Source Attribution of Nontyphoid Salmonellosis in New Zealand Using Outbreak Surveillance Data

Abstract

In this study, 204 New Zealand outbreaks of nontyphoid salmonellosis reported from 2000 to 2009 were analyzed for information on the sources of human infection. Data were extracted from the outbreak module of EpiSurv, New Zealand's notifiable diseases database, and augmented with information from individual case reports and separate investigation reports. The outbreaks involved 1,426 cases, representing an estimated 9% of the total salmonellosis cases reported for the study period. Salmonella Typhimurium was the causative serotype in 78% of 172 outbreaks for which a serotype was available, involving 71% of outbreak cases. The most commonly reported outbreak setting was the home (47% of outbreaks), followed by commercial food operations (31%). Foodborne transmission was reported for 63% of the 123 outbreaks for which only one mode of transmission was reported, followed by person-to-person transmission (32%), waterborne transmission (3%), and zoonotic transmission (2%). However, evidence for the mode of transmission was weak or absent for 107 (63%) of the 169 outbreaks for which a mode of transmission was reported. For only 22 outbreaks was laboratory evidence successfully used to identify a potential source of infection. Of these 22 outbreaks, 7 were foodborne, 11 involved an infected food handler, 2 were attributed to contact with animals, 1 was attributed to consumption of drinking water, and 1 was attributed to multiple sources. The laboratory-confirmed contaminated foods were diverse and included imported and domestically produced foods. The results of this analysis support the hypothesis that nontyphoid salmonellosis is primarily a foodborne disease in New Zealand, but there is insufficient evidence to confirm important food vehicles.