WASHINGTON — Hundreds of people in 18 states have become sick from a salmonella outbreak, linked to raw chicken products made at three California plants, owned by Foster Farms, according to the U.S. Department of Agriculture.

“The outbreak is continuing,” USDA’s Food Safety and Inspection Service said in a statement.

An estimated 278 illnesses, mostly in California, were caused by strains of Salmonella Heidelberg. The chicken products were distributed mostly to retail outlets in California, Oregon and Washington state, the USDA said.

The illnesses were linked to Foster Farms brand chicken through epidemiologic, laboratory and traceback investigations, conducted by local, State and federal officials.

The Centers for Disease Control and Prevention is partnering with state health departments to monitor the outbreak while the Food Safety and Inspection Service (FSIS) continues its investigation.

The most common symptoms of salmonella infection are diarrhea, abdominal cramps and fever within eight to 72 hours. Additional symptoms may be chills, headache, nausea and vomiting that can last up to seven days.