In this study, 141 case subjects were matched with 281 control individuals. Illness was significantly associated with eating at a Mexican-style restaurant (matched odds ratio 4.6), eating pico de gallo (matched OR 4.0), corn tortillas (matched OR 2.3), and freshly prepared salsa (matched odds ratio 2.1), after adjustment for sex, Hispanic ethnic background, and age.
In July 2008, the departments of health in New Mexico and Arizona, the Navajo Nation, the Indian Health Service, and the CDC conducted a household-based study using personal interviews about possible sources of infection and preparation of food. A total of 41 case households and 107 control households were included. In particular, data were collected about how cilantro, jalapeño peppers, serrano peppers, and tomatoes were brought into, stored, prepared, and consumed in the home.
On univariate analysis, illness was associated with having a raw jalapeño pepper in the home (matched OR 2.9).
Epidemiologists also investigated clusters linked to restaurants or events, which were defined as a single location or event in which at least two people with the outbreak strain became ill within 7 days of their meal date and had meal dates within 10 days of each other. Nine of these epidemiologic studies were conducted.
State and local health departments in 14 states and the District of Columbia reported 37 clusters associated with restaurants or events.
Local and state health and agricultural departments, the Food and Drug Administration, and the CDC conducted traceback investigations of distribution pathways for implicated foods associated with ill individuals and restaurant clusters.
"The FDA collected food samples and conducted environmental investigations along the distribution chain, including at distribution centers, packing facilities, and farms, to determine possible sources of contamination," wrote the investigators."
"Salsa and guacamole, both foods typically containing tomatoes and hot peppers, were implicated repeatedly in cluster investigations; these foods may have provided a medium for salmonella growth. Cut or diced tomatoes require prompt refrigeration because of the potential for salmonella growth," the authors wrote. Both salsa and guacamole are often kept at room temperature for several hours in commercial settings.
"On the basis of the FDA investigations, pepper contamination probably occurred on the farm." The outbreak strain was traced back to and identified in jalapeño peppers collected in Texas and in serrano peppers (and agricultural water) on a Mexican farm. Tomato tracebacks did not converge.
The authors reported that they have no relevant financial relationships. Dr. Osterholm reported that he is a consultant to Fresh Express, Hormel, and Alex Lee Inc.