Investigators of a recent salmonella outbreak in the US and Canada concluded that fresh tomatoes served in restaurants were the probable cause. The outbreak sickened at least 183 people, but no deaths were reported.

The outbreak began in mid-September and continued into mid-October but is now considered over. No new cases have been reported since Oct. 13, officials said.

Althougoh 22 people became so ill they required hospitalization, there have been no fatalities reported. Officials pinned the cause to fresh tomatoes served in restaurants based on interviews and surveys conducted with people who fell victim to the food poisoning.

. No particular restaurant or type of restaurant was implicated.

Now that a specific food has been linked to the outbreak, the US Food and Drug Administration will attempt to trace the farm or group of farms, or at least the area in the country where the contaminated tomatoes were grown, according to Dr. David Acheson, chief medical officer of the FDA's Center for Food Safety and Applied Nutrition.

That investigation could take a few weeks, Acheson said.

Salmonella are bacteria that pass from the feces of people or animals to other people or other animals. They cause an estimated 1.4 million cases of foodborne illness and more than 500 deaths annually in the United States.

Symptoms include diarrhea, abdominal cramps, and fever within 8 to 72 hours after eating contaminated food. Victims may also experience chills, headache, nausea, and vomiting. The infection usually runs its course in four to seven days.

While many people recover without treatment, salmonella infections can be life-threatening. Infants and young children, pregnant women and their unborn babies, and older adults are particularly vulnerable. People with weakened immune systems -- such as those with HIV/AIDS, cancer, diabetes, kidney disease and transplant patients -- are also at higher risk.