Introduction

A good starting point for any discussion on emerging foodborne pathogens is to describe what this term is taken to mean. For our purposes we have adopted a definition which draws on several earlier published versions (Morse, 1995; 2004) and classifies emerging pathogens as:

One consequence of having a relatively broad definition is that it is not always easy to decide which organisms to exclude and this is a problem we have encountered on several occasions while trying to prevent this book becoming an unmanageable tome. We have not attempted exhaustive coverage of the topic. The specific organisms or groups of organisms addressed in this book are selected for their topical relevance, their impact on public health and their value in portraying important characteristics of emerging pathogens. For some emerging pathogens such as the protozoan, Cyclospora cayetanenis, which are not covered here, or specific focus on the important topic of antimicrobial resistance in foodborne pathogens, the reader is referred to other sources (Strausbaurgh, 2000; Threlfall et al., 2000; Doyle et al., 2001; Miliotis and Bier, 2003; Molbak, 2004; Velge et al., 2005, Wassenaar, 2005, White et al., 2004).

Emergence or re-emergence of foodborne pathogens results from a combination of several factors. It depends on our ability to detect, identify and recognise new agents but also reflects the dynamic relationship between:

• the changing characteristics and distribution of pathogenic microorganisms

• changes in the people they affect and

• changes in the production and processing of foods that create new ecological niches for microbial survival and growth.

It is possible to distinguish six major trends that currently influence these factors (WHO, 2000; Käferstein and Abudssalam, 1999; Käferstein et al., 2001; Tauxe, 2002; Trevejo et al., 2005) and their impact is illustrated throughout this book.

Mass production and globalisation of the food supply

With changes in the socio-economic status of some people leading to increased demand for foods such as meat and meat products, and a growing human population worldwide, food production systems have changed drastically in the last few decades. Intensified animal production has contributed to the spread of zoonotic pathogens and contamination of foods of animal origin as well as the environment. Countries also increasingly source their food supply on a global basis and this poses an increasing problem of control of foodborne hazards. An early illustration of this was the introduction of exotic Salmonella serovars to the UK during World War II as a result of the large scale importation of dried egg. There have been numerous examples since then, a more recent example being the large outbreak of Cyclospora infection in the United States in 1996 associated with imported raspberries from South America.

When control measures for a particular hazard are implemented as part of the management strategy in one country, it can be undermined by importation from areas where these are not applied, as evidenced by outbreaks of salmonellosis caused by the importation of Salmonella Enteritidis infected eggs into the UK.

The international movement of people

This can take several forms and would include refugees from wars, social conflicts or economic hardship as well as those from more prosperous regions travelling the world for purposes of business or leisure. These have all increased in recent years and offer an alternative route for the acquisition and importation of foodborne (and other) infectious diseases. For instance, Sweden has a good record with regard to the control of indigenous salmonellosis but salmonellosis remains a significant public health problem with 90% of cases estimated to be imported.

The potential double impact of international travel on both the acquisition and transmission of foodborne illness is illustrated by the international outbreak of salmonellosis caused by a food service worker returning to her job preparing airline meals following a holiday abroad where she had contracted a salmonella infection.

The changing character of the population

Certain groups of people – the very young, the old, the very sick, and the immunocompromised, are more susceptible to foodborne (and other) infections and the proportion of the population in some of these groups is increasing, although such changes are not geographically uniform. A number of opportunistic pathogens are specific threats to these people, for instance Enterobacter sakazakii is a particular concern for premature infants or immunocompromised newborns.

The changing demographics of the developed world mean that the proportion of people described as elderly is increasing as life expectancy increases. It is estimated that by the year 2025, 25% of the population will be over the age of 60. In less wealthy countries, the population of children under 5 is particularly at risk of diarrhoeal diseases, further aggravated by the problem of malnutrition and the so-called malnutrition-infection cycle.

The number of people with a compromised immune system has also increased. HIV infection which weakens the body’s resistance to infection is one factor in this but other conditions and therapies that suppress the immune system such as cancer chemotherapy also increase susceptibility to foodborne and other infections.

Lifestyle changes

The economic and social changes responsible for urbanisation and increased travel and tourism also contribute to an increase in eating food prepared away from the home – in restaurants, from street food vendors and in canteens. Inadequate training in food safety or lack of an appropriate infrastructure to support good hygienic practices means an increased risk of larger scale outbreaks occurring. A change in the diet, as a result of international trade in food and tourism has also influenced the emergence of certain diseases in some countries. For instance, the emergence of salmonellosis in Japan can be attributed to the increased consumption (and possibly import) of meat and meat products.

Transfer of recognised pathogens into new geographic areas

This can be a result of the factors already cited which increase global movement and exchange of people and commodities, but it can also be the inadvertent consequence of other actions such as the practice of ships transporting harbour water around the world as ballast and then discharging it in new locations. This was implicated in a major outbreak of Vibrio parahemolyticus infection in Galveston in the United States with oysters harvested from near the site where tankers discharged ballast water and loaded oil destined for Japan. A similar practice is thought likely to have introduced V. cholerae into South America for the first time since the 19th century in 1991.

Microbial evolution

Evolutionary processes are more noticeable in microorganisms where marked changes in an organism’s physiology can occur in a relatively short time scale. If such changes affect the virulence of an organism it can result in the emergence of new pathogens and this is illustrated here in particular by the description of Yersinia and E.coli O157. Such changes can occur completely independently of human activity, but the latter can provide the selective pressure for the new strains to thrive.

The existence of emerging pathogens reflects the fact that the struggle against foodborne illness is in a constant state of flux as biological evolution introduces new combinations of virulence and resistance factors and social and economic developments change the milieu in which pathogenic microorganisms survive, grow and cause illness. Our ability to picture this situation and respond to new threats is dependent on the accuracy and coverage of information available from sources such as microbiological surveys and epidemiological data. It can never be comprehensive but, surprisingly, it is only relatively recently that serious attempts have been made to determine the degree to which our formal epidemiological picture of foodborne illness reflects the objective situation. Improvements in analytical methods which detect and type microorganisms and the development of tools such as microbiological risk assessment have also helped increase our understanding and ability to manage foodborne risks in recent years.

The first part of this book deals with a number of more general issues from the viewpoints of scientists studying emerging foodborne pathogens, regulators and food industry safety specialists. It describes current knowledge of the biology of how new pathogens emerge, the improvements in systems to identify enteric pathogens and recognise outbreaks that might previously have been missed and how concepts developed as part of risk analysis, such as food safety objectives, might be applied to emerging pathogens. In the second part, the ecology, epidemiology, detection and identification, and control measures are described for a selected number of specific emerging...