Surveillance of foodborne disease in the United Kingdom

“Infectious intestinal disease occurs in one in five people each year, of whom one in six presents to a general practitioner.” So wrote Wheeler and colleagues in 1999 (Wheeler et al, 1999). This translated into 9.4 million people suffering from infectious intestinal disease (IID) annually, with around 1.5 million people consulting their GP.

Though a significant proportion of IID is foodborne, it is also transmitted through other routes i.e. person-to-person, animal-to-person and environment-to-person (including through water). Two of the most commonly asked questions are “what proportion of IID is foodborne?” and “what foods contribute to the burden of foodborne disease?” These have proven surprisingly difficult to answer. Part of the reason for this is that the surveillance systems for IID tend not to attribute clinical symptoms or microbiological findings to routes of transmission.

Surveillance of Infectious Intestinal Disease

There are several routinely available sources of data on infectious intestinal disease in the UK:-

• Food poisoning notifications: Every clinician in the United Kingdom has a legal obligation to notify suspected clinical cases of food poisoning to the Proper Officer of the Local Authority. However, from a clinical point of view it can be very difficult to distinguish food poisoning from, e.g. viral gastroenteritis, which is unlikely to be foodborne, since the symptoms can be similar. So the clinician is being asked to make a diagnosis based on symptoms and transmission route. It seems that few clinicians respond to their legal duty and notify too little and too late (Harvey, 1991; Durrheim & Thomas, 1994; Day & Sutton, 2007). These statistics are generally regarded as the least reliable in describing the burden of foodborne disease.
• Surveillance of laboratory reports from clinical microbiology laboratories: Viewed, in the main, as one of the more robust methods of surveillance, laboratory reports of microbiologically-confirmed IID are collated by the Health Protection Agency (HPA) in England, Health Protection Scotland (HPS), the National Public Health Service NPHS) in Wales and the Communicable Disease Surveillance Centre (CDSC) in Northern Ireland. Laboratory report surveillance is passive, relying on voluntary electronic reporting by clinical laboratories to these national centres. As well as underlining the public health impact of IID Wheeler and colleagues (1999) calibrated the national surveillance system in England. They estimated the factor by which the number of cases of infection with specified pathogens needed to be multiplied to establish the actual number of infections in the community. They showed that for every case of IID reported to national surveillance, 136 cases had occurred in the community. For campylobacters the ratio of disease in the community to reports to national surveillance was approximately eight to one and for salmonellas the ratio was around three to one (Wheeler et al, 1999). A limitation of laboratory report surveillance is that the data underestimates the true burden of disease in the community. However, these long-running systems yield useful trend information since they have been established for many years and those laboratories that contribute to the schemes continue to report to them.
• Surveillance of general outbreaks of infectious intestinal disease: All four countries in the United Kingdom collect information on general outbreaks of IID once investigations are completed. General outbreaks are those which affect members of more than one household. Family outbreaks are excluded from the schemes and one of the reasons is that it can be difficult to distinguish co-primary cases (i.e. those that might have been infected at the same time from the same source like contaminated food) from secondary cases (i.e. those where spread has occurred from one person to another and not through contaminated food). As well as including details such as the time and place of the outbreak, number of cases affected, admitted to hospital and died, these schemes capture the mode of transmission. If an outbreak is foodborne the investigator also details the food vehicle(s) implicated, the types of evidence used to identify the food vehicle(s), and the food handling faults thought to have contributed to the outbreak (which are based on the 4Cs). Hughes and colleagues (2007) recently published an overview of foodborne outbreaks of IID in England and Wales as part of a major investigation into Breakdowns in Food Safety. They report that between 1992 and 2003, 7620 general outbreaks of IID were reported to the Health Protection Agency, 1729 (23 per cent) of which were foodborne. In total, 39,625 people were affected and there were 68 deaths. Over half of all foodborne outbreaks were due to Salmonella sp. The proportion of outbreaks due to Salmonella sp., Clostridium perfringens and Vero cytotoxin-producing Escherichia coli O157 decreased over time, whereas the proportion of outbreaks attributed to Campylobacter sp. increased.
• Other sources of data: Though the three data sources described above are used most often to monitor IID several other data sources are also employed. The Royal College of General Practitioners Weekly Returns Service (WRS) operates a sentinel surveillance system through a network of 78 General Practices, providing weekly data on major illnesses diagnosed in general Primary Care across England and Wales.According to the WRS the incidence of IID has declined sharply since 1998 (Fleming & Elliott, 2006). Symptom-based surveillance through NHS Direct and NHS24 is co-ordinated by the Health Protection Agency West Midlands Regional Surveillance Unit. Data from the WRS and NHS Direct/NHS24 are reported routinely in the Health Protection Report. The HPA also collaborates with QRESEARCH, developed by the University of Nottingham in collaboration with Egton Medical Information Systems Ltd, to provide regular reports on the incidence of diarrhoea and vomiting (Smith et al, 2007). Finally hospital episode statistics and mortality statistics give insights into more severe cases of IID that are admitted to hospital and/or die as a result of their illness. Both these sources of information are limited by coding inaccuracies and neither tend to contain very detailed information on causative organisms, save for salmonella deaths.

Recent trends in infectious intestinal disease

Recent trends in some of the major causes of infectious intestinal disease in England and Wales are shown in Figure 1. Campylobacter continues to dominate as the most commonly reported cause of bacterial IID. The great success story is salmonella infection where the incidence has dropped dramatically since 1997. This is largely attributed to the controls in place in the UK to reduce the risk of salmonella infection from hens’ eggs. Salmonella Enteritidis phage type 4, which overshadowed most other phage types since its emergence in the UK in the early 1980s, has dropped dramatically since vaccination of breeder and layer flocks was introduced alongside other controls (Ward et al, 2000; Gillespie et al, 2005). This is a public health triumph. Worryingly, however, the incidence of Listeria monocytogenes infection has increased in recent years (Figure 2). This is predominantly affecting the elderly and the reason for the increase is, as yet, unclear (Gillespie et al, 2006). Recent outbreaks have been linked to sandwiches, particularly hospital sandwiches.

How much infectious intestinal disease is foodborne?

With myriad sources of information about IID the challenge is how to assemble this data to provide good intelligence on the burden of foodborne disease and the foods contributing to illness. In a groundbreaking study in the US, Mead and colleagues (1999) rationalised several sources of surveillance data to derive a point estimate of the burden of foodborne disease in the United States (US). They estimated that there were some 76 million illnesses each year leading to 325,000 hospital admissions and 5,000 deaths. Adak and colleagues (2002) modified slightly the Mead method to describe the burden and trends of UK acquired foodborne disease (IFD) in England and Wales. They estimated that in 2000 there were over 1.3 million cases of foodborne disease resulting in nearly 21,000 hospital admissions and almost 500 deaths. The most important pathogens were campylobacters, salmonellas, Clostridium perfringens, Verocytotoxin producing Escherichia coli (VTEC) O157, and L. monocytogenes. They concluded that reducing the overall level of UK-acquired foodborne disease in England and Wales would mean tackling campylobacter infection. However, lowering mortality rates also means better control and prevention of salmonellas, C. perfringens, L. monocytogenes, and VTEC O157.

What are the foods that contribute to the burden of foodborne disease?

The burden estimate for foodborne disease in England and Wales provided the foundation for attributing the proportion of illness to various types of contaminated food (Adak et al, 2005). The most important cause of UK acquired foodborne disease was shown to be contaminated chicken causing over 398,000 cases of illness, with a risk expressed as cases per million servings of 111, and around 140 deaths. Red meat (beef, lamb, and pork) contributed heavily to deaths whilst eating shellfish was associated with the highest disease risk, reflecting large numbers of cases despite lower levels of consumption. Pre-harvest contamination of oysters with norovirus generated a large number of illnesses. However, reducing the impact of UK acquired foodborne disease still depends mainly on controlling contamination of chicken with campylobacters and non-typhoidal salmonellas. Although by no means perfect these data have, for the first time, allowed us to partition the burden of foodborne disease according to food type.

Conclusion

At its inception one of the Food Standards Agency’s key priorities was reducing foodborne illness by 20 per cent. Knowing not only how much illness exists in the community, but also the contaminated foods that contribute to that burden, should allow the Food Standards Agency to refine further its disease reduction strategy.

References

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