Chris Bell

Outbreaks: Causes and Lessons to be Learnt

A large number of listeriosis outbreaks linked to the consumption of food products has occurred in the last two decades. Table 2.1 summarizes Information concerning a few of these outbreaks, indicating both the worldwide nature of outbreaks and the wide range of foods involved. These have been summarized by McLauchlin (1996).

Factors Affecting the Growth and Survival of Listeria Monocytogenes

Listeria spp., including L. monocytogenes, are present and will continue to be found in a wide variety of raw food materials. The natural exposure of vegetables, fruits, grains, food animals, fish and poiutry to the environmental sources of the organism makes it impossible for such foods to be produced free from L. monocytogenes at source. In order to assure the safety of food products in respect of potential foodborne bacterial pathogens, growing, harvesting, handling, storage, processing and associated food supply Systems must be managed by food producers and processors in such a way as to reliably control the growth of any pathogens likely to be present. L. monocytogenes in particular must be prevented from multiplying to potentially harmful levels. To achieve this, it is necessary to understand the conditions and factors that affect its growth and survival.

Industry Action and Reaction

Listeria monocytogenes as a species is subdivided into serotypes and genetic techniques are used to further differentiate strains of the organism. There has been a great deal of debate concerning the specific pathogenicity of individual strains of L. monocytogenes and their relevance to public health. In 1995, in a case concerning fitness for human consumption of some batches of Lanark Blue cheese allegedly found to contain high levels of L. monocytogenes serotype 3a, the food condemnation orders requested by the local authority under the UK Food Safety Act, 1990 (Anon., 1990a) were refused. This was in part due to evidence presented indicating that this serotype had never been associated with listeriosis caused by the consumption of contaminated food. In 1996, the UK Health and Agriculture Departments asked the Advisory Committee on the Microbiological Safety of Food (ACMSF) for advice on whether all strains of L. monocytogenes were pathogenic or should be regarded as pathogenic. Following consultation and discussion with other expert groups, the Committee advised that, for public health purposes, it would be prudent to treat all strains of L. monocytogenes as pathogenic. In a detailed review of the pathogenicity of L. monocytogenes, McLauchlin (1997) concluded ‘in the interests of public safety and for considerations for public health purposes, all L. monocytogenes, including those recovered from food, should be regarded as potentially pathogenic.’

Industry Focus: Control of Listeria Monocytogenes

Accumulated evidence to date clearly demonstrates the need for the food industry to employ measures to minimize the potential for L. monocytogenes to be present in foods at the point of consumption in numbers considered hazardous to health. Many products can potentially present a risk of causing outbreaks of listeriosis but in most outbreaks a failure in the control Systems can be identified that has lead to the outbreak. In such cases,hazard analysis and implementation of controls at the critical points identified could have prevented the outbreaks, provided the control Systems were operated consistently correctly. It is strongly recommended that all persons involved in the primary production, processing and sale of food adopt a hazard analysis approach to consider all relevant pathogens, including L. monocytogenes.