Recurrent multistate outbreak of Salmonella Newport associated with tomatoes from contaminated fields, 2005

Abstract

Salmonella Newport causes more than an estimated 100,000 infections annually in the United States. In 2002, tomatoes grown and packed on the eastern shore of Virginia contaminated with a pan-susceptible S. Newport strain caused illness in 510 patients in 26 states. In July-November 2005, the same strain caused illness in at least 72 patients in 16 states. We conducted a case-control study during the 2005 outbreak, enrolling 29 cases and 140 matched neighbourhood controls. Infection was associated with eating tomatoes (matched odds ratio 9.7, 95% confidence interval 3.3-34.9). Tomatoes were traced back to the eastern shore of Virginia, where the outbreak strain was isolated from pond water used to irrigate tomato fields. Two multistate outbreaks caused by one rare strain, and identification of that strain in irrigation ponds 2 years apart, suggest persistent contamination of tomato fields. Further efforts are needed to prevent produce contamination on farms and throughout the food supply chain.

Fig. 1

Pulsed-field gel electrophoresis (PFGE) of Salmonella Newport isolates from patients. Lanes 1, 2, XbaI restriction enzyme PulseNet PFGE pattern JJPX01.0061; lanes 3, 4, BlnI restriction enzyme PulseNet PFGE pattern JJPA26.0021; lane S, the PulseNet universal size standard Salmonella Braenderup H9812 digested with XbaI [34].

Fig. 2

Case-patients with Salmonella Newport infections due to outbreak strain who were enrolled (■, n=30) and not enrolled (□, n=42) into the case-control study, by week specimen received in state laboratory, 2005.

Fig. 3

Distribution of culture-confirmed cases in 2002 and 2005 (separated by semi-colon) due to outbreak strain of Salmonella Newport, by state.

Fig. 4

Number of all human Salmonella Newport cases (gray) with subset that were subtyped as the outbreak PFGE pattern (black), by week, 2002–2005. The large tomato-associated outbreaks in 2002 and 2005 are visible. Small clusters of the outbreak pattern were also detected in the same seasons of 2003 and 2004, but were not investigated.