Raw chicken packaged at three Foster Farms facilities in California has been identified as the cause of a salmonella outbreak that has sickened 278 people in 18 states since March, according to federal officials.

Consumers need to cook chicken thoroughly and take other precautions in order to prevent illness, the Department of Agriculture said Monday. The chicken was distributed to retail outlets in California, Oregon and Washington state and most of the illnesses have occurred in California, the Associated Press reported.

Even though the outbreak appears to have begun in March, the USDA was only notified of the illnesses in July, according to Dan Engeljohn of the USDA's Food Safety and Inspection Service. He said investigators had a difficult time pinpointing the source of the salmonella.

No recall is in effect and the salmonella infections were caused by eating chicken that was undercooked or improperly handled, a spokesman for Foster Farms told the AP.

The three facilities that packaged the chicken associated with the outbreak were all in California's Central Valley, one in Livingston and two in Fresno. The USDA had not directly connected the outbreak to a specific product or production period. Suspect packages of chicken would have USDA marks P6137, P6137A and P7632.

California health officials said no recall was planned, but reminded consumers that chicken must be cooked to an internal temperature of 165 degrees F.

"This is the important public health issue," Anita Gore, spokeswoman for the California Department of Public Health, told the AP. "Chicken can carry bacteria, and chicken needs to be fully cooked."

She added that people must thoroughly wash their hands after handling raw meat.

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