The incidence of Listeria monocytogenes in meat, poultry and seafood plants in the Nordic countries

Abstract

During 1998/1999 a total of 36 surveys were carried out in six meat, five seafood and two poultry processing plants. A total of 2522 samples, including processing lines and environment, personnel, raw materials and products (raw or ready-to-eat, RTE), were analysed for the presence of Listeria monocytogenes. The environmental and personnel samples were taken after cleaning and after the process had been running for 2 h. Samples of food products were taken after every important step during processing and samples of poultry carcasses were taken immediately after production and after storage for 21 days. The overall incidence of L. monocytogenes in meat processing plants varied from 0% to 15.1%, in poultry plants from 20.6% to 24.1% and in seafood plants from 5.9% to 22.1%. In raw products the average incidence was 15.6% for meat, 22.2% for poultry and 39.0% for seafood products. The heating steps during the production of RTE products eliminated Listeria. On average, 2.3% of RTE meat and 4.8% of RTE seafood products were recontaminated with L. monocytogenes. In the seafood sector almost all Listeria positive samples also included L. monocytogenes (91.1% of the positive samples), whereas in the meat and poultry sectors other Listeria species (mainly L. innocua) dominated. In most plants, the implemented cleaning procedures were insufficient to eliminate Listeria. The results clearly indicated the problematic sites, which in most plants were conveyer belts and other transporting equipment, floors and drains, but also raw material in the meat industry, cutting boards in the poultry industry and cooking equipment in the seafood industry. In order to solve the problems observed in this study, there is a need for close co-operation between the suppliers of equipment and cleaning agents, the staff of cleaning companies and hygiene specialists from the food industry.

Introduction

Listeria monocytogenes is the causal agent of listeriosis, a disease that can be serious and is often fatal in susceptible individuals. It has been shown by epidemiological investigations that listeriosis is a foodborne illness (Embarek, 1994; Slutsker and Schuchat, 1999). No cases have been reported in Iceland or on the Faroe Islands since 1997, but in the other Nordic countries from 4 to 8 cases per million inhabitants have been reported annually (Anon., 2001) which is in agreement with the incidence reported in France (Goulet et al., 2001). All of the Nordic countries have a laboratory notification system for L. monocytogenes. In Finland, Norway and Sweden physicians must also report all diagnosed cases of listeriosis. L. monocytogenes is ubiquitous in the environment and can grow in vacuum- and gas-packaged products at refrigeration temperatures (Duffy et al., 1994). Numbers of the bacterium can increase in refrigerated products from less than 100 cells per gram, which in many studies is considered to be the highest accepted dose for healthy people (Buchanan et al., 2000; Huss et al., 2000), to more than 100 000 cells per gram, over a period of 3–4 weeks (the shelf-life of many foodstuffs) (Miettinen et al., 1999). Thus, ready-to-eat (RTE) food products which are stored at refrigeration temperature for more than 10–15 days and are consumed without sufficient heating to kill living bacterial cells belong to the risk foodstuffs causing listeriosis. It has also been reported that L. monocytogenes has shorter generation times in seafood (pH 6.1–7.6) than in meat and meat products (pH 5.1–6.2). This is partly due to the effect of pH on the growth of L. monocytogenes, which has higher growth rates at near neutral pH (Shineman and Harrison, 1994).

Although L. monocytogenes and other Listeria species have been isolated from many different types of raw and processed food (Ojeniyi et al., 1996; McCarthy, 1997; Valdimarsson et al., 1998; Gravani, 1999; Nørrung et al., 1999; Capita et al., 2001), the main sources and routes of contamination are still not fully understood.

The general tendency towards increasing numbers of listeriosis cases and also of other foodborne diseases indicates that there is a need for more knowledge and increased data are needed in risk assessment and for improved preventive measures. In order to prevent and control contamination of the environment and products with this pathogen, it is important to detect the most important sources of contamination and to understand the mechanisms leading to the persistence of different strains of L. monocytogenes in the environment. HACCP-based internal control is used to improve the safety of food products, although in many cases the true critical points are often lacking.

The aim of this study is to determine and compare the incidence of L. monocytogenes at various stages during food processing in similar, but geographically separated plants. In order to detect the common contamination sites of the bacterium, six meat, five seafood and two poultry processing plants were surveyed. Knowledge of the common sources of contamination will in turn aid risk assessment and the implementation of preventive measures.