U.S. health officials say they believe a salmonella outbreak is linked to raw tuna found in sushi. In this photo, a man walks past images of sushi displayed outside a sushi restaurant in Tokyo on Feb. 20, 2015. Reuters/Toru Hanai

An outbreak of salmonella in the U.S. has affected 53 people across nine states, prompting an investigation by the Centers for Disease Control and Prevention (CDC).

The exact cause of the outbreak remains unknown but officials said on Thursday that the infections may be linked to raw tuna, after most of the people involved reportedly fell ill after eating sushi. Authorities are trying to trace the source of the outbreak by tracing food supplies through restaurants the patients had eaten at.

"At this time, a common brand or supplier of raw tuna has not been identified," the CDC said on its website, and cautioned the elderly, young children, pregnant women and people with weak immune systems to avoid raw fish or shellfish.

The illnesses were reported between March 5 and May 13 of this year, and the patients' ages ranged from less than one to 83 years. So far, no deaths have been reported from the outbreak, but 10 people have been hospitalized. Thirty-one of the 53 cases were reported in California where the infections have so far been limited to six counties in the state’s south.

Symptoms of salmonella, which is the most common source of food poisoning in the U.S., include diarrhea, fever and abdominal cramps. While most patients eventually recover without needing treatment, some suffer from more severe diarrhea and may need hospitalization to prevent death.