The US Centers for Disease Control and Prevention (CDC) said 53 people infected with the outbreak strain of Salmonella Paratyphi B variant L(+) tartrate(+) were reported from nine states.
Ten ill people have been hospitalized but no deaths have been reported.
Interviews with ill people suggest consumption of sushi made with raw tuna as a possible source of infection.
FDA traceback investigation
The Food and Drug Administration (FDA) said the investigation has not conclusively identified a food source but is increasing its monitoring of tuna.
The agency is conducting a traceback investigation, evaluating and analyzing records to determine whether there is a common source of tuna.
“In this effort, the FDA works with its investigational partners to identify clusters of people made ill in separate geographic areas and works to trace the path of food eaten by those made ill back to a common source,” it said.
“This is labor intensive and painstaking work, requiring the collection, review and analysis of hundreds and at times thousands of invoices and shipping documents.
“If a specific food or source linked to the illnesses is identified, the FDA will work to remove it from the marketplace and alert consumers to avoid it.”
FDA and CDC said as a common brand or supplier of raw tuna has not been identified, there are no specific steps for restaurants, retailers, or consumers to take to protect customers or themselves.
Reported consumption of sushi
Of 37 ill people for whom information is known, 36 (97%) reported eating sushi in the week before they became ill.
This is significantly higher than results from a survey of healthy people in which 5% reported eating "sushi, sashimi, or ceviche made with raw fish or shellfish" in the seven days before being interviewed.
Of the 36 people reporting eating sushi, 34 (94%) ate a sushi item containing raw tuna, and 21 (81%) of 26 said they had a sushi item containing raw “spicy tuna.”
Public health investigators are using PulseNet to identify illnesses that may be part of the outbreak.
DNA "fingerprinting" is performed on Salmonella bacteria isolated from ill people by using pulsed-field gel electrophoresis (PFGE).
This PFGE pattern has never been seen before in the PulseNet database, said CDC.
Illness onset dates range from March 5 to May 3 with people aged younger than 1 to 83 becoming ill.
Salmonella Paratyphi B variant L(+) tartrate(+) bacteria typically causes diarrhea, fever, and abdominal cramps 12-72 hours after being exposed.
It does not lead to paratyphoid fever, enteric fever or typhoid fever.
Illnesses come from Arizona (10), California (31), Illinois (1), Mississippi (1), New Mexico (6), South Dakota (1), Virginia (1), Washington (1), and Wisconsin (1).