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Jeremy G Wheeler a London School of Hygiene and Tropical
Medicine, London WC1E 7HT, b Scottish Centre for Infection and Environmental Health,
Ruchill Hospital, Glasgow G20 9NB, c Food Safety Authority of
Ireland, Lower Abbey Street, Dublin 1, Republic of Ireland, d Leeds Public Health Laboratory, Leeds LS15 7TR, e Centre
for Applied Microbiology and Research, Porton Down, Salisbury,
Wiltshire SP4 0JG, f Southampton
University, Southampton SO6 6YD
Correspondence to: Mr
Wheeler j.wheeler{at}lshtm.ac.uk
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Abstract |
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Objective:
To establish the incidence and aetiology
of infectious intestinal disease in the community and presenting to
general practitioners. Comparison with incidence and aetiology of cases
reaching national laboratory based surveillance.
Design:
Population based community cohort incidence study, general practice based incidence studies, and case linkage to
national laboratory surveillance.
Setting:
70 general practices throughout England.
Participants:
459 975 patients served by the
practices. Community surveillance of 9776 randomly selected patients.
Main outcome measures:
Incidence of infectious
intestinal disease in community and reported to general practice.
Results:
781 cases were identified in the community cohort, giving an incidence of 19.4/100 person years (95% confidence interval 18.1 to 20.8). 8770 cases presented to general practice (3.3/100 person years (2.94 to 3.75)). One case was reported to national surveillance for every 1.4 laboratory identifications, 6.2 stools sent for laboratory investigation, 23 cases presenting to
general practice, and 136 community cases. The ratio of cases in the
community to cases reaching national surveillance was lower for
bacterial pathogens (salmonella 3.2:1, campylobacter 7.6:1) than for
viruses (rotavirus 35:1, small round structured viruses 1562:1). There
were many cases for which no organism was identified.
Conclusions:
Infectious intestinal disease occurs in
1 in 5 people each year, of whom 1 in 6 presents to a general
practitioner. The proportion of cases not recorded by national
laboratory surveillance is large and varies widely by microorganism.
Ways of supplementing the national laboratory surveillance system for
infectious intestinal diseases should be considered.
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Key messages
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Introduction |
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Infectious intestinal disease causes substantial morbidity and economic loss in the United Kingdom and is responsible for over 300 deaths and 35 000 hospital admissions annually in England and Wales. 1 2 Food poisoning notifications and laboratory reports of pathogens responsible for infectious intestinal disease have been rising since the early 1980s, and public awareness has risen following several large outbreaks, culminating in 1996 in the outbreak of Escherichia coli O157 infection in Scotland.3 However, infectious intestinal disease can arise from various sources, of which food is only one.
The national surveillance system provides information about trends in incidence and outbreaks of infectious intestinal disease. 4 5 Sources of data include voluntary reporting of organisms identified by public health and other diagnostic microbiology laboratories and reports of general outbreaks of infectious intestinal disease. The Public Health Laboratory Service Communicable Disease Surveillance Centre collates these data. National surveillance inevitably underestimates disease occurring in the community and seen in primary care. Many people do not seek medical attention, and of those that do only a proportion will have a stool specimen submitted for investigation. Not all of these stools will yield a pathogen, and not all pathogens identified are reported to the Communicable Disease Surveillance Centre. Because presentation rates and the sensitivity of laboratory identification vary according to the pathogen, the spectrum of pathogens reaching national surveillance may be different from that causing disease in the community. National surveillance data may also overestimate the proportion of cases in certain age groups or those who are part of outbreaks.
We studied the incidence and aetiology of cases of infectious
intestinal disease presenting to general practitioners and in the
community and how these related to national surveillance (laboratory reports). The study does not address under-reporting of food poisoning through statutory notifications.
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Participants and methods |
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The methods have been described in full elsewhere.6 The study was set in 70 general practices serving a total population of 459 975. The practices were volunteers selected from the Medical Research Council's general practice research framework to meet specific criteria. Criteria were chosen to make the sample representative of all general practices nationally with respect to geographical location, urban and rural characteristics, and social deprivation index.6 Practice recruitment was staggered over 18 months, and each practice participated for a complete year. Data were collected between 1993 and 1996. Approval was obtained from the Royal College of General Practitioners, participating research bodies, and all local research ethics committees.
The study estimated the incidence of infectious intestinal disease at five levels (community case, case presented to general practitioner, stool sent for test, positive test result, and reported to national surveillance).
Community cohort
We selected at random 200 people of all ages from each
practice list by obtaining computer files of the age-sex registers and
running a random selection program which stratified by age and sex. All
those selected were invited to participate by letter and telephone.
People who agreed to participate returned weekly postcards for six
months declaring the absence of symptoms. Those with symptoms sent a
stool specimen from home to Leeds Public Health Laboratory; the case
definition is described elsewhere.6 A second cohort was
recruited for another six months.
General practice incidence study
All cases of infectious intestinal disease presenting to
general practitioner were eligible. The practice research nurse
recorded details on each case. Patients of all ages were included. Each
practice was randomly allocated to one of two arms. In the first arm
general practitioners asked all patients to provide a stool specimen
for investigation at Leeds Public Health Laboratory (34 practices). In
the enumeration arm the doctors' decision to request stool testing
locally was observed without intervention (36 practices). Patients in
the enumeration arm who had positive stool samples were sought in the
national surveillance database by using names, dates of birth, and
laboratory reference numbers.
Stool investigations
Stools were tested at Leeds Public Health Laboratory and
public health laboratory service reference laboratories for a wide
range of target organisms and bacterial toxins (table 1).
Investigations were generally more extensive than those used in routine
diagnostic laboratories. Selective and enrichment culture techniques
were used for bacteria, except enterovirulent E coli, which were detected by DNA probes. Microscopy was used for protozoa and
helminths and electron microscopy and commercial enzyme linked immunosorbent assays (ELISA) for detection of viruses. Microbiological methods are detailed elsewhere. 5 7
Statistical analysis
We calculated the incidence in the community using
the number of person weeks of follow up of the two consecutive cohorts
as the denominator and the incident cases as the numerator. Data were
excluded from analysis if follow up infromation was incomplete. In the
general practice component the denominator was the practice population
adjusted for list inflation and combined with the exact period of
practice participation (generally one year). Estimates of list
inflation were derived from the community cohort study sampling frames,
where the proportion of patients invited to participate who had died or
moved away was recorded.6 The numerator was the number of
cases presenting to the general practitioner adjusted for suspected
underascertainment
that is, failure to report a case. We
conducted a detailed study of underascertainment to estimate the size
of this adjustment. A researcher independently visited half the
practices and compared computerised diagnostic records with case
ascertainment details from the general practice incidence
study.8
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Results |
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Community incidence
A total of 9776 people were recruited to the cohort
(average of 140 in each practice) with a total follow up of 4026 person
years. The response rate was 40% (9776 of the 24 399 invited; follow
up information complete for 9296), and 82% (7623) of
participants returned over 22 of the 26 weekly postcards. We
ascertained 781 cases of infectious intestinal disease, an incidence of
19.4/100 person years (95% confidence interval 18.1 to 20.8) (table
1).
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General practice incidence
We ascertained 8770 cases presenting to general practitioners, a rate of 3.3/100 person years (2.94 to 3.75) after list
inflation and underascertainment were corrected for. The uncorrected
figure was 1.91 (1.70 to 2.14).
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Discussion |
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Community rates
This study establishes the incidence of infectious intestinal disease in a large, representative population in England. We
found that 1 in 5 people in the general population develop such disease
each year, an estimated 9.4 million cases in England annually. Earlier
studies in North America11-13 found higher community rates but were family based and may reflect higher rates in children and parents compared with other groups. Although case definitions vary,
our result is similar to that found in a recent Dutch
study14 but lower than those of other studies in England
and Wales. 15 16
that is, the tendency to
"telescope" illness events into the recent past. We believe the
prospective, negative reporting method that we used did not underestimate incidence for two reasons: firstly, completeness of
follow up was good,6 and, secondly, participants were
unlikely to send a postcard denying symptoms if they had them. Although the recruitment rate was not high (40%), partly due to difficulties in
contacting people on practice lists, it was comparable with that in
similar studies.14 Moreover our cohort was large and broadly represented the national population in terms of age, sex, and
social class.6
General practice rates
About 1 in 30 patients presented to their general
practitioner with infectious intestinal disease in a year. The rate is
similar to those reported in single practice studies in England,
17 18 a recent study in four practices in
Wales,16 and estimates derived from the Royal College of
General Practitioners sentinel surveillance scheme.19
Reporting pyramid
We estimated that for every case detected by national
laboratory surveillance, there are 136 in the community. One potential
bias in this estimate is that we could not ensure that our sample
contained a representative number of outbreaks. Outbreaks may have been
underrepresented in the community sample because we excluded
residential homes, prisons, universities, and long stay
hospitals
sites where outbreaks are shown to
occur.21 We did, however, include schools. However,
general outbreaks constitute less than 10% of laboratory reports of
salmonella and less than 1% of reports of
campylobacter.22
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Acknowledgments |
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We thank Professor T W Meade and the Medical Research Council EMCU staff, and M Goldsborough, A Williams, L Hands, E Marshall, P Allen, F Symes, and J Elwood and all the participating practices. A list of participating general practices is available on the BMJ's website.
Contributors: JGW contributed to study design, coordinated the fieldwork, analysed the data. DS coordinated the fieldwork and helped in surveillance data linkage, JC contributed to study design, PGW helped with surveillance data linkage, LCR contributed to design and analysis of the study, DST and MJH did the microbiology, and PJR helped with design and coordinating fieldwork. JGW wrote the paper with core contributions from all authors. All authors will act as guarantors.
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Footnotes |
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Members of the study executive and particpating practices are listed on the BMJ's website
Funding: Department of Health.
Competing interests: None declared.
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References |
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(Accepted 18 February 1999)
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