Discussion
To undertake a comprehensive review of the current definition of “clinically vulnerable groups”, this research has consolidated Scottish prevalence data to determine who are the clinically vulnerable groups in Scotland, with regards to foodborne illness. It was determined that despite people over the age of 65 years making up 20% of the population in Scotland, 68% of listeriosis, 31% of campylobacteriosis and 63% of norovirus cases were among older adults. Likewise, 5% of the population consist of children under the age of five, however 14 – 20% of Salmonella, STEC and norovirus cases were among this age group. Data were only available regarding listeriosis for pregnancy, cancer and rheumatoid arthritis associated cases. Additionally, interesting findings were consolidated regarding prevalence of foodborne illness among people prescribed proton pump inhibitors, medication for diabetes, or antibiotics. To enable a better understanding of foodborne illness among these groups, advancements in surveillance methods would be required.
By integrating the narrative review detailing the physiological reasoning of susceptibility with the systematic review of foodborne illness prevalence, we have provided an in-depth and comprehensive understanding of foodborne illness among clinically vulnerable groups in developed countries. In line with the data captured from Scotland, prevalence of pregnancy-associated foodborne illness, other than listeriosis, are seldom captured. Literature suggests that the immune system of a pregnant individual modulates to accommodate foetal development, and that immune response is pathogen specific, thus, the focus of capturing data on intracellular pathogens that can cross the placental barrier such as L. monocytogenes is justified.
At either end of the ageing scale groups are prone to foodborne illness. For example, children have heightened susceptibility to foodborne illness due to limited immune system development, whilst among older adults, ageing leads to progressive deterioration of the immune system. However, among older adults, frequent presence of chronic conditions and the medications used to treat such conditions add additional layers of susceptibility. Data regarding prevalence of all five pathogens of interest were obtained for children and older adults. Differences in prevalence of various foodborne illnesses among children were observed in the reviewed studies.
With regards to older adults, in Scotland, 68% of listeriosis cases were among people aged ≥65 years and 65 – 76% in reviewed studies. Prevalence of other foodborne illnesses among older adults were found to be higher in Scotland than in similar countries. For example, prevalence of campylobacteriosis was 23% in Scotland compared to 3 – 14% in similar countries. Likewise, in Scotland, 60% of norovirus infections were among older adults, while in similar countries prevalence ranged between 14 – 22%. Although trends in ageing across the UK nations suggest that Scotland has a higher median age (42 years) and a higher percentage of people aged ≥65 years (19.1%) than England (40 years, 18.4%) and Northern Ireland (38.9 years, 16.6%) (Office for National Statistics, 2020); it is reported that 21.3% of the population of the European Union was aged ≥65 years (Eurostat, 2024). Italy and Portugal were the European countries with the largest percentage of older adults, with 24% of the total population aged ≥65 years. Bulgaria, Czechia, and Finland had 23%. Ireland, Iceland, Luxembourg, and Türkiye had the lowest percentage, with 15.2 – 9.9% of their population in the ≥65 years age category (Eurostat, 2024). In the US, 16.8% of the population are aged ≥65 years (United States Census Bureau, 2023), therefore high prevalence of Campylobacter and norovirus are unlikely to be as a direct result of variations in proportions of people aged ≥65 years in Scotland. It is not known if this is due to differences in surveillance systems, consequently this warrants further exploration in future.
In this study, we have identified several clinically vulnerable groups and documented the physiological reasons that increase their susceptibility to foodborne illness. Although prevalence studies provided evidence of L. monocytogenes associated with such groups, there is a dearth of data detailing prevalence of Salmonella, E. coli, Campylobacter, and norovirus among several clinically vulnerable groups, other than children and older adults.
Therefore, based on available data, we identified groups that should be acknowledged as being clinically vulnerable to foodborne illness, including PPI users, people with diabetes, rheumatoid arthritis, cancer, and IBD. Although the prevalence of some of these conditions may be relatively low, these groups are disproportionately affected by foodborne illness particularly listeriosis.
The review of definitions used by other food safety organisations worldwide indicates that the YOPI (Young, Old, Pregnant, Immunocompromised) grouping is most frequently referred to. While some definitions acknowledge people with cancer as being susceptible, the groups disproportionately affected by listeriosis, such as PPI users, people with diabetes, peoplewith rheumatoid arthritis, people with cancer, and people with IBD, are seldom mentioned in these definitions.