Evolving public health approaches to the global challenge of foodborne infections.

The landscape of foodborne infections is in flux. New pathogens emerge, established pathogens may acquire new characteristics and appear in unexpected food vehicles, while many existing problems remain unsolved. Consumers want more fresh foods year round, populations age and migrate, and the technologies and trade practices that produce foods change. Protecting the public health and minimizing the burden of foodborne illness mean expecting the unexpected, and being prepared to understand it when it occurs, so that prevention can be improved. Public health surveillance is also constantly evolving, as new diseases emerge and are judged worthy of notification, as new diagnostic tests change the ease and specificity of routine diagnosis and as social interest in particular issues waxes and wanes. Accurate health information, including reliable estimates of the burden of foodborne disease, can improve foodborne disease prevention, foster global health security, promote economic growth and development and strengthen evidence-based policy making.

Emerging food-borne zoonoses.

Diarrhoeal diseases, almost all of which are caused by food-borne or waterborne microbial pathogens, are leading causes of illness and death in less developed countries, killing an estimated 1.9 million people annually at the global level. Even in developed countries, it is estimated that up to one third of the population are affected by microbiological food-borne diseases each year. The majority of the pathogens causing this significant disease burden are now considered to be zoonotic. The occurrence of some of these zoonotic pathogens seems to have increased significantly over recent years. The factors involved in such increases have not been well studied, but they are generally agreed to include changes in animal production systems and in the food production chain. Both types of changes can cause corresponding changes in patterns of exposure to the pathogens and the susceptibility pattern of the human population. This paper will not attempt a more in-depth analysis of such factors. The authors briefly describe five of the most important emerging food-borne zoonotic pathogens: Salmonella spp., Campylobacter spp., enterohaemorrhagic Escherichia coli, Toxoplasma gondii and Cryptosporidium parvum. The paper does not include a full description of all important emerging food-borne pathogens but instead provides a description of the present situation, as regards these globally more important pathogens. In addition, the authors describe each pathogen according to the new framework of a Food and Agriculture Organization (FAO)/World Health Organization (WHO) microbiological risk assessment, which consists of hazard identification and characterisation, exposure assessment and risk characterisation. Moreover, the authors provide a brief account of attempts at risk mitigation, as well as suggestions for risk management for some of these pathogens, based on thorough international FAO/WHO risk assessments. The authors emphasise the importance of science-based programmes for the continued reduction of pathogens at relevant points of the 'farm-to-fork' food production chain, as this is the only sustainable basis for further reducing risks to human health in the area of preventable food-borne diseases.

Quantitative risk assessment of Listeria monocytogenes in ready-to-eat foods: the FAO/WHO approach.

Quantitative microbiological risk assessment is a very new and unique scientific approach able to link, for the first time, data from food (in the farm-to-fork continuum) and the various data on human disease to provide a clear estimation of the impact of contaminated food on human public health. The Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) have recently launched risk assessment studies of a number of pathogen-food commodity combinations (Salmonella in eggs and in broiler chickens, Listeria monocytogenes in ready-to-eat foods, Campylobacter in broiler chickens, Vibrio in seafood) to be used to lower the risk associated with these food-borne diseases and ensure fair practices in the international trade of food. The FAO/WHO Listeria risk assessment was undertaken in part to determine how previously developed risk assessments done at the national level could be adapted or expanded to address concerns related to L. monocytogenes in ready-to-eat foods at an international level. In addition, after initiation of the risk assessment, the risk assessors were asked by the Codex Committee on Food to consider three specific questions related to ready-to-eat foods in general, which are: (1). estimate the risk for consumers in different susceptible populations groups (elderly, infants, pregnant women and immunocompromised patients) relative to the general population; (2). estimate the risk for L. monocytogenes in foods that support growth and foods that do not support growth under specific storage and shelf-life conditions; (3). estimate the risk from L. monocytogenes in food when the number of organisms ranges from absence in 25 g to 1000 colonies forming units per gram or milliliter, or does not exceed specified levels at the point of consumption. To achieve these goals, new dose-response relationships and exposure assessments for ready-to-eat foods were developed. Preliminary data indicate that eliminating the higher dose levels at the time of consumption has a large impact on the number of predicted cases.

Emerging food-borne pathogens.

The epidemiology of foodborne diseases is changing. While in many developing nations the efficient treatment of diarrhoeal diseases through oral rehydration has probably led to the prevention of many food related deaths, the underlying problems have not been solved. In these countries, the disease incidence of food-and waterborne disease is still increasing, and now data from other parts of the world indicate that new foodborne pathogens have emerged as important public health problems. Over the last two decades, bacterial infections caused by Campylobacter and enterohaemorrhagic Escherichia coli have emerged, well-recognized pathogens, such as Salmonella eoteritidis, have increased dramatically in disease incidence and important foodborne pathogens have become increasingly resistant to antimicrobial agents. The future strategy for prevention of foodborne disease should be founded in scientifically based evaluations of the whole food production chain 'from farm to table', including a description of the most important (risk) factors. Epidemiological studies of outbreaks as well as sporadic cases should be aimed at a better understanding of these factors. In terms of public health importance of the problem, the focus should be on the size of the problem, and the potential for improvement. The indications are that both in developed and developing countries there is significant potential for an improvement of the situation. With this aim in mind, international effort should focus on scientific assessments of the potential for risk reduction under different regional conditions.

Comparison of microbiological risk assessment studies published.

The FAO/WHO risk analysis framework and principles are in the process of being implemented in different national and international settings. The importance of a further development of international co-operation based upon agreed principles in this area is stressed in the Codex Alimentarius system, as well as under the WTO/SPS agreements. Key input in this context will come from the presentation of formalised microbiological risk assessments. The number of peer-review published microbiological risk assessments is low and the format is still variable. This paper presents a comparison of selected risk assessments in the microbiological area, and suggests a number of lessons to be learned from these.

Incidence and control of Listeria monocytogenes in foods in Denmark.

The Danish regulatory policy on Listeria monocytogenes in foods is based on the principles of HACCP and was developed using a health risk assessment approach. The Danish policy focuses examinations and criteria for L. monocytogenes in ready-to-eat foods and is based on a combination of inspection and product-testing. Based on current epidemiological information from several countries, a concentration of L. monocytogenes not exceeding 100 cfu/g of food at the time of consumption, seems to be of low risk to the consumers. In Denmark, ready-to-eat foods have been placed into six categories where absence of L. monocytogenes in 25 g is required in foods heat treated in the final package and in heat-treated as well as preserved, non heat-treated foods which can support growth within the shelf life. This level is necessary in foods capable of supporting growth, in order not to exceed 100 L. monocytogenes per g at the point of consumption. In heat-treated and preserved foods, which are not supportive of growth within the shelf-life and for raw, ready to eat foods, a level below 10 L. monocytogenes per g is regarded acceptable. A level between 10 and 100 L. monocytogenes per g is not satisfactory and a level above 100/g is not acceptable. Data on the qualitative and quantitative occurrence of L. monocytogenes in foods in Denmark are presented and discussed. In 1997 and 1998, greater than 15,000 samples from different categories of food were examined (semi-quantitatively) for the presence of L. monocytogenes. A significant difference could be seen in the number of samples containing more than 100 L. monocytogenes per g, between different categories of foods (1997, P = 0.001; 1998, P = 0.016). In 1997, preserved meat products and preserved fish products and to a lesser extent vegetables and meat or vegetable mayonnaise were more likely to contain high numbers (i.e. above 100 cfu/g) of L. monocytogenes than other food categories. In 1998, preserved meat products, but also heat-treated meat products, vegetables and meat or vegetable mayonnaise had the highest frequency of samples with > 100 L. monocytogenes per g. In a survey performed in 1994 and 1995, 1.3% of ready-to-eat food samples (heat-treated meat products, preserved meat and fish products) were found to be contaminated with L. monocytogenes at a level above 100 cfu/g. The samples included in this survey were primarily products produced by authorized companies and were comprised mainly of vacuum packed products or products packed in modified atmosphere and with long shelf lives, typically above several weeks. The corresponding percentages of positive samples primarily processed in the retail outlets (heat-treated meat products, preserved meat and fish products) in 1997 and 1998 were 0.3% and 0.6%, respectively. The results suggest that ready-to-eat meat and fish products with extended shelf-lives produced by authorized companies are more likely to contain high numbers (> 100 cfu/g) of L. monocytogenes than products processed in the retail sector which often have a shorter shelf life.

Development and testing of improved suicide functions for biological containment of bacteria.

We have developed very efficient suicide functions for biological containment based on the lethal Escherichia coli relF gene. The suicide functions are placed in duplicate within a plasmid and arranged to prevent inactivation by deletion, recombination, and insertional inactivation. The efficiency of this concept was tested in a plasmid containment system that prevents transfer of plasmids to wild-type bacteria. Protection against plasmid transfer was assayed in test tubes and in rat intestine. Protection was efficient and refractory to inactivation by mutation and transposons. The efficiency of the suicide system was also tested in soil and seawater. We show that unprecedented suicide efficiency can be achieved in soil and seawater after suicide induction by IPTG (isopropyl-beta-D-thiogalactopyranoside). More than 7 orders of magnitude reduction in suicide bacteria was achieved.

Use of indicators for monitoring the reduction of pathogens in animal waste treated in biogas plants.

The possibility of including bacterial indicators in programmes for monitoring the disinfection of animal waste treated in biogas plants was examined. Five full-scale plants receiving waste from several farms were monitored. Of these, two used thermophilic and one mesophilic digestion exclusively whereas two employed a two stage process involving thermophilic pretreatment followed by mesophilic digestion. For each plant the reducing effect on bacterial pathogens present in the incoming waste and on E. coli and enterococci indigenous to the waste was followed over long periods of time (six to 16 months). It was found that both E. coli and enterococci could be used as indicators to monitor the reduction of vegetative bacterial pathogens, but the use of enterococci would give the broadest "safety margin" in this respect. It is concluded that thermophilic as well as mesophilic digestion with thermophilic pretreatment will result in a sufficient reduction of both vegetative pathogenic bacteria and intestinal parasites occurring in concentrations usually found in animal waste to allow for unrestricted use of the de-gassed material in agriculture. This cannot be said for mesophilic digestion used alone. To achieve a sufficient reduction in the number of pathogens during the digestion process, a decrease in the enterococcal concentration of at least 3-4 log10 units should be accomplished, which for normal waste would correspond to a maximum concentration of these bacteria in treated material in the order of 10(2) cfu/ml. It is suggested that enterococcal counts in newly digested material should be one of the parameters in a monitoring programme aimed at controlling the hygienic state of the end-product from large biogas plants.

A comparison of the efficiency of Rappaport-Vassiliadis, tetrathionate and selenite broths with and without pre-enrichment for the isolation of Salmonella in animal waste biogas plants.

A total of 481 samples of biomass from biogas plants treating slurry and other types of animal waste were examined for the presence of salmonellae by means of five different isolation methods. In 131 samples, Salmonella was isolated by means of one or more methods. A statistical evaluation of the isolation frequencies showed that Rappaport-Vassiliadis broth was significantly better than selenite broth with and without pre-enrichment and tetrathionate broth with pre-enrichment, whereas tetrathionate broth without pre-enrichment was significantly poorer than the other four methods. For each of the thirty different Salmonella serotypes, the isolation frequencies for the five methods are presented. Remarkably, Rappaport-Vassiliadis broth had very high isolation frequencies and tetrathionate broth with and without pre-enrichment had very low isolation frequencies for Salmonella typhimurium as well as Salmonella dublin.

Use of norfloxacin to study colonization ability of Escherichia coli in in vivo and in vitro models of the porcine gut.

The colonization resistance conveyed by the intestinal microbiota can prevent colonization of the intestinal system by new strains. In this study, this resistance was partly circumvented by use of the antimicrobial drug norfloxacin. The colonization abilities of two closely related Escherichia coli strains, which were resistant to nalidixic acid and rifampin, respectively, were investigated in minipigs and a two-stage continuous-flow in vitro gut model. Whereas both strains were unable to colonize the intact enteric system in vivo and in vitro, a 3-day norfloxacin treatment modified both systems to allow colonization by the nalidixic acid-resistant strain but not the rifampin-resistant strain. The results indicate the usefulness of norfloxacin to circumvent the normal colonization resistance while keeping a fairly normal microbiota in the gut. The results also indicate that it could be possible to construct in vitro gut models which could distinguish between strains with different gut colonization abilities. Both of these possibilities could come to be used in the study of the colonization and effects in the gut of new bacterial strains, i.e., genetically modified microorganisms.

Effect of water coagulation by seeds of Moringa oleifera on bacterial concentrations.

The effects of a Sudanese water purification method traditionally used in Sudan to treat turbid waters were studied with respect to turbidity reduction and removal of faecal indicator bacteria as well as selected enteric bacterial pathogens. Water treatment was performed at 30 degrees C with Moringa oleifera seed material as a coagulant, and the technique employed corresponded closely to that used to clarify turbid water in Sudanese villages. A turbidity reduction of 80.0-99.5% paralleled by a primary bacterial reduction of 1-4 log units (90.00-99.99%) was obtained within the first 1 to 2 h of treatment, the bacteria being concentrated in the coagulated sediment. During the 24 h observation period a secondary bacterial increase due to regrowth in the supernatant water was consistently observed for Salmonella typhimurium and Shigella sonnei, in some cases for Escherichia coli, but not for Vibrio cholerae, Streptococcus faecalis and Clostridium perfringens. The potential of the method when compared with some alternative for the improvement of rural drinking water supplies is discussed.