Shiga Toxin 1-Producing Shigella sonnei Infections, California, United States, 2014-2015.

Shiga toxins (Stxs) are cytotoxins that mediate severe gastrointestinal disease caused by Shiga toxin–producing Escherichia coli (STEC) and Shigella dysenteriae serotype 1 (1). S. dysenteriae 1 produces the prototype Shiga toxin (Stx), and STEC can produce 2 groups of Stxs: Stx1 and Stx2. Vascular damage caused by Stxs in the colon, kidneys, and central nervous system may result in hemorrhagic colitis, or more severe conditions such as hemolytic uremic syndrome (HUS) (2). Stx2 has been shown to be more virulent than Stx1 (3), and adverse clinical outcomes such as HUS are more frequently associated with Stx2-producing strains of STEC than Stx1-producing strains (4). Antimicrobial drug treatment for STEC infections and late or inappropriate antimicrobial drug treatment for S. dysenteriae 1 infections have been associated with an increased risk for HUS (5,6). Although Shiga toxins have been associated with STEC and S. dysenteriae 1, infections caused by other types of Stx-producing Shigella spp. have been recognized in recent years. Sporadic infections with Stx1-producing S. dysenteriae serotype 4 and S. flexneri in persons with a history of travel to the Caribbean island of Hispaniola have been characterized in the United States and Canada (7–9). A recent survey of Shigella isolates from persons with a history of travel to the Caribbean found that 21% of isolates encoded and produced Stx; positive strains were S. flexneri 2a, S. flexneri Y, and S. dysenteriae 4 (10). The same Stx-converting bacteriophage was identified among these isolates, suggesting the emergence of Stx-producing shigellae in this region was caused by spread of the phage to multiple Shigella species and serotypes. One case of infection with Stx1-producing S. sonnei in a patient from Germany who had a history of travel to Ukraine (11) and one instance of isolation of stx2a-encoding S. sonnei from a patient from Finland who had a history of travel to Morocco have been described (12). Although these novel strains of Stx-producing shigellae have been reported recently, data regarding the clinical characteristics and epidemiology of these infections remain limited. In the United States, infections with Stx-producing organisms are primarily caused by STEC; S. dysenteriae 1 infections are rare. Of laboratory-confirmed cases of shigellosis reported in the United States, ≈75% are caused by S. sonnei (13). S. sonnei infections are typically less severe than infections with S. dysenteriae 1 and are characterized by diarrhea, which may be bloody and accompanied by fever, nausea, and abdominal cramps. Illnesses are usually self-limited and resolve within 5–7 days of onset. Extraintestinal complications such as bacteremia and urogenital infections are rare but have been documented (14–17). Although antimicrobial drug treatment is generally unnecessary for patients with uncomplicated S. sonnei infections, antimicrobial drugs are often used to limit the duration of illness and communicability and to reduce illness severity (18). In August 2014, Stx1-positive fecal samples from 2 patients tested by using enzyme immunoassay were reported to the County of San Diego Health and Human Services Agency (COSD HHSA; San Diego, CA, USA); the same clinical specimens showed positive S. sonnei culture results. COSD HHSA initiated an investigation and notified the California Department of Public Health (CDPH). Surveillance was enhanced retrospectively and prospectively in California to confirm Stx-producing S. sonnei isolates, identify additional cases, describe the clinical and epidemiologic characteristics of infected persons, monitor for severe clinical outcomes such as HUS, and to investigate for potential sources of infection. We subsequently identified 2 clusters of Stx-producing S. sonnei; the second cluster continued into late 2015. We report our findings on the initial 56 confirmed cases of Stx1-producing S. sonnei identified during June 2014–April 2015.