WEDNESDAY, Aug. 18 (HealthDay News) -- A nationwide outbreak of salmonella in eggs has sickened more than 250 people and led to a recall of more than 228 million eggs, according to news reports.

The outbreak, which apparently began in May, appears to be ongoing, the U.S. Centers for Disease Control and Prevention said.

On Aug. 13, the CDC said: "There have been confirmed Salmonella enteritidis illnesses relating to the shell eggs and traceback investigations are ongoing."

The outbreak has now been tracked to in-shell eggs from Wright County Egg in Galt, Iowa, which launched the recall. Its eggs were distributed to wholesalers and food service companies nationwide under multiple brand names: Lucerne, Albertson, Mountain Dairy, Ralph's, Boomsma's, Sunshine, Hillandale, Trafficanda, Farm Fresh, Shoreland, Lund, Dutch Farms and Kemps.

According to state health officials, contaminated eggs have sickened at least 266 Californians and seven in Minnesota, the Associated Press reported.

The recall covers eggs in their shells packed between May 16 and Aug. 13. They come in cartons ranging from six to 18 eggs and are marked with plant numbers P-1026, P-1413 and P-1946. The eggs should be returned for a refund.

"This certainly has the potential to be a very large outbreak, both given the apparent number of reported cases so far and also the fact that many of these eggs may still be in consumer refrigerators," Caroline Smith DeWaal, food safety director of the Center for Science in the Public Interest, told USA Today.

The U.S. Food and Drug Administration currently has teams on site at Wright County Egg, spokeswoman Patricia El-Hinnawy said.

"Wright County Egg is fully cooperating with FDA's investigation by undertaking this voluntary recall," the company said in a statement, according to ABC News. "Our primary concern is keeping salmonella out of the food supply and away from consumers."

Salmonella can cause fever, abdominal cramps and diarrhea and usually lasts four to seven days.

The FDA advised consumers to:

• Toss recalled eggs or return them to the store for a refund.
• See a doctor if you think you are ill after eating recalled eggs.
• Keep eggs refrigerated at all times.
• Throw out cracked or dirty eggs.
• Wash hands, utensils and preparation surfaces with soap and water after contact with raw eggs.
• Cook eggs until both the white and the yolk are firm and eat promptly after cooking.

The agency also warned consumers not to keep eggs warm or at room temperature for more than two hours, and not to eat raw eggs or restaurant dishes made with raw, undercooked, or unpasteurized eggs.

Eating undercooked eggs should also be avoided, especially by young children, the elderly and people with weakened immune systems or debilitating illness, the agency added.

More information

For more information on salmonella, visit the U.S. Centers for Disease Control and Prevention.