Foodborne pathogens in milk and the dairy farm environment: Food safety and public health implications

Milk and products derived from milk of dairy cows can harbor a variety of microorganisms and can be important sources of foodborne pathogens. The presence of foodborne pathogens in milk is due to direct contact with contaminated sources in the dairy farm environment and to excretion from the udder of an infected animal. Most milk is pasteurized, so why should the dairy industry be concerned about the microbial quality of bulk tank milk? There are several valid reasons, including (1) outbreaks of disease in humans have been traced to the consumption of unpasteurized milk and have also been traced back to pasteurized milk, (2) unpasteurized milk is consumed directly by dairy producers, farm employees, and their families, neighbors, and raw milk advocates, (3) unpasteurized milk is consumed directly by a large segment of the population via consumption of several types of cheeses manufactured from unpasteurized milk, (4) entry of foodborne pathogens via contaminated raw milk into dairy food processing plants can lead to persistence of these pathogens in biofilms, and subsequent contamination of processed milk products and exposure of consumers to pathogenic bacteria, (5) pasteurization may not destroy all foodborne pathogens in milk, and (6) inadequate or faulty pasteurization will not destroy all foodborne pathogens. Furthermore, pathogens such as Listeria monocytogenes can survive and thrive in post-pasteurization processing environments, thus leading to recontamination of dairy products. These pathways pose a risk to the consumer from direct exposure to foodborne pathogens present in unpasteurized dairy products as well as dairy products that become re-contaminated after pasteurization. The purpose of this communication is to review literature published on the prevalence of bacterial foodborne pathogens in milk and in the dairy environment, and to discuss public health and food safety issues associated with foodborne pathogens found in the dairy environment. Information presented supports the model in which the presence of pathogens depends on ingestion of contaminated feed followed by amplification in bovine hosts and fecal dissemination in the farm environment. The final outcome of this cycle is a constantly maintained reservoir of foodborne pathogens that can reach humans by direct contact, ingestion of raw contaminated milk or cheese, or contamination during the processing of milk products. Isolation of bacterial pathogens with similar biotypes from dairy farms and from outbreaks of human disease substantiates this hypothesis.