Historically, most foods have been produced and consumed locally. The internationalization of the food industry since World War II, made possible by the advent of refrigerated freighters, has changed this simple relationship. Now fresh fruits and vegetables that previously would have been available for only a few weeks can be obtained all year long. Wide-scale importation of food products has greatly increased dietary options for many consumers. At the same time, there is concern that these foods may come from regions in which hygienic practices are not on a par with those in the United States. With this concern in mind, the movement of the South American cholera epidemic into Mexico is being monitored closely by U.S. public health officials, since Mexico supplies the United States with much of its fresh produce during the winter months.

International trade has become so pervasive that it is virtually impossible to screen most of the food entering the country for known microbial hazards, let alone for new microbiological threats. This situation is likely to continue as political agreements remove barriers between trading partners. There is already virtually unrestricted movement of foods among the member nations of the European Economic Community, for example. Similarly, the U.S.-Canada-Mexico free-trade agreement is likely to lead to reduced inspection of imported foods, a subject that is currently being hotly debated. For foods entering the United States, as in Europe, there is likely to be increasing reliance on inspections conducted by the country of origin. This will necessitate the further development and implementation of international standards, such as the Codex Alimentarius of the Joint Food and Agricultural Organization/World Health Organization Food Standards Programme.

International commerce can affect food safety, even when the food itself is not being transported. This was the case in a 1986 outbreak of shellfish-related paralytic poisonings in South Australia and Tasmania. The poisonings were caused by Alexandrium catenella, A. minutum, and Gymnodinium catenatum, three dinoflagellate species not normally found in that part of the world. These marine plankton, which produce potent neurotoxins, are concentrated by shellfish as they filter particulate matter from seawater. The toxins do not affect shellfish, but they can cause serious neurological problems when consumed by warm-blooded animals. The likely source of the organisms was the bilge water of ocean-going freighters. Typically, bilge water is changed while ships are in port, releasing the microscopic stowaways. Studies have indicated that entire miniature ecosystems are transported around the world in this manner (Hallegraeff and Bolch, 1991; Jones, 1991).

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