Consumers should fully cook green onions as a precaution against hepatitis A, US health officials said on Saturday as they probed the cause of an outbreak that has struck more than 500 people in Pennsylvania.

Raw or lightly cooked green onions were linked to cases of the liver disease in Tennessee, North Carolina and Georgia in September, but the source of the Pennsylvania outbreak is still unknown, the Food and Drug Administration said. Hepatitis A is usually mild but can cause fever, exhaustion, vomiting, abdominal pain and in rare cases death.

As of Saturday, 510 cases of the illness had been confirmed in Pennsylvania, the state health department said on its Web site. The Chi-Chi?s Mexican restaurant chain, which voluntarily pulled green onions off the menu at all its more than 100 restaurants after employees and patrons of a Beaver Valley, Pennsylvania, restaurant became ill from hepatitis A, said on Saturday it was told by state authorities three people had died from the outbreak.

The FDA said it appeared ?only a few shipments? of green onions were likely to have been contaminated with the hepatitis A virus. The agency advised consumers worried about contracting the illness that they can minimize the risk by thoroughly cooking green onions by boiling or frying them, or using them in foods that will be cooked, such as casseroles. At restaurants and delicatessens, concerned consumers should ask if any foods are made with green onions. Salsa, green salads and tuna salad often contain green onions, the FDA said.

The green onions implicated in the Tennessee outbreak apparently came from Mexico, the FDA said. Investigators are working with Mexican officials and monitoring green onion imports for evidence of contamination, the agency said. Anyone with symptoms of hepatitis A should consult a doctor immediately, the FDA advised.

Louisville, Kentucky-based Chi-Chi?s, a subsidiary of Prandium Inc, has said none of its other restaurants have been involved in the outbreak. The Beaver Valley location has been closed.