In the summer and early fall of 1994, several hundred persons reported developing diarrheal disease -- later confirmed as salmonellosis. The outbreak traced to batches of Schwan's ice cream, which are distributed throughout the United States. Because most food poisoning goes unreported, government scientists investigated how many such untallied cases might be associated with this incident. Their analysis suggests that here too, the true magnitude of the outbreak was considerably larger than had at first been anticipated. They now estimate the tainted ice cream probably sickened some 224,000 people in the United States.
These scientists have also confirmed earlier suspicions: that this food poisoning -- the largest single salmonella-poisoning outbreak ever recorded in the United States -- occurred because ingredients for the ice cream had been transported to the manufacturer in tainted tanker trailers.
Between 800,000 and 4 million cases of salmonella poisoning occur each year in the United States. Roughly one-quarter of the cases involve the same strain of the bacteria (S. enteritidis) implicated in the 1994 ice cream incident.
That outbreak initially puzzled researchers because although homemade ice cream and unpasteurized commercial ice cream have proven sources of food poisoning in the past, Schwan's products had been made from ingredients that had undergone pasteurization to kill any disease-causing microbes.
Investigators from a team of local, state and federal agencies combed Schwan's Marshall, Minn., plant for clues on how the Salmonella had gotten into its ice cream. In the May 16 New England Journal of Medicine, Thomas W. Hennessy of the Minnesota Department of Health, in Minneapolis, and his colleagues now report finding only sound manufacturing practices and good sanitation there.
During their investigation, these researchers did learn, however, that between two and 19 tanker trailer shipments of ice cream premix were used in the production run of each type of ice cream product. So they turned their attention to the trucking firm that delivered this premix.
Upon questioning, the company volunteered that as of July 1, 1994, it had greatly stepped up its hauling of unpasteurized eggs to Minnesota from egg-breaking plants elsewhere in the Midwest. Almost immediately, the trucking company made it a common practice to follow up each such egg-hauling trip with the transport of premix to Schwan's ice cream factory.
Because fresh eggs are a leading source of foodborne Salmonella, health regulations require that trucking firms wash and sanitize tankers after each shipment of liquid eggs. However, federal officials uncovered evidence that the trucker hauling the ice cream premix had a less than sterling record.
Its trailers had not been routinely inspected, record keeping on cleaning proved inadequate, and tanker equipment that came into contact with food sometimes was soiled. Indeed, federal inspectors not only found egg residues inside a trailer after it had undergone a cleaning, but also cracks in the lining of five trailers which could harbor bacteria. Finally, the investigators learned that as a time-saving measure, the trucker allowed its drivers to bypass a cleaning of tankers after they had hauled eggs.
The fact that contamination associated with the food-poisoning in this outbreak now appears small -- on the order of 6 microbes per half-cup serving of ice cream -- points to the need for high levels of vigilance throughout food processing, the researchers say. Observes Martin J. Blaser of the Vanderbilt University School of Medicine, in Nashville, this is especially true where centralized food production, as is common in the United States, allows a small quantify of pathogens to taint large batches of food that will travel long distances from its source.
Indeed, he points out in an editorial accompanying the study on the ice cream outbreak, salmonellosis is rare in developing countries, even in those where sanitation is poor and diarrheal diseases are endemic. The reason, he suspects, is that most of their food is produced and consumed locally.
In developed countries, he notes, "food production is often so complex that many points at which contamination could occur are simply not recognized." He points out, for instance, that the hauling of both pasteurized and unpasteurized food products by a single vehicle "was not identified until [this latest] investigation was undertaken."
To prevent further outbreaks, Hennessy's team recommends that unless foods will be pasteurized again at some later date, they should never be shipped in vehicles that are ever permitted to carry unpasteurized products.
References:
Hennessy, T.W., et al. 1996. A national outbreak of Salmonella enteritidis infections from ice cream.New England Journal of Medicine 334(May 16):1281.
Blaser, M.J. 1996. How safe is our food? New England Journal of Medicine 334(May 16):1324.
Further readings:
Raloff, J. 1996. Tracking and tackling foodborne germs. Science News 149(May 25):326.
1996. Food safety: Information on foodborne illnesses. Report RCED-96-96 (May). U.S. General Accounting Office, Washington, DC 20548-0001.
1994. Foodborne pathogens: Risks and consequences. CAST Task Force Report No. 122 (September), Council for Agricultural Science and Technology (4420 W. Lincoln Way, Ames, IA 50014-3447. Ph: 515-292-2125; FAX: 800-375-CAST. E-mail:cast@cast-science.org).
Blumenthal, D. 1990. Salmonella enteritidis. FDA Consumer 24 (April):6.
Cliver, D.O. 1993. Eating safely: Avoiding foodborne illness. American Council on Science and Health, (1995 Broadway, 2nd Fl., New York, NY 10023-5860. Ph: 212-362-7044; FAX: 212-362-4919. E-mail:acshmail@aol.com).
This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.
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