case for four outbreaks of human listeriosis, a bacterial infection that occurred in the United States, Canada, and Switzerland in the early to mid-1980s. Careful monitoring of disease incidence data by medical facilities allowed these epidemics to be detected, even though the actual number of cases was relatively low. Subsequent epidemiologic investigations implicated cole slaw (Schlech et al., 1983), milk (Fleming et al., 1985), and soft cheeses (Office of Federal Public Health, Switzerland, 1988; Linnan et al., 1988) as the vehicle of infection. Of particular concern is that listeriosis, caused by Listeria monocytogenes, is most often diagnosed in pregnant women or their newborns and in immunosuppressed individuals, in whom it can be fatal. The CDC has recently published recommendations for the prevention of food-borne listeriosis; for those at high risk (immunocompromised individuals, pregnant women, and the elderly) the recommendations cite additional foods to avoid (Centers for Disease Control, 1992h).
Any change in the conditions or practices associated with the production of agricultural commodities can affect the safety of the food supply. A virtually uncontrollable factor, like the weather, can have a substantial impact. For example, drought can make grains more susceptible to mycotoxin-producing fungi, and toxic fungal metabolites, such as aflatoxin, can threaten the health of both humans and livestock. This particular risk has been substantially lessened by an ongoing U.S. Department of Agriculture (USDA) program that monitors the status of major agricultural commodities. Once identified, contaminated grain is destroyed.
New agricultural procedures can also have unanticipated microbiological effects. For example, the introduction of feedlots and large-scale poultry rearing and processing facilities has been implicated in the increasing incidence of human pathogens, such as Salmonella, in domestic animals over the past 30 years. The use of antibiotics to enhance the growth of and prevent illness in domestic animals has been questioned because of its potential role in the development and dissemination of antibiotic resistance (Cohen and Tauxe, 1986; Institute of Medicine, 1989). Approximately half the tonnage of antibiotics produced in the United States is used in the raising of animals for human consumption. Thus, concerns about the selection of antibiotic-resistant strains of bacteria and their passage into the human population as a result of this excessive use of antibiotics are realistic (Institute of Medicine, 1989). It is conceivable that surveillance of feedlot animals for the development of resistant organisms might be a means of early warning for the emergence of newly drug-resistant pathogens.
Broad-based societal events indirectly related to agriculture may also affect food safety. Recent concerns about bovine spongiform encephalopathy