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Unpasteurized Milk: A Continuing Public Health Threat
Abstract & Commentary
By Michele Barry, MD, FACP
Dr. Barry is a Professor of Medicine and Public Health; Director of the International Health Program, Department of Medicine, Yale University School of Medicine, New Haven, CT.
Dr. Barry receives research support from Johnson & Johnson and is a retained consultant for the Ford Foundation.
Synopsis: Although milk and dairy products are a crucial component of a healthy diet, if these products are consumed unpasteurized they can present a serious health hazard due to possible contamination with pathogenic organisms.
Source: LeJeune J, Rajala-Schultz P. Unpasteurized milk: A continued public health threat. Clin Infect Dis 2009;48:93-100.
Pasteurization, the process of heating milk for a predetermined time and maintaining it at a set temperature, has become the cornerstone of milk and dairy product safety. Prior to both pasteurization and a cattle pathogen eradication program in the United States, bovine tuberculosis and brucellosis were the classic zoonotic milk-borne diseases in the United States. Both of these programs were successful; M. bovis and B. abortus are rarely found in domestic cattle. Organisms predominant in raw milk are those that are excreted by asymptomatic cattle and goats or introduced by contamination from environmental sources. Common pathogens now encountered are Coxiella burnetti, Listeria species, Mycobacterium avium subspecies paratuberculosis, Campylobacter species, and various coliforms, including E. coli and Salmonella species. Current guidelines for temperature and time combinations used during pasteurization are based on the ability to destroy Coxiella burnetii, present in 94% of pooled milk collected from farms in the United States. With the advent of a new food movement advocating consumption of raw milk for health purposes, the number of serious milk-borne disease outbreaks has increased. This publication reviews mechanisms of contamination and formulates public health recommendations.
Milk and other dairy products, primarily from cows but also less frequently from goats and sheep, are important components of the American diet. With the current "back to basics" organic farming and food sustainability movement, consumption of raw milk has become more popular. Consumption of raw milk has always been common among farm families with rates varying between 35–60%.1 According to the U.S. Centers for Disease Control and Prevention's FoodNet Population Survey in 2002, 3.5% of respondents reported having consumed unpasteurized milk in the 7 days prior to the survey.2
In the United States, the U.S. Food and Drug Administration is responsible for providing oversight of dairy products. In 1924 the U.S. Public Health Service developed a model regulation known today as the Grade A Pasteurized Milk Ordinance (PMO), which all 50 states have voluntarily adopted. These guidelines provide guidance for all aspects of production, handling, transportation, testing, and processing of milk. In addition, the PMO establishes maximum allowable bacterial limits in raw milk destined for pasteurization as well as in pasteurized milk. Individual states enforce the PMO within their borders, and the FDA in 1987 prohibited interstate shipment of raw milk for human consumption. In 2006 the sale of raw milk was illegal in 26 states.3 Various schemes and marketing strategies in these states circumvent these laws by "selling shares" or "leasing cows," thus permitting consumers to obtain raw milk from "their" cows without buying or selling the actual raw milk.
With the advent of pasteurization, the incidence of milk-borne disease dropped dramatically from 25% of all food outbreaks in 1938 to less than 1% at the beginning of the 21st century. With the trend of increased raw milk consumption, the rate of disease outbreaks associated with raw milk consumption has begun to increase dramatically (see Figure 1), with several serious outbreaks of disease due to salmonellosis, campylobacteriosis, and infections with E. coli 0157:H7. One notorious outbreak report by Blaser et al. involved an outbreak of C. jejuni among 19 of 31 college students who consumed unpasteurized milk on a visit to a farm. The authors emphasize the key factor in the prevention of milk-borne disease is consumer avoidance of raw milk consumption, as no microbiologic assay has been as reliable for prevention as the pasteurization process.
Pasteurization is the most effective method of enhancing the microbiologic safety of milk. Despite the recent lay public opposition to pasteurization by claiming that pasteurization changes the nutritional value of milk, there is no scientific evidence to substantiate these claims, and the authors offer an excellent table in this review listing the nutrients in milk and the effects of pasteurization on each nutrient. The increasing popularity of raw milk has prompted a few states to pass legislation, and the states of Vermont, Missouri, Maryland, and Alaska have initiated bills expanding the sale and distribution of raw milk. Time magazine published an article describing the growing contingent of natural food fans who have pushed 6 states to allow the sale of raw milk in stores and 28 states to permit consumers to buy from a farm.4 This raw milk movement sets the stage for an increase in disease outbreaks. (See Table 1.)
Milk can become contaminated at any stage from the production to consumption phases. Typically, unless there is mastitis, milk in the mammary gland itself is sterile. However, as milk is excreted it is exposed to commensal microflora on the teat skin (in cattle, staphylococci, streptococci, Bacillus species, corynebacteria, and some coliforms predominate). Systemic diseases of cattle can result in localization of their pathogens in the mammary gland or lymph nodes and consequent excretion of such pathogens into milk. Bovine tuberculosis and brucellosis were the common contaminants before a massive eradication program occurred. Now Coxiella and Listeria species, Mycobacterium avium, subspecies paratuberculosis (a suspected agent in triggering Crohn's disease), Campylobacter species, and coliforms such as E. coli 0157:H7 and Salmonella enterica are more prevalent. For example, C. burnetti, the causal agent of Q fever, is prevalent in 94% of all pooled milk, and M. avium, subspecies paratuberculosis, is found in 68% of all U.S. dairy herds. Most of these organisms appear due to environmental contamination following the excretion of milk and during collection, processing, and distribution.
Very young, aged, pregnant, and immunocompromised hosts are the most susceptible to severe sequelae from the pathogens in raw milk. The key factor in prevention of milk-borne disease is consumer avoidance of raw milk. AMA policy H-150.980 clearly asserts that raw milk is dangerous and should be pasteurized, as does the American Veterinary Medical Association. Testing raw milk for contamination as an alternative to pasteurization has been proposed for the dedicated raw milk drinkers, but testing schemes are limited by assay sensitivity, sampling strategies, and recontamination.