Food Poisoning from Alfalfa and Clover Sprouts

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The summary below is from the full report titled "Escherichia coli O157 and Salmonella Infections Associated with Sprouts in California, 1996–1998." It is in the 21 August 2001 issue of Annals of Internal Medicine (volume 135, pages 239-247). The authors are JC Mohle-Boetani, JA Farrar, SB Werner, D Minassian, R Bryant, S Abbott, L Slutsker, and DJ Vugia, for the Investigation Team.

What is the problem and what is known about it so far?

Food poisoning is an illness that results from eating food contaminated with an infectious organism (bacteria, virus, parasite) or a toxin. More than 6 million cases of food poisoning occur every year in the United States. The most common symptoms of food poisoning are vomiting, diarrhea, and stomach pain. People usually recover quickly from food poisoning; some people, however, may need hospitalization and fluids to treat dehydration, and a few people die of food poisoning every year. A wide variety of foods can cause food poisoning if they are spoiled, are improperly prepared, or are contaminated in the field and served raw. Several instances of food poisoning related to sprouts have been reported, but detailed information on such outbreaks has been lacking.

Why did the researchers do this particular study?

To describe outbreaks of food poisoning related to sprouts.

Who was studied?

The study included 600 people with food poisoning and an approximately equal number of people without food poisoning.

How was the study done?

The researchers studied five outbreaks of food poisoning that occurred in the United States between 1996 and 1998. They obtained information from local health departments about people with food poisoning. All of these people had cultures that were positive for E. coli O157 (a harmful strain of E. coli) or Salmonella, two types of bacteria that commonly cause food poisoning. The researchers also contacted people in the outbreak areas who did not have diarrhea or food poisoning. Both groups of people were asked questions about what they had eaten around the time of the food poisoning outbreaks. The researchers then looked to see whether food that was more commonly eaten by the people with food poisoning was purchased from particular producers, and whether it was contaminated with Salmonella or E. coli O157.

What did the researchers find?

People with Salmonella or E. coli O157 food poisoning had consumed alfalfa or clover sprouts about 5 to 10 times more frequently than people without food poisoning. Three particular sprout growers were found who either used seeds or had crops that were contaminated with Salmonella or E. coli O157. The researchers identified several possible causes of contamination, such as using infected seeds, irrigating crops with unclean water, fertilizing crops with animal manure, or having livestock graze on crops.

What were the limitations of the study?

This study does not tell us how often sprouts are contaminated with bacteria that may cause food poisoning, nor does it address how often food poisoning occurs after eating sprouts.

What are the implications of the study?

Alfalfa or clover sprouts can be contaminated with bacteria and cause food poisoning. The authors particularly advise against serving sprouts in facilities where food poisoning outbreaks can occur among high-risk persons, such as nursing homes, hospitals, schools, and child care centers.