Researchers from the Interagency Food Safety Analytics Collaboration (IFSAC) said if examined characteristics are different, then the assumption of similar exposure pathways is less plausible.
This would help assess the usefulness of outbreak data in estimating which categories are most often linked to foodborne illnesses, they added.
The assumption of similar exposure pathways for outbreak and sporadic illnesses cannot be tested directly because the food sources of sporadic illnesses typically are unknowable.
Using data from the CDC’s Foodborne Diseases Active Surveillance Network (FoodNet) from 2004-2011, the work compared demographic, clinical, temporal, and geographic characteristics of outbreak and sporadic illnesses.
This was done for Salmonella, E. coli O157, Listeria monocytogenes and Campylobacter and concluded that characteristics of outbreak and sporadic cases vary for the four pathogens.
The six characteristics
- the FoodNet site reporting the case;
- year in which a case occurred;
- the season in which it occurred;
- age of patient;
- sex of patient;
- hospitalization status of patient
“The research findings are important because they indicate that, with the exception of Salmonella illnesses among children younger than three years, it is reasonable to use outbreak data to estimate which foods are most often linked to foodborne illnesses,” said the researchers.
FoodNet data includes only a portion of reported US illnesses (about 15% of the population); so the number of outbreaks and illnesses analysed was limited.
For example, fewer Campylobacter illnesses were associated with outbreaks compared with the other three pathogens, which limits the strength of conclusions about Campylobacter attribution.
The analysis found no evidence that laboratory-confirmed outbreak and sporadic cases are dissimilar with respect to sex or hospitalization status of patients.
During the study period, <1% of Campylobacter infections from FoodNet but ≈20% of E. coli O157 infections were outbreak cases. These represented ≈5% of Listeria and Salmonella infections.
Outbreak and sporadic illnesses
Campylobacter, Listeria monocytogenes and E. coli O157 outbreak illnesses are not significantly different from sporadic ones with respect to patients’ illness severity, gender, and age.
Salmonella outbreak illnesses are not significantly different from sporadic illnesses on illness severity and gender. For age, the percentages of outbreak and sporadic illnesses that occur among older children and adults are also similar.
However, the percentage of outbreak illnesses in the youngest age category (0-3 years) was substantially lower compared with the other age groups.
Researchers did a random forest and boosted tree analyses to gauge the relative importance of six characteristics in distinguishing between outbreak and sporadic cases before doing logistic regression modelling.
Seasonal quintiles were similar across pathogens except for E. coli O157; the first season was longer compared with the other pathogens, extending from January through the end of May.
IFSAC was created in 2011 by the Centers for Disease Control and Prevention (CDC), the US Food and Drug Administration (FDA) and the US Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS).
“Comparing Characteristics of Sporadic and Outbreak-Associated Foodborne Illnesses, United States, 2004–2011”
Authors: Eric D. Ebel, Michael S. Williams , Dana Cole, Curtis C. Travis, Karl C. Klontz, Neal J. Golden, and Robert M. Hoekstra