A salmonella outbreak tied to potentially tainted Foster Farms chicken is continuing to expand, with at least 621 now sick in 29 states, even as government health officials have urged the firm to recall days' worth of poultry produced and sold in March.

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Officials at the Centers for Disease Control and Prevention took the unusual step of updating cases in the ongoing salmonella Heidelberg outbreak on Friday, a federal holiday. The outbreak began in March 2013 and has not been able to be stopped, despite Foster Farms claims that the firm has dramatically reduced salmonella levels in its three California plants.

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Late Thursday, Foster Farms officials issued a voluntary recall for all chicken parts produced at three California plants with freeze-by or use-by dates of March 16 to March 29, 2014. Although the chicken is not still on the retail shelves of Costco, Safeway, FoodMax, Kroger and other stores that sold the meat, consumers could have chicken potentially tainted with salmonella Heidelberg in their home freezers.

U.S. Department of Agriculture officials said they have evidence for the first time of a direct link between an illness and the outbreak. Officials with the USDA's Food Safety and Inspection Services said they received a report of illness from CDC on June 23 associated with consumption of boneless chicken breast produced by Foster Farms. Using epidemiological and traceback investigations, one case of illness tied to the outbreak was identified in California. The person fell ill on May 5, 2014, FSIS officials said.

Until now, the agency had said it had no direct evidence of a link between Foster Farms chicken and the outbreak, despite the growing numbers of illnesses, including hospitalizations of more than a third of victims sickened.

The products affected by the recall include fresh chicken with use or freeze by dates of March 16 to March 29, 2014, and plant codes of P-6137, P-6137-A or P-7632.

IN-DEPTH

— JoNel Aleccia