Incidence of Foodborne Illness and Its Risk Factors

In this article we will discuss about the incidence of foodborne illness and its risk factors.

Incidence of Foodborne Illness:

Statistics covering foodborne illnesses are notoriously unreliable. Simply quantifying the problem of those diseases initiated by infection through the gastrointestinal tract is difficult enough, but to determine in what proportion food acted as the vehicle is harder still.

Many countries have no system for collecting and reporting data on gastro­intestinal infections and even where these exist the reported data is acknowledged to represent only a fraction of the true number of cases. Studies have suggested that the ratio of actual to reported cases can be between 25:1 and 100:1.  

One should also be circumspect about using published national statistics for comparative purposes since apparent differences can often simply reflect differences in the efficiency of the reporting system. In the United States, reporting of foodborne illness outbreaks to the Center for Infectious Diseases, is not compulsory so that some States report rates 200 times those of other States.

In the early 1980s reported outbreaks of foodborne disease for the United States were roughly twice those reported by Canada which has a population only one tenth the size. It seems unlikely that Canadians are markedly more susceptible to foodborne illness or more careless about food hygiene than their neighbours; more probably the disparity reflects a higher level of under-reporting in the United States.

Some support for this appears if the statistics for all gastrointestinal disease are compared. These are a much closer reflection of the relative population sizes since these figures are officially notifiable in the USA. Such statistical problems are not unique to North America.

The WHO Surveillance Programme for Control of Foodborne Infections and Intoxications in Europe which reports data from more than 30 countries has noted the different national systems of notification and reporting.

These include:

1. notification of cases of foodborne disease without any specification of the causative agent or other epidemiologically important details;

2. reporting only laboratory-confirmed cases of foodborne disease collated by a central agency;

3. reporting cases of gastrointestinal infection which, in some cases, are regarded as being foodborne regardless of whether the involvement of food has been established;

4. reporting only cases of salmonellosis.

If all these different types of data are treated as equivalent, 276 469 cases of foodborne disease became known to official health agencies in participating countries in 1983 and 1984. This amounted to a mean overall incidence in 1984 of 38.3 cases per 100 000 inhabitants. However, because of differences in the reporting systems between countries, this figure ranges from 2.0 to 915.8 per 100 000.

Most cases of foodborne illness are described as sporadic; single cases which are not apparently related to any others. Sometimes two or more cases are shown to be linked to a common factor in which case they constitute an outbreak. Outbreaks can be confined to a single family or be more generalized, particularly when commercially processed foods are involved.

In England and Wales, information on sporadic cases of foodborne disease comes from a number of different sources. The Office of Population Censuses and Surveys (OPCS) publishes statistics on clinical cases of food poisoning which comprise notifications by medical practitioners and those cases identified during the course of outbreak investigations but not formally notified by a doctor.

Although notification is statutory, i.e. required by law, this data is acknowledged to be incomplete as a result of significant under-reporting. There is no agreed definition of food poisoning and the diagnosis is often made purely on the basis of symptoms, without recourse to any microbiological investigation which could establish both the causative agent and the food vehicle.

Similarly, it is probably significant that the league table of the most commonly reported causes of food poisoning in England, Wales and Scotland (Table 6.4) also reflects the relative severity of symptoms (with the notable exception of C. botulinum). It is reasonable to assume that the more ill you feel the more likely you are to seek medical attention and the more likely your case is to figure in official statistics.

The situation can be represented as a pyramid, where the large base reflects the true incidence of food poisoning which is reduced to a small apex of official statistics by the various factors that contribute to under-reporting (Figure 6.2).

A second source of statistics is the CDSC of the Public Health Laboratory Service which collects information under a voluntary, non-statutory reporting system from public health and hospital laboratories on isolations of gastrointestinal pathogens.

The statistics generated in this way include cases where food was not the vehicle but the pathogen was acquired by some other means such as person-to-person spread or from domestic pets.

Information on outbreaks is collected by the CDSC from microbiologists and environmental health officials around the country. Sometimes the existence of an outbreak is impossible to ignore if it involves a large number of people or a readily defined commercial or institutional context, for example a large public reception, diners at the same restaurant or passengers in the same airliner.

Sometimes the existence of an outbreak may emerge from follow-up investigations on sporadic cases. This is often possible where highly discriminating typing schemes are available that enable the pathogen strain causing an outbreak to be distinguished from strains responsible for the statistical background ‘noise’ of sporadic cases.

Even so, it is probable that many outbreaks remain undetected, submerged in the numbers for sporadic cases.

Annual reports of statistics on food poisoning and isolations of Salmonella have been published for England and Wales since 1949 (Figure 6.3) and have shown no discernible trend until the 1980s when a steady increase was apparent.

A similar but smaller increase was noted in Scotland. Nearly all European countries have reported an increase in foodborne illness since the mid-1980s (Figure 6.4), and in the United States over the period 1983-87 more cases, although fewer outbreaks, were reported.

In Austria, the rate of foodborne salmonellosis has increased from 19 cases per 100 000 inhabitants in 1985 to 62 in 1989. In Poland and Spain, the incidence of foodborne disease increased from 30 (1983) and 25 (1982) respectively, to 94 and 116 per 100 000 in 1989.

This apparent increase may be a result of improved reporting and data collection procedures, better methods of isolation and heightened awareness of foodborne illness, but is also held to reflect a real underlying upward trend in the incidence of foodborne illness.

Risk Factors Associated with Foodborne Illness:

Outbreaks of food poisoning involve a number of people and a common source and are consequently more intensively investigated than the numerous sporadic cases that occur. Valuable information is derived from these investigations about contributory factors and the common faults in food hygiene that can occur.

Specific examples will be given when bacterial pathogens are considered individually, but analysis of this information does allow a number of generalizations to be made.

The foods that are most frequently incriminated in foodborne disease in Europe and North America are those of animal origin: meat, poultry, milk, eggs, and products derived from them. This is particularly true of illness caused by Salmonella and Clostridium peifringens. Data on the association of particular foods with foodborne disease for England and Wales is presented in Figure 6.5.

The same general picture is true of most industrialized countries although the relative importance of some animal products does differ. For example, in Spain between 1985 and 1989 eggs and egg products such as mayonnaise were incriminated in 62% of outbreaks for which a cause was established.

Fish and shellfish are less commonly implicated but can be an important vehicle in some countries, often reflecting local dietary habits. Between 1973 and 1987, 20% of food-poisoning outbreaks in the USA and 10% of outbreaks in France in 1988 were associated with fish and shellfish, though for most other countries the figures are lower.

Outbreaks can result from the distribution of a contaminated food product or from situations where meals are being produced for large numbers of people. Evidence from numerous countries has shown that mass-catering is by far the most frequent cause of outbreaks, whether it comes under the guise of restaurants, hotels, canteens, hospitals or special events such as wedding receptions.

There are a number of reasons why this should be, but inadequacies of management, staff training and facilities are often identified.

Analyses of the specific failures in food hygiene that have contributed to outbreaks have been conducted on a number of occasions and results of two of these, from the United States and from England and Wales are presented in Table6.5. Comparing the two is not entirely straightforward since, in most outbreaks more than one contributory factor has been identified so that the columns do not add up neatly to 100%.

Also, the surveys differ in the categories used and even where they are nominally the same they may still not be equivalent in all respects. Even so, inspection of Table 6.5 reveals two major contributory factors; temperature and time.

Failure to cool foods and hold them at temperatures inimical to microbial growth, or to heat them sufficiently to kill micro-organisms, coupled with prolonged storage giving micro-organisms time to multiply to dangerous levels. An interesting difference between the two sets of data is the lower incidence of infected food handlers contributing to illness in England and Wales.