BACKGROUND: Each year in the United States, infections resulting from Salmonella enterica cause an estimated 1.2 million illnesses resulting in 19,000 hospitalizations and 390 deaths. Illnesses occur sporadically throughout the year, but might also occur as part of an outbreak. In the US, Salmonella illness outbreaks are most commonly linked to a food source; however, contact with live animals can also result in outbreaks of human illness. We sought to characterize zoonotic Salmonella outbreaks and examine differences compared to foodborne outbreaks.
METHODS: Data reported to the Centers for Disease Control and Prevention through the National Outbreak Reporting System (NORS) from 2009–2014 with a primary mode of transmission listed as animal contact or food were analyzed. Missing data for age or sex categories were recoded as age or sex unknown. Chi-square tests were conducted to compare proportions of categorical variables. Logistic regression was used to calculate odds ratios for age, sex, health outcomes and multistate exposure. Wilcoxon rank-sum tests were used to compare medians for outbreak size and duration. P-values of <0.05 were considered statistically significant. Analyses were conducted using SAS 9.3.
RESULTS: From 2009–2014, a total of 484 outbreaks with an implicated food or animal contact source were reported through NORS; of these, 99 (20.5%) resulted from Salmonella transmission through animal contact and 385 (79.5%) resulted from foodborne transmission, resulting in 3,604 (19.8%) and 13,568 (80.2%) illnesses, respectively. A significantly higher proportion of outbreak-associated illnesses among children aged <1 year and children aged 1–4 years resulted from animal contact transmission compared to foodborne transmission (15.2% vs. 1.4%, p<0.01 and 24.5% vs 5.6%, p<0.01, respectively). Outbreak-associated illnesses resulting in hospitalizations (OR 1.81, 95% CI: 1.62, 2.02), emergency room visits (OR 1.58, 95% CI: 1.14, 2.18), or healthcare provider visits (OR 2.67, 95% CI: 2.07, 3.43) were more likely to be associated with animal contact compared to food. Animal contact outbreaks reported to NORS were more likely to be multistate compared to foodborne outbreaks (OR 5.43, 95% CI: 3.37, 8.76) and had a longer median duration in days (98 days vs. 8 days, p< 0.01).
CONCLUSIONS: Outbreaks of Salmonella infections resulting from animal contact frequently have characteristics that are distinct from food outbreaks. Characterizing the differences between outbreaks linked to animal contact and outbreaks linked to food might help investigators distinguish between the two outbreaks when a vehicle isn’t initially clear and can provide useful information to improve public health responses.