BACKGROUND: Sprouts are often implicated in Salmonella outbreaks. In 2016, Colorado noted an increase in cases of two rare Salmonella serotypes (S. Reading and S. Abony), prompting a public health investigation.
METHODS: Cases were defined as Colorado residents with laboratory confirmed S. Reading or S. Abony infection with the outbreak pulsed-field gel electrophoresis (PFGE) patterns and symptom onset in June-August 2016. We conducted case finding via passive surveillance and PulseNet. We conducted a case-control study by assessing food exposures among cases and controls who dined at Chain A; online order records with phone numbers were used to recruit controls who consumed food from the same Chain A locations on the same dates as cases. We conducted environmental investigations at all implicated Chain A and B locations and the common alfalfa sprout distributor. Records were obtained from the sandwich chains and food distributors to conduct a traceback. The CDPHE laboratory tested food samples.
RESULTS: Seventeen Colorado cases were identified with onset dates from June 16 to August 5. Three cases had co-infection with S. Reading and S. Abony, and a variety of specimen sources were seen among cases, including stool (12), urine (3), stool and urine (1), and urine and blood (1). Additional PFGE-matched cases were identified in eight states. Eight Colorado cases ate sandwiches prepared at one of four Chain A locations in June 2016. Three Colorado cases ate sandwiches prepared at one of two Chain B locations in June-August 2016. We enrolled 7 cases and 43 controls. Cases were more likely than controls to have consumed deli turkey (odds ratio undefined, p=0.004), and 19 times more likely than controls to have consumed alfalfa sprouts (p=0.003). Environmental investigations and traceback activities indicated a common Colorado-based supplier of alfalfa sprouts to both chains, resulting in a product recall in August 2016. No other food products were common among both chains. Sprouts collected from one of the implicated locations were tested by the CDPHE laboratory and tested negative for Salmonella.
CONCLUSIONS: The likely cause of this outbreak was alfalfa sprouts produced by a Colorado firm and served at multiple sandwich chain restaurants. Sprout-associated Salmonella outbreaks involving multiple serotypes are uncommon. Consuming raw sprouts are a known risk factor for foodborne illness, and outbreaks linked to these products continue to be reported. Public health and food safety agencies should continue efforts to promote sprout food safety messages and actions.