Because FDA officials have not been able to pinpoint the exact source of the contamination, they are warning consumers not to eat fresh spinach regardless of the brand. Grocers and restaurants have responded by pulling spinach from their shelves and menus.
According to the FDA, there have been 19 other food-poisoning outbreaks linked to lettuce and spinach since 1995, and last November, officials called on California growers to increase the safety of their products.
Robert Brackett, FDA: "I think fast enough progress has not been made. The big part is that we're going to need a lot of research to do this but in fact we're going to have to take a much more aggressive look at protecting the public."
The FDA believes the contamination was the result of polluted irrigation or flood waters. Test results from produce packing plants aren't due back for a week or more.
According to the California Farm Bureau Federation, nearly 75 percent of the fresh market spinach grown in the U.S. comes from California. USDA estimates the farm value of the U.S. spinach crop at a little more than $210 million annually.
Given the crippling effect an outbreak can have on sales, growers are anxious to learn the results of the tests.