Multistate Outbreak of Salmonella Paratyphi B Variant L(+) Tartrate(+) and Salmonella Weltevreden Infections Linked to Imported Frozen Raw Tuna – United States, March – July 2015

BACKGROUND: Foodborne salmonellosis causes an estimated 1 million illnesses and 400 deaths annually in the United States (US). In April 2015, PulseNet, the national molecular subtyping network for foodborne disease surveillance, identified a cluster of 11 Salmonella Paratyphi B variant L(+) tartrate(+) infections from 4 states with an indistinguishable pulsed-field gel electrophoresis (PFGE) pattern which was novel in the US. State and local health departments, FDA, and CDC initiated an investigation to determine the source of the outbreak.

METHODS:  A case was defined as infection with an outbreak strain of Salmonella Paratyphi B variant L(+) tartrate(+) or Salmonella Weltevreden reported to PulseNet occurring during March 1­–July 31, 2015. Case-patients were interviewed to identify common exposures. Investigators conducted environmental and traceback investigations and tested product samples for Salmonella. Selected clinical and food isolates were further characterized by whole-genome sequencing (WGS).

RESULTS: We identified 69 cases in 11 states. The median age of case-patients was 31 years (range < 1–83); 53% were male. Twelve (18%) of 66 case-patients were hospitalized, and no deaths were reported. Of those with available information, 49 (92%) of 53 case-patients reported consuming sushi in the week before illness onset; 46 (98%) of 47 reported eating a sushi item containing raw tuna, and 28 (88%) of 32 reported eating a sushi item containing raw “spicy tuna.” Traceback from 7 of 8 points of service associated with illnesses in 4 states identified Importer A as a common source of frozen raw tuna. Minnesota investigators collected 15 intact samples of frozen raw tuna from a point of service reported by a case-patient, which received product from Importer A. Nine samples yielded the outbreak strain of Salmonella Paratyphi B variant L(+) tartrate(+) and one sample yielded a strain of Salmonella Weltevreden rarely reported to PulseNet. Epidemiologic, traceback, and WGS information linked an additional strain of Salmonella Paratyphi B variant L(+) tartrate(+) and an additional strain of Salmonella Weltevreden to the outbreak. Importer A issued three voluntary recalls of frozen raw tuna supplied by one processor in Indonesia.

CONCLUSIONS: Epidemiologic, traceback, and laboratory evidence implicated frozen raw tuna from Indonesia as the source of the outbreak. Due to the long shelf-life of this product, recalls likely prevented additional illnesses. This is the fourth multistate outbreak of Salmonella linked to frozen raw tuna reported since 2007 and highlights the continuing risk for foodborne illness associated with raw seafood consumption.