Outbreak of Multidrug-Resistant Salmonella I 4,[5],12:I:- Infections Linked to Pork in Washington State, 2015

BACKGROUND: In June 2015, Public Health—Seattle & King County and the Washington State Department of Health detected an outbreak of Salmonella enterica I 4,[5],12:i-, a serotype commonly isolated in humans and pigs. It can be multidrug-resistant and has been linked to animal products in previous outbreaks. It is one of the most frequently reported serotypes in the United States, but was seen infrequently in Washington in the five years prior to this outbreak (average: 33 cases/year).

METHODS: Cases were defined as patients with one of 6 pulsed-field gel electrophoresis (PFGE) patterns for Salmonella I 4,[5],12:i:- or Infantis and illness onset from April 25–October 15, 2015. Patient interviews, traceback investigation, environmental sampling, and comparison of food exposures to FoodNet Population Survey data were used to identify the source of the outbreak. Whole genome sequencing (WGS) was performed on isolates to assess relatedness of the samples and identify cases outside of Washington. Antimicrobial resistance testing was performed on selected clinical samples.

RESULTS: There were 192 cases from 5 states with serotype I 4,[5],12:i:- (188) or Infantis (4). The majority (184) were Washington residents. Median age was 34 (range <1–90), half (50%) were female, 16% were hospitalized, and there were no deaths. Seventy-six percent of patients (94/123) consumed pork, which is significant compared to the FoodNet Population Survey (43% p<0.001). Of those that consumed pork, 73% (65/89) traced back to a single pork-slaughter facility (Establishment A) located in Washington State. Environmental samples acquired at Establishment A were positive for serotype I 4,[5],12:i:-. Establishment A recalled 639,642 pounds of pork products and voluntarily suspended operations on August 27, 2015.

Most (96/153) patients available for interview reported eating pork or other dishes from markets or restaurants that received pork from Establishment A, attended pig roasts, or had live pig exposure. Environmental samples collected from the site of a live pig exposure also PFGE-matched one of the outbreak strains. WGS discerned two separate clades of highly related isolates. All ten samples tested yielded multidrug resistance.

CONCLUSIONS: Epidemiologic, laboratory, and traceback evidence clearly implicated pork processed at Establishment A in this outbreak. However, investigation at commercial farms was not able to be completed and is an area for future research. This outbreak emphasizes the need for Salmonella control measures along all levels of the supply chain, as well as the need for investigation into heat resistance, pathogenicity, and drug resistance of serotype I 4,[5],12:i:-.