L.monocytogenes is considered emerging because the role of food in its transmission has only recently been recognised. As such the joint FAO/WHO Secretariat on Risk Assessment of Microbiological Hazards in Foods co-ordinated a risk assessment of this food pathogen, aiming to answer key questions posed by the Codex Committee on Food Hygiene (CCFH).
"The risk assessment provides a valuable resource for risk managers in terms of the issues to be considered when managing the problems associated with L.monocytogenes," concluded the Food and Agriculture Organisation (FAO) and the World Health Organisation (WHO) in the final report issued recently.
The disease is most often associated with consumption of foods such as soft cheese and processed meat products that are kept refrigerated for a long time because Lm can grow at low temperatures. Outbreaks of listeriosis have been reported from many countries, including Australia, Switzerland, France and the US. Two recent outbreaks of Listeria monocytogenes in France in 2000 and in the USA in 1999 were caused by contaminated pork tongue and hot dogs respectively.
In pregnant women, infections with L.monocytogenes can cause abortion and stillbirth, and in infants and persons with a weakened immune system it may lead to septicemia (blood poisoning) and meningitis.
Risk assessment is the science-based component of risk analysis. Over the past decade, risk Analysis - a process consisting of risk assessment, risk management and risk communication - has emerged as a structured model for improving food control systems with the objectives of producing safer food, cutting the numbers of foodborne illnesses and facilitating domestic and international trade in food.
"Science today has allowed us to accumulate a wealth of knowledge on microscopic organisms, their growth, survival and death, even their genetic make-up. It has given us an understanding of food production, processing and preservation, and of the link between the microscopic and the macroscopic world and how we can benefit from as well as suffer from these microorganisms," writes the FAO/WHO report.
According to the report, the many scientists involved in assessing recent scientific data on L.monocytogenes to formulate a risk assessment set out to primarily answer three key questions. These were to estimate the risk of serious illness: from L. monocytogenes in food when the number of organisms ranges from absence in 25 grams to 1000 colony forming units (CFU)/gram or millilitre, or does not exceed specified levels at the point of consumption, and secondly, for consumers in different susceptible population groups - elderly people, infants, pregnant women and immuno-compromised patients - relative to the general population.
The final question set out to estimate risk from L. monocytogenes in foods that supports its growth and foods that do not supports its growth under specific storage and shelf-life conditions.
Four RTE foods were chosen for the research - pasteurised milk, ice cream, fermented meat, and cold smoked fish.
The risk assessment suggests that most cases of listeriosis result from the consumption of high number of L. monocytogenes in RTE foods which do not meet the suggested criteria of 0.04 or 100 CFU/g. Low numbers of the bacterium had a low probability of causing illness.
Their findings also suggest that the probability of becoming ill after eating the food pathogen was also greater for members of 'susceptible' population groups, such as pregnant women and infants.
In parallel to the FAO/WHO risk assessment, a recent study from food scientists at Cornell University in the US found that strains of the deadly pathogen can persist for up to a year or longer.
Scientists led by Brian Sauders and Martin Wiedmann concluded that food retailers have a harder time controlling Listeria than food processors. While food processors can control people entering the plant, retailers cannot always control the pathogens introduced by customers and employees.
Listeria used in the Cornell study was detected in ready-to-eat delicatessen foods like ham, chicken, roast beef and smoked fish. It was also found in hummus, imitation crab, cheeses and in foods requiring cooking before consumption, such as hot dogs and raw foods including beef, turkey, lobster tails and shrimp.
The UN-backed World Health Organisation (WHO) claims that changes in farm practices, more extensive food distribution systems and the increasing preference for meat and poultry in developing countries all have the potential to increase the incidence of foodborne illness.
And the incidence can also be quite an expense for society. WHO estimates that the medical costs and the value of the lives lost during just five foodborne outbreaks in England and Wales in 1996 were estimated at £ 300-700 million and the cost of the estimated 11 500 daily cases of food poisoning in Australia has been calculated at AU$ 2.6 billion annually.