to reduce the risk of drinking waterborne gastrointestinal
illnesses, but the effectiveness of the risk reduction is affected by many factors. These include
the fact that after the contamination is detected and boiling water notice issued and received,
people have drunk the contaminated water. Some of the persons might not even receive the
advice and of those who receive it, not all follow the instructions (precisely). The notice may
be taken more seriously and then followed precisely in cases when the water is truly
contaminated. On the contrary, in proactive cases, when there is no contamination present
(only increased risk of contamination), the notice might not have so good efficiency. The
length of the contamination event may also affect the interest of people to boil their water.
Problem formulation: In this group study, we needed to assess how effective boil water
advisory is in reducing the risk of infection of water consumers.
2
SCOPE
In this chosen city surface water is used as source of drinking water after treatment. We are
modelling if there is a breakdown, for one day, at the water treatment plant. All surface water
is, untreated, led directly to all taps. In our case the population of the city is 100.000 people,
they all have water from the same treatment plant.
Surface water is often fecally contaminated and may contain a wide range of faecal
pathogens,
such
as
Campylobacter
jejuni,
E.
coli
O157,
rotavirus,
norovirus,
Cryptosporidium and Giardia. In this study, based on the known prevalence in the surface
water, being common causes of waterborne outbreaks, the pathogens Campylobacter jejuni,
Cryptosporidium were selected to be included in this assessment.
Boiling the water for 3 minutes is considered effective to kill all pathogens – no infection risk
if boil water notice is followed precisely.
2.1
Hazard indentification
Campylobacter
Campylobacter is considered the most important bacterial agent in waterborne diseases in
many European countries (Stenström et al. 1994; Furtado et al. 1998) - a large number of
outbreaks of Campylobacter have been reported in Sweden for example, involving over 6000
individuals (Furtado et al. 1998). Issues with Campylobacter in drinking waters tend to be a
post-treatment problem, due to situations such as broken sewers.
Campylobacters have been isolated from rivers (Arvantidou et al. 1996; Obiri-Danso and
Jones, 1999), lakes (Arvantidou et al. 1996), groundwater (Savill et al. 2001) as well as
drinking water (Alary and Nadeau, 1990; Savill et al. 2001; Vogt et al. 1982). The
occurrence of the organisms in surface waters has also proved to be strongly dependent on
rainfall, water temperature and the presence of waterfowl (WHO, 2004).
Cryptosporidium
Cryptosporidium has been well described as one of the most important pathogen in
waterborne outbreaks in many different countries (Proctor et al 1998; MacKenzie et al 1995;
The most important one is the well known outbreak in Milwakee (USA) in 1993, where more
than 400.000 people were infected.
Cryptosporidium is a protozoan parasite which can infect both animals and humans. The
parasite infects the epithelial cells of the digestive and respiratory tract causing diarrhoea.
The incubation period is around 7 days (range 1-14 days) and the disease is often selflimiting with a duration of 6-9 days. Longer duration is often found in AIDS patient where
the effect of the infection can be lethal (WHO 2009). All age group can be infected.
Infection can happen through consumption of infected water or food, from animals and
humans can give the infection to other humans (WHO 2009 and Fayer 2004). After ingestion
of a oocyte four sporozoites are released and infect the epithelial cells. Cryptosporidium only
survives outside a host in the oocyst state where it can live month in cold water (Fayer 2004).
3
ASSUMPTIONS
For the modelling, due to the given time, the next assumptions are made:
Dilution effect
Mixing between treated and untreated water is not considered: so we are dealing with a worst
case event.
Boiling is effective in killing pathogens
Campylobacter and Cryptosporidium are effectively destroyed (MMWR, 1994)
All isolates have equal chance of causing illness
All inhabitants are equally exposed
Concentrations of reference pathogens were sourced from literature
The only route of exposure is drinking water
The purpose of the exposure assessment is to determine the amount, or numbers of organisms
that correspond to a single exposure (dose) or the total amount or number of organisms that
constitute a set of exposures (Haas et al. 1999). Amongst three main routes of exposure
(ingestion, inhalation, and dermal absorption), only the ingestion route was taken into
consideration in this study as faecal-oral transmission is considered the main route of
spreading of enteric diseases. The volume of tap water ingested via drinking is much higher
than that for bathing, brushing teeth or preparing food.
Volume ingested via drinking
In this study, we used data published Teunis et al. (1997). Based on the data from
Netherlands, the human daily consumption of unboiled drinking water is an average 0.27
liter, and the 95%-interval of the consumption is 0.017 – 1.27 liter.
Fraction of population following the boil water notice
In this study, it was assumed that 100 % of the population will know about the boil water
advisory after 24 hours from the beginning of the contamination. Furthermore, it was
assumed that 80 % of the population will comply with the advisory and 20 % will not. This
assumption was based on the data presented by Karagiannis et al. (2009) were 81.8% of
population responded to comply with the advisory after a faecal contamination detected by
the presence of E. coli in tap water in Netherlands. Most respondents still used unboiled
water to brush teeth, wash salads and fruits; a result that is supported by the findings from
UK reported by O’Donnell et al., (2000) where only 38 % of the population followed the
given advices precisely. However, as there was no data available of the tap water volumes
ingested through these activities, there were excluded from the assessment.
4
SCENARIOS
Two different scenarios are chosen, a reactive and proactive scenario. The scenarios are
chosen to give an idea of the risk due to different circumstances,- knowing on beforehand
about the breakdown and not knowing.
Reactive scenario
In this scenario there is a breakdown at the water treatment plant, without knowing it on
before hand. So all the citizens will drink, for one day, polluted tap water.
Proactive scenario
The watercompany knows on beforehand that there is a increased risk of water contamination
due to the repairs. So in this scenario, boil water notice is already issued and on going when
the catastrophe and water treatment system breakdown at the waterworks occur.
5
MODELLING
The risk of infection was calculated using @RISK, applying 10,000 iterations in the Monto
Carlo simulations. The modelling is done for campylobacter and cryptosporidium.
The dose-response models used were β-Poisson model for rotavirus and EHEC infections and
the exponential model for helminth ova (Haas et al. 1999).
The equations are:
(a) β-Poisson dose-response model
(Equation 1)
where
Pinf is the risk of infection of an individual exposed to a single pathogen dose d
ID50 is the median infective dose
α is a pathogen “infectivity constant”
5.1
Campylobacter
In the next table the model input for campylobacter is given.
Parameters
Data input
Source
Distribution
value
Counts of campy in a random 1
AM de Roda Husman et Geometric
L of surface water
al 2006 Microbial water
quality of River Meuse
Data from
water at the intake of
Dutch surface
Brabantsche Biesbosch,
water
RIVM report 2006
Amount of water consumed/ 1 Min: 0.017
Teunis et al, 1997
Triangular
Medema, 1996
Fixed value
Probability of illness given Fixed value:
Nauta et al. (CARMA
Fixed value
infected
project on risk
random person
Mode: 0.27
Max: 1.27
Dose-Response model
Bata-Poisson
model
Alpha: 0.145
Beta: 7.59
0.33
assessment of
Campylobacter in
chicken meat)
Table. parameters input campylobacter model
5.2
Cryptosporidium
In the next table the input for cryptosporidium is given:
Parameters
Data input
Source
Distribution
value
Concentration of
Risk assessment of
Cryptosporidium oocysts in a
Cryptosporidium in
random 1 L of surface water
surface water - WHO
Normal
(2009)
Amount of water consumed/ 1
Min: 0.017
random person
Mode: 0.27
Teunis et al, 1997
Triangular
WHO, 2004
Fixed value
Fixed value
Max: 1.27
Dose-Response model
Beta-Poisson
model
Alpha: 0.115
Beta: 0.176
Probability of illness given
Fixed value:
Risk assessment of
infected
0.7
Cryptosporidium in
surface water – WHO
(2009)
Table. parameters input cryptosporidium model
6
RESULTS
The risk of infection was calculated using @RISK and the results are presented as probability
of infection (Pinf) per exposure and number of infected people in the studied population.
6.1 Campylobacter
In the graph the results are shown.
Number of campylobacteriosis cases (proactive model) /100,000 inhabitant
5.0%
2.0%
0.01
90.0%
10.0%
0.22
5.0%
88.0%
2.76
0.008
0.006
Number of campylobacteriosis cases
(proactive model) /100,000 inhabitant
@RISK Student Version
Number of campylobacteriosis cases
(reactive model) /100,000 inhabitant
For Academic Use Only
0.004
0.002
16
14
12
10
8
6
4
2
0
-2
0
Values in Thousands
Graph : results campylobacter
The results are also shown in the table.
Output
Mean ± SD
5% percentile
95% percentile
Probability of infection with
0.258± 0.116
0.0333
0.417
1705± 765.89
220
2756
8527± 3829
1100
13779
Campylobacter given
consumption of contaminated
dose
Number of cases (proactive
scenario)
Number of cases (Reactive
scenario)
Table. results campylobacter
From the table and the graph is shows that probability of infection, given the consumption of
contaminated water is 0,258. For the reactive scenario it means that 8.527 people, out of
100.000 people, get infected. In the proactive scenario the number of people is lower, 1.705
people will get infected from drinking the contaminated water.
6.2
Cryptosporidium
In the graph the results are shown.
Graph. results cryptosporidium
The results are also shown in the table.
Output
Mean ± SD
5% percentile
95% percentile
Probability of infection with
0.075
0.0009
0.28
1042 ± 1307
12.06
3873.25
5212 ± 6534
60.29
19366.25
0.093
Cryptosporidium given
consumption of contaminated dose
Number of cases (proactive
scenario)
Number of cases (Reactive
scenario)
Table**: results cryptosporidium
From the table and the graph is shows that probability of infection, given the consumption of
contaminated water is 0,075. For the reactive scenario it means that 5.212 people, out of
100.000 people, get infected. In the proactive scenario the number of people is lower, 1.042
people will get infected from drinking the contaminated water.
7
DISCUSSION
It was assumed beforehand that the risk for infection for those persons who will receive the
boil water advice in time and also followed the advice, the risk of infection is only theoretical
and in practice zero. We have seen important differences between the dose of both
microorganisms (Campylobacter and Cryptosporidium) that are due to the fact that we took
in to account the infectious oocysts of Cyptosporidium instead the total number.
For risk mitigation, minimizing the presence of the most hazardous microbes in the
population would decrease their prevalence in the source waters as well. In cases when the
contamination happens despite the prevention efforts, measures increasing the effectiveness
of the boil water notice may be applied: e.g. as much as possible information should be
provided at the beginning and people need to be informed also about the expected date of
lifting the notice. Example of information given can be found from the internet:
http://www.health.gov.on.ca/english/public/pub/watersafe/watersafe_boiled.html.
It is important that false alarms (boil water notices without contamination) are prevented as
much as possible as they may frustrate people and decrease the efficiency in real
contamination cases, but it is important that there is no delays in issuing the notice (no time to
wait laboratory results).
Uncertainties
Choice of organisms
In this study only the infection risk caused by Cryptosporidium and Campylobacter was
calculated. It is presumably, that following the surface water consumption the prevalence of
viral infections would have been numerous as well. Due to the limit of time, viral infections
and the effect of multiple infections were left out from this risk assessment.
Concentrations of organisms
Concentration of certain pathogens differs considerably between different water bodies and
seasons. Moreover, the prevalence of certain pathogens is varying greatly between
geographical areas and the sensitivity of persons and populations to certain infections differ
as well. In this study, quantitative data on Cryptosporidium and Campylobacter numbers in
surface waters were used.
Break down of system
The assumption made that tap water of the whole distribution system was contaminated at
equal concentrations will unlikely describe the real situation. A case of total water treatment
system breakdown should in the reality be very rare and extreme event. More probable might
be that only a small portion of water is contaminated or that the contaminant is diluted with
clean water.
Delay
Depending on the characteristics of the contamination case, duration of the consumption of
the contaminated water before the boil water notice is issued will vary. It is possible that it
will take several days before the water quality problem is discovered e.g. after complaints or
when water quality analysis shows the presence of the fecal indicator such as Escherichia
coli, instead of one day as it was in our case.
Intake
Daily intake of unboiled tap water (based on data of Teunis et al., 1997) may underestimate
the real consumption, depending on the population structure and climatic conditions.
Boiling advise
In this study, we assumed that heat inactivation at 100 °C for 3 minutes is able to completely
kill all pathogenic orgamisms from the water. However, the instructions given regarding the
length of boiling differ from 1 minute to 15 minutes. Authorities have recommended water to
be microbiologically safe after bringing it to a rolling boil for 1 minute (MMWR, 1994). The
WHO technical note for emergency recommends to boil at least 5 minutes and preferably up
to 20 minutes (WHO, 2005).
Following advise
Fraction of population exposed is also a variable with great uncertainty. For example, during
a large outbreak of cryptosporidiosis in UK, when households were advised to boil tap water
before consumption, 88% of the respondents believed that they were following the advice,
but still 20% washed food that would be eaten raw in unboiled tap water and 57% used
unboiled water to clean their teeth (Willocks et al., 2000). This means that the simple advice
to boil water may be followed only partly due lack of information or due the
misunderstandings (Mayon-White and Frankenberg, 1989). Therefore it is important that
after boil water notice is delivered by media, it is followed with a detailed instructions letter
to the households and further inquiries are handled by giving the basic information through
telephone service and via Internet.
8
CONCLUSION
For both the microorganism the risk of getting infected is higher than the acceptable level
(1/10.000). Looking into the difference between the proactive and the reactive scenario we
saw that it is very important to give all the essential information to the residents fast since a 1
day delay will cause around 5 times as many infected people.
9
PERSPECTIVES
In further analysis, the variability of contamination events (surface water/sewage, level and
duration of contamination) need to be taken into consideration. Also the uneven distribution
of contaminants in the water network need to be taken into acoount. Moreover, DALY’s
instead of probability of infection should be used and more attention is needed for risk
perception and risk communication.
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