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Scientific assessment support
Urgent advice on the public health risk of Shiga-toxin producing Escherichia coli in fresh vegetables
No abstract available
The scope of this Scientific Report of EFSA is to provide a fast-track assessment of the exposure of the consumer to STEC through consumption of raw vegetables, and to suggest possible mitigation options.
The German outbreak strain seems to share virulence characteristics of STEC and EAEC strains. STEC strains usually have an animal reservoir, while EAEC have a human reservoir. Infections in humans caused by similar strains (same serotype, same phylogroup, same MLST type, and with similar virulence gene array) have been reported in the past, and as such, the strain could not be regarded as “new”. However this outbreak strain is rare, and until now it has never been found to be responsible for the rate of infection and severity of disease seen during the current outbreak. Sequence analysis and comparative genomics will be able to show if the outbreak strain is an EAEC that acquired EHEC virulence determinants, or vice versa. The antimicrobial resistance genotype of the outbreak strain, and the molecular typing of the blaCTX-M-15-containing plasmid, could provide some clues on the epidemiology of this pathogen.
Regarding the Exposure Assessment, many different types of foods have been identified as a potential source of STEC. These are usually raw or undercooked foodstuffs contaminated with faeces from ruminants, either during primary production (e.g. slaughtering, milking, fertilised vegetables) or further processing and handling. Data on STEC are reported annually on a mandatory basis by EU Member States to the European Commission and EFSA. When interpreting this data it is important to note that they are not directly comparable due to differences in sampling strategies and applied analytical methods. In the scientific literature, outbreaks of STEC infection are becoming increasingly recognised as associated with vegetables, particularly contaminated sprouting seeds and green leafy salad vegetables. Outbreaks may have more than one exposure route involved. For example, primary human infection may originate from consumption of contaminated food or direct contact with an animal carrying STEC, while secondary infection may occur by the faecal-oral route, after contamination of food through handling by an infected person shedding the bacteria. As a result, especially during the late stages of an outbreak multiple exposure routes are likely. Contamination of fresh produce with STEC is rare but has been linked to some severe outbreaks. In some outbreaks, the origin of contamination was suspected to be contaminated irrigation water and access of farm animals to the immediate environment of fresh-produce. In most outbreaks however, the origin of contamination was not elucidated. Contamination of vegetables with STEC can occur in different steps of the food chain: during primary production; during harvest and post-harvest including handling and processing, at marketing and retail and during catering and in the care of the consumer after sale during transport and in domestic settings. Bacterial contamination of vegetables occurs mostly on the surface of the tissues of the plants but it may also be internal. Although there is no conclusive data, in theory internalisation of STEC would result in increased survival both in pre-harvest and post-harvest due to protection from exposure to UV and desiccation, as well as increased protection to surface decontamination treatments. Pre-harvest contamination can derive from infected farm animals. The possible routes of contamination are irrigation water contaminated with animal waste as well as sewage, application of organic fertilizers of animal and/or human origin and direct contact of animals with fresh produce growing fields. There is considerable debate on the possibility of STEC being present internally in leafy vegetables, particularly if exposed during the pre-harvest phase. It should be noted that internalisation of E. coli in plants has been only shown in experimental laboratory conditions specially in the case of root inoculations with very young plants and using high inoculation doses. Processing of vegetables involves many points of contact with people, surfaces, water and the environment (soil, dust) and this represents potential opportunities for contamination with food-borne pathogens. Minimally processed vegetables and sliced fruit exhibit a characteristic high humidity. This fact, together with the high number of cut surfaces, can provide ideal conditions for microbial growth, including that of food-borne pathogens and spoilage micro-organisms. Cutting practices increased the chance of bacterial cross-contamination and can result in increased susceptibility of bacterial attachment. There is scarce information on the prevalence and quantity of STEC in vegetables both from surveillance and outbreak investigations. It is currently not possible to estimate the relative exposure to humans from pre-harvest or post-harvest contamination of vegetables by STEC. At the same time, scarcity of data also hampers the estimation of the relative significance of surface or internal contamination of vegetables by STEC for human exposure.
Regarding Mitigation Options, the use of Good Agricultural Practices (GAPs), Good Manufacturing Practices (GMPs), and Hazard Analysis and Critical Control Point (HACCP) in the fresh fruit and vegetable industry provide the basic framework for safe products for the consumer. GAPs describe preventive measures implemented in farming operations to reduce product contamination and provide guidance for food-safety practices in the field. Implementing HACCP programs in processing and packaging facilities is a requisite for food safety. Since there is evidence of asymptomatic carriers of STEC in humans, screening of humans involved in food handling is relevant. The monitoring and/or exclusion of STEC carriers from food handling could be considered as a mitigation option. In reference to Pre-harvest mitigation options: the application of mitigation strategies reflected in GAPs in line with codes available from international organisations is recommended. In particular, to avoid access of farm animals (in particular ruminants) to the immediate environment of fresh produce; to use of irrigation and of agricultural water which are of adequate microbiological quality; and to control the sourcing, handling and treatment of manure and slurry that are to be used for fertilising fields intended to grow produce for human consumption. In reference to Post-harvest mitigation options: current technologies or practices do not effectively eliminate any microbiological hazard acquired during post-harvest processing of fresh vegetables. Therefore the main focus should be on prevention of contamination both during pre-harvest and post-harvest. The only effective method of eliminating STEC from foods is to introduce a bactericidal treatment, such as heating (e.g. cooking or pasteurization) or irradiation. Adhesion of pathogens to surfaces and internalisation of pathogens limits the usefulness of conventional processing and chemical sanitizing methods in preventing transmission from contaminated produce. The application of mitigation strategies reflected in GMPs and GHPs in line with codes available from international organisations is recommended. In particular, the use of water of adequate microbiological quality during further processing; to ensure basic training on food hygiene practices to food handlers; to ensure adequate design and hygiene management of food premises including pest control plans and the correct management of cold chain seems of particular importance for those products processed for ready-to-eat consumption (e.g. cut vegetables, unpasteurised vegetable juices). In reference to catering and home: GHPs when preparing food e.g. wash hands before and after preparing foods, wash all fruit and vegetables with potable running water, avoid cross-contamination, keep storage temperatures low for food. Peeling or cooking fruit and vegetables can also remove microbes. Although these measures have been proven to be useful they cannot completely eliminate the risk.
VTEC, STEC, E. coli, produce, vegetables, O104:H4