Prevalence of Listeria monocytogenes in foods: Incidence in dairy products

Abstract

Listeria monocytogenes is ubiquitous and is recognized as an important foodborne pathogen. Epidemiological evidence and outbreaks have pointed to dairy products as alleged vehicles for cases of listeriosis. Incidents involving dairy products resulted in a number of surveillance programmes, the discovery of this pathogen in contaminated dairy products, and numerous product recalls. Results of raw milk sampling surveys indicate only 3–4% of the raw milk supply can be expected to contain Listeria species. These surveys have also shown that the levels in most samples are low (less than 10 cfu/ml). As pasteurization effectively destroys the Listeria organism, the presence of Listeria in finished product must be the result of post-pasteurization contamination from environmental sources in the plant. There are a number of mitigating factors in both product characteristics and processing which have an impact on the organism's ability to survive and multiply. Results of finished product sampling surveys, such as those conducted for frozen products (ie, ice cream), show low numbers of the organism (less than 15 cfu/g) in contaminated samples. In fact, no clinical evidence exists which associates frozen dairy products with listeriosis outbreaks. A number of preventive measures specifically targeted to those areas of likely exposure have been developed and implemented in the frozen dessert industry.