Of 77 995 people invited in the cohort study, 8336 (10.5%) responded positively and 7033 were recruited (9.0%) (). Some people recruited near the study end date had no eligible follow-up time (n=184), and two people withdrew consent. We analysed data from 6836 participants. Compared with the 2001 census population, cohort participants were more likely to be older, female, employed in managerial and professional occupations, less deprived and in rural areas (online appendix 1). The median follow-up duration was 39 weeks (IQR 27–45 weeks); overall, 86% of the maximum achievable follow-up time to 31 August 2009 was completed. Six hundred and ten (9.5%) participants dropped out, accounting for 219 (4.5%) lost person-years of follow-up.
Recruitment in the cohort study, Infectious Intestinal Disease 2 Study, UK 2008–9.
In the GP presentation study, 2233 eligible symptomatic patients were referred and 2203 invited to participate. A total of 1392 people (63.2%) responded positively, and 1254 (56.9%) were recruited (). We excluded 140 people reporting recent foreign travel, 77 with illness lasting over 2 weeks, and 46 because of missing or inconsistent information on symptoms and/or travel. Ultimately, we analysed data from 991 cases.
Recruitment in the general practice presentation study, Infectious Intestinal Disease (IID) 2 Study, UK 2008–9.
Rate of overall IID
There were 4658 person-years of follow-up and 1201 definite IID cases in the cohort. The crude IID incidence rate was 258 cases per 1000 person-years. The age- and sex-standardised rate was 274 cases per 1000 person-years (95% CI 254 to 296). When both definite and possible cases were considered, this rose to 523 cases per 1000 person-years (95% CI 497 to 551). There was little evidence that rates varied by socioeconomic characteristics or between urban and rural areas (data not shown).
In the under-ascertainment analysis, approximately one case was recruited into the GP presentation study for every six identified in the medical records, although this varied by age group, Read code category and practice. After adjustment for under-ascertainment, there were 5546 IID cases and 312 232 person-years of follow-up, yielding a consultation rate of 17.7 per 1000 person-years (95% CI 14.4 to 21.8). This was lower than that estimated from definite cases in the cohort who reported consulting their GP for their illness (25.3 cases per 1000 person-years, 95% CI 20.7 to 31.3). The age-specific rates of GP consultation in the two studies were similar except for young children and older adults (). Among children <5 years, the rate was 133 consultations per 1000 person-years in the cohort study (95% CI 92 to 199) and 85 consultations per 1000 person-years in the GP presentation study (95% CI 59 to 122), while among those aged 65 and over, the corresponding rates were 30 consultations per 1000 person-years (95% CI 22 to 42) and 20 consultations per 1000 person-years (95% CI 15 to 27), respectively.
Age-specific rates of infectious intestinal disease general practice (GP) consultations—estimates from the cohort and general practice presentation studies, Infectious Intestinal Disease 2 Study, UK 2008–9.
Call rates to NHS Direct in England for diarrhoea and vomiting were 6.1 per 1000 person-years, similar to that estimated from cohort study patients in England who reported contacting NHS Direct for their illness (5.5 per 1000 person-years, 95% CI 3.4 to 9.5).
Rates of IID by organism
Rates of IID by organism in the community, presenting to the GP and reported to national surveillance are shown in . For organisms tested by more than one method, rate estimates are presented separately for routine diagnostic methods and for routine and PCR methods combined.
Incidence rates of infectious intestinal disease in the community and presenting to general practice by organism, Infectious Intestinal Disease 2 Study, UK 2008–9
Rates in the community
Viruses predominated among IID cases in the community: the estimated rates (cases per 1000 person-years) were 47 for norovirus, 26 for sapovirus, 13 for rotavirus and 10 for adenovirus. The most common bacteria were Campylobacter (11 cases per 1000 person-years) and enteroaggregative E coli (six cases per 1000 person-years). The Salmonella rate was less than one case per 1000 person-years. Based on ELISA, Cryptosporidium and Giardia rates were around one case per 1000 person-years, although PCR-based estimates were slightly higher.
E coli O157 was present in only one sample, and there were no cases of CDAD or L monocytogenes IID in the community cohort.
GP presentation rates
Norovirus was the most common organism among cases presenting to the GP (two consultations per 1000 person-years ()); approximately one in every 23 people with norovirus IID consulted a GP. Rotavirus and sapovirus were also common (~1.5 consultations per 1000 person-years). One in seven patients with campylobacteriosis consulted their GP, resulting in approximately one consultation per 1000 person-years based on culture diagnostics. Other organisms occurred at rates of less than one consultation per 1000 person-years. Salmonellosis was uncommon (<0.2 consultations per 1000 person-years), although one in three patients consulted their GP.
Only one case of CDAD occurred in the GP presentation study, and no cases of L monocytogenes IID were identified.
Ratios to national surveillance
presents, by organism, the rates of IID in the community and presenting to GP, and the ratios of these rates to those estimated from national surveillance data. and illustrate reporting patterns for all IID and for the four major pathogens, Campylobacter, Salmonella, norovirus and rotavirus. In each diagram, the rates in the community, presenting to GP and reported to national surveillance are represented as ellipses, with the area of each ellipse proportional to the rate.
Incidence rates of infectious intestinal disease (IID) in the community and presenting to general practice by organism, and ratios to national surveillance, IID2 Study, UK 2008–2009
Figure 4 Patterns of reporting to national surveillance for all infectious intestinal disease (IID), UK 2008–9. Black numbers represent the rates (with 95% CIs) in the community, presenting to general practice and reported to national surveillance. Red (more ...)
Figure 5 Patterns of reporting to national surveillance for Campylobacter, Salmonella, norovirus and rotavirus, UK 2008–9. Black numbers represent the rates per 1000 person-years (with 95% CIs) in the community, presenting to general practice and reported (more ...)
The ratios of community and GP presentation rates to national surveillance rates were much higher for viruses than for bacteria and protozoa. For every national surveillance case of norovirus IID, there were 12.7 GP consultations (95% CI 8.8 to 18.3) and 288 community cases (95% CI 239 to 346). The corresponding ratios for rotavirus were one in five (95% CI 3 to 7) and one in 43 (95% CI 30 to 62). By contrast, for every national surveillance case of campylobacteriosis, there were 1.3 GP consultations (95% CI 0.9 to 1.8) and 9.3 community cases (95% CI 6.0 to 14.4). For Salmonella, the corresponding ratios were 1.4 GP consultations (95% CI 0.6 to 3.3) and approximately five community cases (95% CI 1.2 to 18.2). Among the protozoa, 2.3 GP consultations (95% CI 1.0 to 5.6) and 8.2 (95% CI 2.1 to 31.7) community cases occurred for every case of cryptosporidiosis reported to national surveillance. The corresponding figures for giardiasis were somewhat higher, although there was considerable uncertainty in the estimates.
Estimated annual cases and GP presentations
In 2009, there were approximately 16.9 million cases of IID and over 1 million IID-related GP consultations. Campylobacter accounted for over 500 000 cases and approximately 80 000 GP consultations (). Norovirus caused nearly three million sporadic (non-outbreak-related) IID cases and approximately 130 000 GP consultations. The burden from sapovirus was also considerable, with an estimated 1.6 million sporadic cases and nearly 100 000 GP consultations, while rotavirus caused more than 750 000 cases and 80 000 GP consultations.
Estimated annual numbers of infectious intestinal disease (IID) cases in the community and presenting to general practices by organism, IID2 Study, UK 2008–2009