Underreporting is hiding the true cost to local authorities of infectious intestinal disease. Alfred Barker looks at the surveillance and economic evaluation of ID

The strategic plan 2001 - 2006, recently published by the Food Standards Agency (FSA), calls for a reduction in foodborne illness of 20 per cent over the next five years.1 In order to achieve this target, which is regarded as a tough one, the FSA has committed to improving existing surveillance systems in order to monitor its performance. Underreporting of infectious intestinal disease (IID) is of particular concern, as it affects the ability of the national surveillance system to report the true number of cases. Foodborne illness, including underreporting, has also been shown to have considerable economic implications on both local authority and public sector finances.2

UNDERREPORTING
Certain bacteria, protozoa and viruses are recognised as pathogens causing intestinal disease. These IIDs are typically associated with one or more of the following symptoms: diarrhoea; abdominal pain; vomiting; nausea and fever. Pathogens associated with IID may be food or waterborne, eg salmonella or campylobacter, or acquired by other means, eg person to person contact for pathogens such as rotavirus or shigella.3 A recent FSA study3 supported the theory that routine laboratory reporting of the above pathogens detects the tip of the clinical iceberg of infection only.4 This underreporting of cases of IID is well established, and has been referred to by Sockett and Roberts (1991)5 and by the UK centre responsible for collating reporting.6 It is estimated that 9.5 million cases of IID occur annually in England (20 per cent of the population) of which 1.5 million (3 per cent) present to their General Practitioner (GP).3 The above cases may be either sporadic, or part of a general outbreak. A general outbreak is defined as: "affecting members of more than one private residence or residents of an institution". General outbreaks are distinct from family outbreaks affecting members of the same private residence only.7 Specimens tested as part of investigations into general outbreaks of IID account for only a small proportion of total laboratory reports. Reports unaccounted for by general outbreaks may be genuine sporadic cases, may arise from family outbreaks, or from unrecognised or unreported general outbreaks.

SURVEILLANCE OF IID
Nationally, there are three principal routine sources of data on foodborne disease and IID in use in England and Wales:6 - statutory notifications from clinicians of cases of food poisoning; - voluntary reports from diagnostic laboratories of laboratory confirmed infections; and - standard report forms on general outbreaks of IID submitted by consultants in communicable disease control (CsCDC) on general outbreaks of IID. Data from the first are produced by the Office of National Statistics (ONS) and the other two by the Public Health Laboratory Service (PHLS) and the Communicable Disease Surveillance Centre (CDSC). All doctors in clinical practice have a statutory duty to notify the proper officer of the local authority of cases, or suspected cases, of certain infectious diseases and of food poisoning. When a registered GP suspects that an individual is suffering from IID, they must formally notify the proper officer - usually the CsCDC. The CsCDC is responsible for the surveillance of IID within the population of the District Health Authority (DHA). That person also co-ordinates epidemiological investigations of cases and outbreaks of IID and offers medical advice to the local authority environmental health department. Upon receipt of a formal notification from a GP, the CsCDC informs the environmental health department which is responsible for the detailed epidemiological investigation. This is known as a "passive" or self-reporting surveillance system. It is significant in the underreporting of disease, since it relies upon the voluntary reporting of foodborne illness by physicians and other health authorities.8 Wheeler et al (1999) found that only one in 30 patients with symptoms of IID visited their GP each year.4 Where a patient does not present to their GP, the case will not be confirmed or included in national statistics. Warden (1998) notes that there are also discrepancies in the number of people visiting their GP because of symptoms of FBD and the number of formal notifications.9 Notification is not contingent on laboratory confirmation of infection and delaying notification until laboratory confirmation is available defeats the purpose of a rapid notification system, designed to enable effective and timely intervention at local level.10 In addition to those cases of food poisoning formally notified by clinicians, the local authority ascertains cases by use of environmental health officers, who are responsible for investigating incidents of IID and gathering epidemiological data, eg patient food histories, and establishing causal factors in the food processing system. EHOs are also responsible for the enforcement of food safety legislation. They are therefore able to instigate remedial actions which will prevent a recurrence of the incident. The factors in food production, which contributed to the IID incident need to be established. To achieve this, the EHO should inspect the suspect food process and may also collect food samples which are a possible source of the pathogen. The Advisory Committee on the Microbiological Safety of Foods (ASMSF) (1990) suggested that proving that a suspect food is responsible for a case of foodborne illness is a balance of probabilities.11

ECONOMIC EVALUATION OF IID
Estimates of national costs of salmonellosis requested by the Richmond Committee, were submitted based on certain studies.5,12 It was estimated that the costs of salmonella in England and Wales in 1992, were between £350m and £502m. The FSA study estimated that IID in England costs at least three-quarters of a billion pounds a year.3 Economic evaluation is a technique that enables efficient choices to be made about the use of resources. Although there is a long history of economic evaluations in the field of public health13 evaluations of intestinal infection are comparatively recent, stimulated by the increase in reported infections, particularly of salmonella in the late 1980s.14 The major types of economic evaluation are cost benefit analysis, cost effectiveness analysis, cost utility analysis and cost of illness studies, sometimes referred to as studies of the socio-economic burden of disease. Costs of illness studies are the earliest forms of economic evaluation and these are widely used in studies of intestinal infection.5,15,16,17,18,19,20 All costs or burdens of illness are estimated and benefits are expressed as avoided costs that would arise from an intervention to prevent or contain the disease. Studies of outbreaks include a study of milkborne salmonella in Scotland in 1981, estimated to have cost £11.3m.21 Costs of an outbreak in 1982 of S. napoli were estimated as £504,808, the intervention having saved some £1,673,826.17 As surveys are rare, estimates of national costs are usually based on projections of numbers from epidemiological sources and costs from outbreaks or from informed opinions about clinical severity. As a basis of estimating the socio-economic costs of disease, each estimation procedure has advantages and limitations and needs to be interpreted with caution. It is estimated that sporadic cases represent two-thirds of all reported infections of salmonellosis in England and Wales; projections from outbreaks might not represent sporadic cases adequately, but it is easier to estimate opportunity costs, the forgone benefits that arise because of the infection, the impact on industry and local health and community services from outbreak studies. The cost of IID is borne by local authorities and the public sector. Costing will vary for different organisms, due to the severity of illness, duration, whether or not hospitalised, days off work and the impact of morbidity on normal activities within the home or on education. Thus, underreporting of IID will have a significant effect on the overall economic evaluation of IID.3

REFERENCES
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2 Barker AV, Powell SC, (2001) "The economic impact of surveillance of foodborne disease". MSc Thesis, Manchester Metropolitan University.
3 Food Standards Agency (2000) A report of the study of infectious intestinal disease in England. The Stationery Office.
4 Wheeler J G, Sethi D, Cowden J M, Wall P G, Rodrigues L C, Tompkins D S, Hudson M J and Roderick P J (1999). "Study of infectious intestinal disease in England: rates in the community, presenting to general practice, and reported to national surveillance. BMJ 318, 1046-1050.
5 Sockett P N and Roberts J A, (1991). "The social and economic impact of salmonellosis. A report of a national survey in England and Wales of laboratory-confirmed salmonella infections". Epidemiol Infect 107, 335-47.
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11 Advisory Committee on the Microbiology Safety of Food (1990). Report of the Committee on the Microbiological Safety of Food, The Microbiological Safety of Food, Part 1. London: The Stationery Office.
12 Sockett P N (1993). "The economic and social impact of human salmonellosis in England and Wales. A study of the costs and epidemiology of illness and the benefits of prevention". PhD Thesis, University of London.
13 Cullens (1891) "A report of the study of infectious intestinal disease in England" 2: 31, Food Standards Agency.
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15 Sockett P N and Stanwell-Smith R (1986). "Cost analysis of the use of healthcare services by sporadic cases and family outbreaks of salmonella typhimurium and campylobacter infection. In Proceedings of Second World Congress Foodborne Infections and Intoxications. Volume II: 1036-39. Berlin: WHO.
16 Sockett P N and Pearson A D (1987). "Cost implications of human campylobacter infection". In Goldring N. (Ed). EAG Scientific Report. Salmonella and Listeria: implications for food safety. Surrey, UK: EAG Scientific Ltd.
17 Roberts JA, Sockett P N and Gill N (1989)."Economic impact of a nationwide outbreak of salmonellosis: cost-benefit of early intervention". British Medical Journal 296(6682): 1227-1230.
18 Roberts T (1988). "Salmonellosis control: Estimated economic costs". Poultry Science 67(6): 936-43.
19 Roberts T (1989). "Human illness costs of foodborne bacteria". American Journal of Agricultural Economics 71(2): 468-74.
20 Roberts T, and Marks S (1995). "Valuation by the cost of illness method: the social costs of Escherichia coli O157: H7 - foodborne disease". In Caswell Jad. (Ed.) Valuing food safety and nutrition. Boulder, Colorado: Westview Press. Pp 173-205.
21 Cohen DR, Porter IA, Reid TM, Sharp JC, Forbes GI, Paterson GM (1983). "A cost benefit study of milk-borne salmonellosis". Journal of Hygiene (Lond) 91(1): 17-23. Alfred V Barker BA, MSc, MCIEH, Senior Environmental Health Officer, St Helens MBC Tel: 01744 456364 e-mail: alfredbarker@sthelens.gov.uk

Acknowledgements The author would like to thank the following for their help, Dr. Susan Powell, University of Central Lancashire, for her continuing support, the staff of the Environmental Health Section, St. Helens MBC and Lorraine Barker for her secretarial support.