Notes from the Field: Rapidly Linking an Outbreak of Salmonella Typhimurium Infections to Domestically Grown Cantaloupes Through Early Collaboration — United States, 2022

Investigation and Outcomes

On August 4, 2022, PulseNet identified 12 S. Typhimurium infections that were genetically related within seven allele differences by WGS to the 2020 environmental strain. Cases were defined as infections with isolates that were related to the 2020 strain within 10 allele differences and that occurred during July 7–September 11, 2022. In total, 87 outbreak cases from 11 states were identified in 2022.^† The median patient age was 65 years (range = 1–93 years); 67% of patients were female. Thirty-two (37%) patients were hospitalized; none died.

Upon outbreak detection, investigators worked with state and local agencies to assess cantaloupe and watermelon exposure, which were vehicles of interest based on previous outbreak investigations. In 2022, cantaloupe consumption was reported significantly more frequently by ill persons (36 of 47; 77%) than during a 2018–2019 survey of healthy persons conducted on FoodNet sites (29%, p<0.001) (2). FDA traced the source of cantaloupes purchased by ill persons to a common geographic region close to where the 2020 Salmonella environmental strain was identified. By August 25, 2022, the combination of epidemiologic and traceback data and relationship to the 2020 environmental strain indicated that cantaloupes grown in the Midwest were the likely outbreak source. At the time cantaloupes were identified as the source, the 2022 cantaloupe growing season (May–July) had already ended (3). As a result, contaminated melons were unlikely to be on the market; therefore, a recall was not initiated because ongoing foodborne illness risk had ceased. In 2022, the time from outbreak detection to determining melons were the likely source was 14 days shorter compared to the 2020 outbreak investigation, which ranged from September 18, 2020–October 23, 2020.

Preliminary Conclusions and Actions

Although the risk for foodborne illness from contaminated melons had ended before definitive public health action could be taken, this investigation highlights how WGS-based surveillance combined with rapid epidemiologic data collection by state and local agencies can be used to reduce the time to outbreak detection and response. The time from outbreak detection to source identification was 2 weeks shorter in 2022 compared with that during the 2020 outbreak. This shortened time frame is attributable to collaboration with partners to prepare to rapidly assess food exposures after illnesses with the 2020 environmental strain were identified. In the future, these activities, paired with prospective melon sampling and Salmonella testing might identify melon-associated outbreak strains earlier, further speeding outbreak investigations by quickly narrowing to a likely source. The strategies detailed in this report might increase the likelihood of public health action during future outbreaks.