FINAL REPORT:
The investigation of an outbreak of
diarrhoeal illness in participants of the
Builth Wells Mountain Bike Marathon
July 2008
Report published … December 2008.
Acknowledgements
The work of numerous environmental health professionals from Powys
County Council and other members of the outbreak control team who made
this rapid epidemiological investigation possible is gratefully acknowledged.
We would also like to record our thanks to the event organisers and
participants for their support which enabled this study to be completed rapidly.
Introduction
On Thursday 17th July 2008, the ‘feedback’ facility on the National Public
Health Service for Wales (NPHS) website received a report of a possible
outbreak of diarrhoeal illness. The report was made by a participant of the
Builth Wells Merida Bikes Mountain Bike Marathon, which had taken place on
the 5th and 6th July. The individual knew of eight people who had become
unwell following the event. The same individual also reported that there were
many others complaining of similar symptoms on a post-event internet
discussion forum. In some cases, the illness was said to have been caused by
Campylobacter.
Powys County Council Environmental Health Department had also been
informed of the cases and began an investigation. Officers spoke to the event
organiser, inspected the campsite used for the event and took samples of the
mains water supply. Efforts were also made to contact the caterer who had
provided the onsite meal service.
An Outbreak Control Team (OCT) meeting was convened between the NPHS
and Powys County Council Environmental Health Officers (EHOs) on Monday
21st July. At this meeting it was decided that a cohort study should be
conducted using an internet based questionnaire housed on the NPHS
website. The questionnaire was made available between Thursday 24 th July
and Monday 11th August and a preliminary report of the study was published
on the NPHS website on 15th August, 2008. This final report, which has been
scrutinised by members of the OCT, replaces the latter and presents the
recommendations made by the OCT.
Background
The Mountain Bike (MTB) Marathon
The Merida Bikes MTB Marathon is one of a series of six mountain biking
events held all over the country1. The event has been held in Builth Wells for
a number of years and takes part over two days. On Saturday 5th July,
organisers estimate that 130 participants completed either a 40 mile or an 80
mile road ride. On Sunday 6th July, 947 participated in an off road ride of
either 25k, 50k, 75k or 90k in length. The proportions participating in each
category were estimated to be 10%, 30%, 30% and 30% respectively.
Conditions were said to be poor on Sunday following heavy overnight rain
which caused the course to become very muddy and led to significant
amounts of mud being splashed over participants. The course was also
reported to be contaminated with sheep faeces in areas.
Feed Stations
Feed stations were available on both days. The feed stations provided riders
with a choice of drinks and food. The riders could chose from either mains
water or an energy drink solution. The latter had been made up from powder
with mains water. Both the water and energy drink were stored in large
containers and jugged out into the competitors own containers by the event
stewards. The food choice was either banana or pre-wrapped Hi 5 energy
bars. It is understood that the bars were removed from their wrappers the day
before the event and placed in a box for competitors to help themselves.
2
Two feed stations operated on Saturday, one at Cefn Gorwydd and one at
Devil’s Staircase near Tregaron. The Cefn Gorwydd feed station was used by
riders attempting both distances, acting as the second feed station in the
circuit completed by the 80 mile riders, but was the first and only feed station
for the 40 mile riders.
Three feed stations operated on Sunday. The first, sited at Pant y Llyn, was
used by riders attempting all distances. Riders reported to Environmental
Health Officers that this area of the hill was heavily contaminated with sheep
faeces. The second feed station was sited at Aberedw and was used by
participants of the 50k, 75k and 90k events. The third feed-station was sited at
Forest Fields and was only used by 75k and 90k participants.
Equipment
In addition to feed-stations, the bikers also carried their own supplies of food
and drink with them. Drinks would either be carried in bottles, usually
positioned on the A-frame of their bike, or in rucksack style camel packs
which are usually carried on the riders’ backs. Food would most often be
carried in the form of low volume, high-energy pre-packed gels or bars.
Event facilities
Many of the competitors stayed on the event site, camping on the fields of
White House Farm. The farm is a licensed Caravan Site and is served with
mains water and sewage. The catering facilities were provided by Extreme
Hospitality from Nottinghamshire. The catering for the weekend consisted of
breakfasts, burgers, cakes etc. as well as a Saturday night ‘Pasta Party’.
Campylobacter
Campylobacter is one of a group of diseases known as zoonoses, meaning
that they move into humans from an animal source2. Campylobacter is a
bacterium found in the gastrointestinal tract of birds and mammals3 4.
Transmission from animals to humans usually occurs through ingestion of
faecally contaminated food or water. The bacterium causes an acute enteritis
of varying severity, leading to symptoms of diarrhoea, abdominal pain, fever,
malaise and nausea. A third to a half of sufferers also have blood in their
stools. Most cases settle within 2-3 days, although symptoms may last up to a
week. Rarer complications include reactive arthritis, Guillan-Barré Syndrome
and haemolytic uraemic syndrome. The incubation period is usually 2-5 days
(average 3), but a range of 1-10 days has been reported. Person to person
transmission is uncommon.
Methods
A cohort study was conducted to test the hypothesis that the outbreak of
diarrhoeal illness was caused by drinking from frame mounted drink bottles
which had been contaminated with sheep faeces during the race. A case was
defined as a participant of the Builth Wells Merida Bikes MTB Marathon event
who developed gastrointestinal symptoms within 2 weeks of the event.
Information about the organisation of the event, the conditions affecting it and
symptoms experienced by participants were gathered by EHOs. This, along
with the epidemiology of Campylobacter and the results of a previous similar
outbreak5, was used to inform the design of a questionnaire. The
3
questionnaire was circulated amongst members of the OCT and selected
NPHS staff for comments. The on-line version was piloted by members of the
OCT. Paper and electronic versions of the questionnaire were also produced
for those who might have difficulty accessing the on-line version.
The EHOs obtained a list of the people who had registered for the event from
the organisers. The majority of records were held in an electronic form (Excel
Spreadsheet), but paper entry forms were the only record for a proportion.
The questionnaire, which was protected by a pass key, went live on Thursday
24th July. Information on the investigation was posted on the NPHS website
and a press release issued. E-mails inviting event participants to complete the
questionnaire were sent to all those for whom an address was held in an
electronic form (i.e. e-mail addresses from the paper entry forms were not
entered manually). The e-mail explained the reason for the study, gave a link
to the questionnaire and the pass key. The same pass key was used for all
competitors. The questionnaire could be completed anonymously. However
participants were asked to leave an e-mail address if they were prepared to
be contacted for follow up. Responses were automatically entered and stored
in a secure database.
A reminder e-mail was sent on 30th July to those who had not already
responded. An update on progress was posted on the NPHS website on
Wednesday 6th August and a final reminder e-mail sent on 7th August. The
questionnaire was taken off line at midday on Monday 11th August.
Results
664 e-mail invitations were sent in the first mailing, 476 in the second and 400
in the third. 2 paper questionnaires were sent out as well as 1 electronic
version. A total of 355 responses were received (52.7% response rate). Of
these, 8 did not participate in the event and so 347 were entered into the final
analysis.
Of those who responded, 29 participated in Saturday’s events and 344 in
Sunday’s events. Of these, 2 participated on Saturday only and 318 on
Sunday only.
161 individuals (46.5 %) reported that they had been ill. The most frequently
reported symptoms were tiredness (159), diarrhoea (151) and abdominal pain
(131). Other symptoms included fever (94), nausea (91), vomiting (31) and
blood in stools (15).
10 riders reported having stool samples which were positive for
Campylobacter.
177 entrants reported that people travelled with them who did not participate
in the event. The number of companions varied from 1 to 10. Only 6 were
reported to have been ill. This confirms that the outbreak appears to have
been limited to event participants.
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DESCRIPTIVE STATISTICS
This section gives an epidemiological description of the cohort of participants
who completed the on-line questionnaire.
Person
The cohort comprised 299 males and 45 females (3 sex not recorded). The
mean age was 36.8yrs (minimum 17, maximum 63). The mean age of the
males was 37.2 years and 33.6 years for the females. Table 1 displays
numbers affected and attack rates by sex.
Table 1: Attack rates by sex
Sex
Ill
Male
142
Female
17
Not ill
157
28
Attack rate (%)
47.5
37.8
It can be seen that the attack rate in males is higher than in females.
However, the difference is not statistically significant (p=0.22).
The participants were divided into five age groups. Table 2 details numbers
affected and attack rates by age group.
Table 2: Attack rates by age group
Age group
n
15-24
25-34
35-44
45-54
55-64
15
128
140
51
10
Ill
Not ill
7
68
62
19
3
8
60
78
32
7
Attack rate
(%)
46.7
53.1
44.3
37.2
30.0
It appears that attack rates are higher in younger participants, peaking in the
25-34 year age group. Existing immunity is likely to be a factor in this
observation.
5
Place
This section describes the distribution of illness amongst the riders who
participated in the study in relation to the events they participated in and the
feed stations they used.
Tables 3 and 4 detail the attack rates for each of the events held over the
weekend.
Table 3: Attack rates for the events held on Saturday
Race
n
Ill
Not ill
40 mile
80 mile
18
11
11
6
7
5
Table 4: Attack rates for the events held on Sunday
Race
n
Ill
Not ill
25k
50k
75k
90k
43
192
54
55
16
76
32
37
27
116
22
18
Attack Rate
(%)
61.1
54.5
Attack Rate
(%)
37.2
39.6
59.3
67.3
The attack rate on Saturday seems to be higher for the 40 mile race.
However, the numbers of respondents are small and the difference is unlikely
to be statistically significant. On Sunday, there is an increasing gradient of
attack rates with increasing distance, up to a maximum of 67.3% in the 90k
race. This might suggest a dose response effect, with those completing the
longer distances being exposed to the greatest risk.
To investigate this observation further, Tables 5 and 6 detail the attack rates
based on how long it took the bikers to complete the course.
Table 5: Attack rates for time spent completing the course on Saturday
Time to
n
Ill
Not ill
Attack Rate
complete
(%)
<2hrs
2
0
2
0.0
2 - 2.5hrs
3
2
1
66.7
2.5 - 3hrs
8
6
2
75.0
3 – 3.5hrs
4
3
1
75.0
3.5 – 4hrs
1
0
1
0.0
4 – 4.5hrs
2
1
1
50.0
4.5 – 5hrs
0
0
0
0.0
5 – 5.5hrs
7
3
4
42.9
5.5 – 6hrs
6
3
3
50.0
6 – 6.5hrs
1
0
1
0.0
6.5 – 7hrs
0
0
0
0.0
>7hrs
1
0
1
0.0
6
Table 6: Attack rates for time spent completing the course on Sunday
Time to
n
Ill
Not ill
Attack Rate
complete
(%)
<2hrs
2
0
2
0.0
2 - 2.5hrs
11
4
7
36.4
2.5 - 3hrs
13
6
7
46.2
3 – 3.5hrs
17
9
8
52.9
3.5 – 4hrs
20
14
6
70.0
4 – 4.5hrs
49
29
20
59.2
4.5 – 5hrs
51
24
28
46.2
5 – 5.5hrs
54
21
34
38.2
5.5 – 6hrs
45
13
32
28.9
6 – 6.5hrs
36
17
19
47.2
6.5 – 7hrs
22
11
11
50.0
>7hrs
20
12
8
60.0
There is no clear gradient with increasing time to compete the course on
either day, which suggests that there is not a simple dose response
association. It is possible therefore that the increased risk is associated with
exposure to a particular area. We therefore looked at attack rates by feed
station.
Tables 7 and 8 detail attack rates by feed stations used on both days.
Table 7: Attack rates by feed stations used on Saturday
Feed station
n
Ill
Not ill
1
2
1+2
Neither
Can’t
remember
9
4
10
17
4
6
2
6
10
1
3
2
4
6
3
Table 8: Attack rates by feed stations used on Sunday
Feed station
n
Ill
Not ill
1
2
3
1+2
2+3
1,2+3
None
Can’t
remember
42
48
10
111
30
59
24
11
18
29
4
45
20
30
7
5
24
19
6
66
10
30
18
7
Attack Rate
(%)
66.7
50.0
60.0
62.5
25.0
Attack Rate
(%)
42.9
60.4
40.0
40.5
66.7
50.0
28.0
41.7
Attack rates are very similar for all the feed stations used on Saturday. It must
be remembered that in our sample, most of the bikers who took part on
Saturday also competed on Sunday. It is possible therefore that any illness
experienced may have been a result of exposures on either day.
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In table 8 it can be seen that the lowest attack rate was seen for those who
did not use any of the feed stations (29.2%). The highest attack rates are
seen for feed stations 2 and 3. This observation will be confounded by the fact
that these two stations are used only by those on the longer races.
Time
Chart 1 presents the timing of onset of symptoms in our sample population.
Day 0 is Saturday 5th July. It can be seen that there is a clear peak in cases
reported between days 2 and 5. The maximum number of cases was reported
on day 3 (Tuesday 8th July). The final case was reported some 15 days after
the event on Sunday 20th July.
Chart 1
MTB Marathon Outbreak Curve
60
Number of incident cases
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Days after event
This profile is characteristic of a point source outbreak. The timing of cases is
consistent with Campylobacter infection.
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ANALYTICAL STUDY
To investigate the observational findings further, a cohort study was
conducted in order to produce relative risks of illness for a variety of
exposures. For the purposes of the study, the significance level was set at
95%, meaning that p values less than 0.05 would be accepted as being
significant. Table 9 presents the results of an analysis of the data for all
participants over both of the days. Significant results are highlighted.
Table 9: Relative risks of various exposures for all participants over both days
Exposure
Exposed
Not Exposed
RR
95%
p
CI
Ill
Not ill
Ill
Not Ill
Water from
72
72
88
113
1.14
0.910.25
station
1.43
Energy Drink
127
126
33
59
1.40
1.040.018
from station
1.89
Energy bar
97
86
63
99
1.36
1.080.009
from station
1.73
Banana from
112
134
48
51
0.94
0.740.62
station
1.20
Drink from
74
70
86
115
1.20
0.960.11
own bottle
1.50
Drink from
140
156
20
29
1.16
0.810.44
Camel pack
1.66
Other
45
32
114
152
1.36
1.070.016
food/drink
1.72
during event
Ingested mud
124
103
36
82
1.79
1.33- <0.0001
2.41
Stayed at
88
105
72
78
0.95
0.760.66
camp
1.19
Drank camp
54
58
84
93
1.02
0.790.90
water
1.30
Ate food from
96
110
42
42
0.93
0.720.59
camp
1.21
Attended
67
79
92
105
0.98
0.780.88
pasta party
1.24
RR= Relative Risk
AR=Population Attributable Risk
Importantly, no increased risk of illness was associated with residence at the
camp site, eating from the food outlet or drinking the mains water.
Additionally, there was no increased risk associated with attendance at the
Saturday night pasta party.
However, significantly increased risks are associated with factors associated
with the event itself. The greatest risk is associated with the inadvertent
ingestion of mud (Relative Risk [RR] 1.79, p=<0.0001, Attributable Risk [AR]
34.2%). Drinking an energy drink from the food station was the next highest
risk (RR= 1.40, p=0.018, AR=22.7%), followed by eating an energy bar from
the feed station (RR=1.36, p=0.009, AR=16.18%) and eating other food
(RR=1.36, p=0.016, AR=7.5). Drinking from a frame mounted bottle also
9
AR
(%)
5.6
22.7
16.1
(4.3)
7.7
12.0
7.5
34.2
(2.8)
0.60
(4.8)
(0.7)
presented a raised risk (RR=1.20) but this did not achieve significance
(p=0.11, AR=7.7). Other in-event factors (drinking from a camel pack, drinking
water from the feed station) did not present a statistically significant increased
risk. Interestingly eating a banana was associated with a reduced risk
(RR=0.94), although this was not significant (p=0.62).
The above analysis was repeated using the Saturday and Sunday competitors
separately, but it did not affect the results or the conclusions and so has not
been included.
Finally, the statistically significant factors were put into a logistic regression
model with the risk of being ill. This determines whether there is any
interaction between the factors. Table 10 displays the results.
Table 10: Logistic regression of significant factors
Risk factor
Odds Ratio
Ingestion of mud
2.72
p
<0.001
Energy Drink from
station
Bar from station
1.70
0.058
1.44
0.138
Other food/drink during
event
2.16
0.006
It can be seen that eating an energy bar and drinking energy drink from the
food stations no longer achieve significance, although the latter only just fails
to do so. However, ingestion of mud and consumption of other food or drink
remain highly significant. It appears therefore, that these two factors act
independently of one another. Overall, inadvertent ingestion of mud presents
the greatest risk.
Conclusions
It is clear that the risk factors associated with illness in this outbreak occurred
during the race and were confined to race participants. No increased risk was
associated with residence at the campsite, drinking the water at the campsite,
eating at the food outlet or attending the pasta party.
The most statistically significant risk was the inadvertent ingestion of mud.
Drinking energy drinks and eating energy bars from the feed stations was also
associated with a significantly increased risk of illness, as was the
consumption of other food and drink during the event. This picture is
consistent with the widespread contamination of hands and utensils with mud
providing a vehicle for infection. Although microbiological confirmation has not
been received for the majority of participants, the results of the study are
consistent with a point source Campylobacter outbreak.
The nature of this sport means that riding through muddy, agricultural land is
unavoidable. The risk of infection from zoonotic organisms such as
Campylobacter will therefore always be present. Clearly the weather
conditions on the day of this event compounded the problem by making
contamination by mud inevitable.
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Recommendations
Whilst it will be impossible to eliminate completely, the following actions
should be considered in order to minimise the risk of contracting
Campylobacter and other zoonotic infections in future mountain bike events.
Recommendations to participants
1. The use of obviously soiled drink and food containers should be avoided.
2. Pre-packaged food should be eaten out of the wrapper.
3. Where possible, hands and utensils should be washed before consuming
food and drink.
Recommendations to event organisers
1. No open food should be served.
2. To avoid the risk of contamination, drinks produced in large volumes for
consumption by participants should be dispensed using a method which
does not require the repeated immersion of utensils.
3. Provision of facilities to wash hands with clean, running water should be
considered.
4. Provision of facilities to wash drinks bottles with clean, running water
should be considered.
5. Wherever possible, courses should be re-routed to avoid areas which are
heavily contaminated with animal faeces.
6. Mountain bikers should be alerted to the potential risk of acquiring
zoonotic illnesses from participation in events which cross land used by
agricultural and other animals. The information would be of particular
relevance to riders who are vulnerable to infection as a result of illness or
medical treatments.
Bibliography
1 Welcome to the 2008 season of the Merida Bikes MTB Marathon Series
accessed at http://www.mtb-marathon.co.uk/ last accessed 12/08/08.
2 NPHS Infections from Animals accessed at
http://www.wales.nhs.uk/sites3/page.cfm?orgid=457&pid=25225 last
accessed 22/09/08
3 J Hawker, N Begg, I Blair, R Reintjes, J. Weinberg (2005) Communicable
Disease Control Handbook Blackwell Publishing: Oxford
4 D L Haymann (Ed) (2004) Control of Communicable Diseases Manual,
American PublicHealth Association: Washington
5 Infectious Disease News. Emerging Diseases: Campylobacter outbreak
among mountain bikers linked to mud. Accessed at
http://www.infectiousdiseasenews.com/200805/outbreak.asp Last accessed
13/8/08
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