# The Third Study of Infectious Intestinal Disease (IID3 Study) in the Community: Protocol for UK-Based Prospective Cohort Studies Investigating the Disease Burden

## TL;DR

This study aims to better understand the true burden of infectious intestinal disease in the UK by tracking cases in the general population and healthcare settings.

## Contribution

The study introduces updated methods and modern diagnostics to more accurately estimate the disease burden and underreporting of infectious intestinal disease in the UK.

## Key findings

- The study will compare incidence rates of infectious intestinal disease across three population-based cohorts.
- Molecular assays will detect a wider range of pathogens than standard NHS testing.
- Results will help revise national surveillance and inform public health policy.

## Abstract

There is a significant hidden burden of infectious intestinal disease (IID) in the UK community, which has increased over time. In the late 2000s, the Second Study of Infectious Intestinal Disease (IID2 study) estimated 17 million IID cases annually in the United Kingdom. However, only a small proportion of cases present to health care, and even those are often not tested for causative organisms.

The Third Study of Infectious Intestinal Disease (IID3 study) aims to determine the IID burden in the UK community, estimate the underreporting level in routine practice and the general population, and recalibrate UK national surveillance based on the new incidence rates.

We will follow methods of previous studies, along with modern pathogen detection methods and digital platforms for recruitment and follow-up. Participants will be recruited to three population-based prospective cohorts: cohort 1 (the general population), cohort 2 (patients with IID presenting to general practices [GPs]), and cohort 3 (enumeration study of IID cases presenting to GPs). Microbiological analysis of stool samples in cohorts 1 and 2 will include testing for a wide range of causative organisms using molecular assays, including pathogen targets not routinely sought by National Health Service (NHS) laboratories. Additional characterization of pathogens will be conducted at national reference laboratories. The incidence rates of IID and organisms detected in cohorts 1-3 will be compared to national surveillance systems, both laboratory and syndromic. Descriptive statistics and analysis will allow comparison of IID rates within each cohort, estimate the overall burden of disease caused by different pathogens, and compare findings to earlier IID studies.

A favorable ethical opinion was obtained from the UK Health Research Authority on August 4, 2022. A pilot phase to test the sampling process was conducted from January to August 2023. Participant recruitment commenced on September 1, 2023, for cohort 2 and on March 16, 2024, for cohort 1; recruitment ceased on August 31, 2025. Data collection is complete, and data analysis is to begin. The study is expected to end in September 2026.

Since the first and second IID studies, changes have occurred within national surveillance systems, the NHS structure, and public recommendations about when to consult a GP and where to seek health care advice, which may have altered the extent of IID reporting and the perceived burden in the community, creating greater uncertainty about the representativeness of IID rates. The IID3 study results will provide insight into trends in disease incidence over time and help quantify inequalities in IID in the UK community. Revised estimates can inform policy related to prevention, including food standards and disease management. Furthermore, advances in molecular diagnostics will significantly enhance pathogen detection, increasing our understanding of the causes of IID.

DERR1-10.2196/88759