2008 Salmonella Outbreak Illustrates Difficulty Identifying Source

Tomatoes first got the blame for a nationwide outbreak of salmonella in 2008, but epidemiologic investigation and microbiologic analysis revealed that jalapeño and serrano peppers played a key role.

Photo credit: Flickr user Choose_Freewill (Creative Commons)

A team of national, state, and local investigators identified jalapeño peppers as the major vehicle for transmission of the Saintpaul serotype of Salmonella enterica that lasted from April to September 2008, according to a study published in the New England Journal of Medicine on Feb. 23 (doi: 10.1056/NEJMoa1005741). Serrano peppers were also identified as a vehicle of transmission.

"This outbreak of foodborne disease in the United States was one of the largest salmonella outbreaks ever identified," wrote Casey Barton Behravesh, D.V.M., Dr.P.H., on behalf of the Salmonella Saintpaul Outbreak Investigation Team. Dr. Behravesh is with the National Center for Emerging and Zoonotic Infectious Diseases, Atlanta.

In all, the investigators identified 1,500 individuals who were infected with the outbreak strain of the salmonella Saintpaul serotype in 43 states, the District of Columbia, and Canada. The greatest incidence rates occurred in New Mexico and Texas – 58.4 cases/million population and 24.5/million population, respectively. Illnesses peaked between mid-May and mid-June. Among case subjects, 21% were hospitalized, and the infection may have contributed to two deaths.

"This outbreak investigation highlights the recurring challenges of epidemiologic identification of ingredients in foods that are commonly consumed, rapid identification and investigation of local clusters, the need to continue exploring hypotheses during an outbreak, and produce tracing in the supply chain," the authors noted.

In an accompanying editorial, Michael T. Osterholm, Ph.D., pointed out the importance of considering all possible sources and the difficulty of identifying outbreaks related to raw produce. "Barton Behravesh et al. remind us that previously unrecognized vehicles for foodborne disease, such as jalapeño peppers can cause large nationwide outbreaks," he said (doi: 10.1056/NEJMp1010907). Dr. Osterholm is with the Center for Infectious Disease Research and Policy at the University of Minnesota, Minneapolis.

"Outbreaks associated with raw produce are among the most difficult ones for public health officials to identify and control, since produce from a single farm may be distributed widely and consumed rapidly because it is perishable," he noted.

The authors also noted that based on their experience with this outbreak, "improvements in product-tracing systems and the ability of the systems to work together are needed for more rapid tracing of implicated products through the supply chain."

The investigation began on May 22, when the New Mexico Department of Health notified the Centers for Disease Control and Prevention about 19 cases of salmonella infection. All seven isolates with complete serotyping were S. enterica serotype Saintpaul. The next day the CDC identified three additional isolates with the same pulsed-field gel electrophoresis (PFGE) pattern in Colorado and Texas.

Outbreaks were identified via clinical laboratories that sent salmonella strains from ill people to state public health laboratories for serotyping. State laboratories routinely submit PFGE patterns to the CDC’s PulseNet (a national molecular subtyping network).

For this outbreak, cases were defined as laboratory-confirmed infection with a specific PFGE XbaI pattern from April 1, 2008 through Sept. 4, 2008. Diarrhea was defined as at least three loose stools in a 24-hour period.

Three case-control epidemiologic studies were conducted to investigate cases that were not linked to restaurant clusters. The first study was conducted in May 2008 by the departments of health in New Mexico and Texas, the Navajo Nation, the Indian Health Service, and the CDC.

In this hypothesis-generating study, case individuals were interviewed about food consumption – particularly red and green bell peppers and other peppers. Raw tomatoes were the most commonly reported food consumed by ill individuals (84%). Roughly a quarter (26%) reported eating peppers other than red or green bell peppers.

In this study, 51 case subjects were matched with 106 control individuals. Illness was significantly associated with eating raw tomatoes (matched odds ratio 5.6) after adjustment for consumption of tortillas. However, illness was not significantly associated with eating salsa or any other food item.

In subsequent studies, investigators included all food items that were reported to have been consumed by more than half of case individuals in this first study (in addition to avocado and guacamole).

In June 2008, the CDC and state and local health departments in 29 states conducted a case-control study to better identify sources. In particular, data were collected on patterns of eating at Mexican-style restaurants and the consumption of raw produce.

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.

Tomatoes first got the blame for a nationwide outbreak of salmonella in 2008, but epidemiologic investigation and microbiologic analysis revealed that jalapeño and serrano peppers played a key role.

Photo credit: Flickr user Choose_Freewill (Creative Commons)

A team of national, state, and local investigators identified jalapeño peppers as the major vehicle for transmission of the Saintpaul serotype of Salmonella enterica that lasted from April to September 2008, according to a study published in the New England Journal of Medicine on Feb. 23 (doi: 10.1056/NEJMoa1005741). Serrano peppers were also identified as a vehicle of transmission.

"This outbreak of foodborne disease in the United States was one of the largest salmonella outbreaks ever identified," wrote Casey Barton Behravesh, D.V.M., Dr.P.H., on behalf of the Salmonella Saintpaul Outbreak Investigation Team. Dr. Behravesh is with the National Center for Emerging and Zoonotic Infectious Diseases, Atlanta.

In all, the investigators identified 1,500 individuals who were infected with the outbreak strain of the salmonella Saintpaul serotype in 43 states, the District of Columbia, and Canada. The greatest incidence rates occurred in New Mexico and Texas – 58.4 cases/million population and 24.5/million population, respectively. Illnesses peaked between mid-May and mid-June. Among case subjects, 21% were hospitalized, and the infection may have contributed to two deaths.

"This outbreak investigation highlights the recurring challenges of epidemiologic identification of ingredients in foods that are commonly consumed, rapid identification and investigation of local clusters, the need to continue exploring hypotheses during an outbreak, and produce tracing in the supply chain," the authors noted.

In an accompanying editorial, Michael T. Osterholm, Ph.D., pointed out the importance of considering all possible sources and the difficulty of identifying outbreaks related to raw produce. "Barton Behravesh et al. remind us that previously unrecognized vehicles for foodborne disease, such as jalapeño peppers can cause large nationwide outbreaks," he said (doi: 10.1056/NEJMp1010907). Dr. Osterholm is with the Center for Infectious Disease Research and Policy at the University of Minnesota, Minneapolis.

"Outbreaks associated with raw produce are among the most difficult ones for public health officials to identify and control, since produce from a single farm may be distributed widely and consumed rapidly because it is perishable," he noted.

The authors also noted that based on their experience with this outbreak, "improvements in product-tracing systems and the ability of the systems to work together are needed for more rapid tracing of implicated products through the supply chain."

The investigation began on May 22, when the New Mexico Department of Health notified the Centers for Disease Control and Prevention about 19 cases of salmonella infection. All seven isolates with complete serotyping were S. enterica serotype Saintpaul. The next day the CDC identified three additional isolates with the same pulsed-field gel electrophoresis (PFGE) pattern in Colorado and Texas.

Outbreaks were identified via clinical laboratories that sent salmonella strains from ill people to state public health laboratories for serotyping. State laboratories routinely submit PFGE patterns to the CDC’s PulseNet (a national molecular subtyping network).

For this outbreak, cases were defined as laboratory-confirmed infection with a specific PFGE XbaI pattern from April 1, 2008 through Sept. 4, 2008. Diarrhea was defined as at least three loose stools in a 24-hour period.

Three case-control epidemiologic studies were conducted to investigate cases that were not linked to restaurant clusters. The first study was conducted in May 2008 by the departments of health in New Mexico and Texas, the Navajo Nation, the Indian Health Service, and the CDC.

In this hypothesis-generating study, case individuals were interviewed about food consumption – particularly red and green bell peppers and other peppers. Raw tomatoes were the most commonly reported food consumed by ill individuals (84%). Roughly a quarter (26%) reported eating peppers other than red or green bell peppers.

In this study, 51 case subjects were matched with 106 control individuals. Illness was significantly associated with eating raw tomatoes (matched odds ratio 5.6) after adjustment for consumption of tortillas. However, illness was not significantly associated with eating salsa or any other food item.

In subsequent studies, investigators included all food items that were reported to have been consumed by more than half of case individuals in this first study (in addition to avocado and guacamole).

In June 2008, the CDC and state and local health departments in 29 states conducted a case-control study to better identify sources. In particular, data were collected on patterns of eating at Mexican-style restaurants and the consumption of raw produce.

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.

Tomatoes first got the blame for a nationwide outbreak of salmonella in 2008, but epidemiologic investigation and microbiologic analysis revealed that jalapeño and serrano peppers played a key role.

Photo credit: Flickr user Choose_Freewill (Creative Commons)

A team of national, state, and local investigators identified jalapeño peppers as the major vehicle for transmission of the Saintpaul serotype of Salmonella enterica that lasted from April to September 2008, according to a study published in the New England Journal of Medicine on Feb. 23 (doi: 10.1056/NEJMoa1005741). Serrano peppers were also identified as a vehicle of transmission.

"This outbreak of foodborne disease in the United States was one of the largest salmonella outbreaks ever identified," wrote Casey Barton Behravesh, D.V.M., Dr.P.H., on behalf of the Salmonella Saintpaul Outbreak Investigation Team. Dr. Behravesh is with the National Center for Emerging and Zoonotic Infectious Diseases, Atlanta.

In all, the investigators identified 1,500 individuals who were infected with the outbreak strain of the salmonella Saintpaul serotype in 43 states, the District of Columbia, and Canada. The greatest incidence rates occurred in New Mexico and Texas – 58.4 cases/million population and 24.5/million population, respectively. Illnesses peaked between mid-May and mid-June. Among case subjects, 21% were hospitalized, and the infection may have contributed to two deaths.

"This outbreak investigation highlights the recurring challenges of epidemiologic identification of ingredients in foods that are commonly consumed, rapid identification and investigation of local clusters, the need to continue exploring hypotheses during an outbreak, and produce tracing in the supply chain," the authors noted.

In an accompanying editorial, Michael T. Osterholm, Ph.D., pointed out the importance of considering all possible sources and the difficulty of identifying outbreaks related to raw produce. "Barton Behravesh et al. remind us that previously unrecognized vehicles for foodborne disease, such as jalapeño peppers can cause large nationwide outbreaks," he said (doi: 10.1056/NEJMp1010907). Dr. Osterholm is with the Center for Infectious Disease Research and Policy at the University of Minnesota, Minneapolis.

"Outbreaks associated with raw produce are among the most difficult ones for public health officials to identify and control, since produce from a single farm may be distributed widely and consumed rapidly because it is perishable," he noted.

The authors also noted that based on their experience with this outbreak, "improvements in product-tracing systems and the ability of the systems to work together are needed for more rapid tracing of implicated products through the supply chain."

The investigation began on May 22, when the New Mexico Department of Health notified the Centers for Disease Control and Prevention about 19 cases of salmonella infection. All seven isolates with complete serotyping were S. enterica serotype Saintpaul. The next day the CDC identified three additional isolates with the same pulsed-field gel electrophoresis (PFGE) pattern in Colorado and Texas.

Outbreaks were identified via clinical laboratories that sent salmonella strains from ill people to state public health laboratories for serotyping. State laboratories routinely submit PFGE patterns to the CDC’s PulseNet (a national molecular subtyping network).

For this outbreak, cases were defined as laboratory-confirmed infection with a specific PFGE XbaI pattern from April 1, 2008 through Sept. 4, 2008. Diarrhea was defined as at least three loose stools in a 24-hour period.

Three case-control epidemiologic studies were conducted to investigate cases that were not linked to restaurant clusters. The first study was conducted in May 2008 by the departments of health in New Mexico and Texas, the Navajo Nation, the Indian Health Service, and the CDC.

In this hypothesis-generating study, case individuals were interviewed about food consumption – particularly red and green bell peppers and other peppers. Raw tomatoes were the most commonly reported food consumed by ill individuals (84%). Roughly a quarter (26%) reported eating peppers other than red or green bell peppers.

In this study, 51 case subjects were matched with 106 control individuals. Illness was significantly associated with eating raw tomatoes (matched odds ratio 5.6) after adjustment for consumption of tortillas. However, illness was not significantly associated with eating salsa or any other food item.

In subsequent studies, investigators included all food items that were reported to have been consumed by more than half of case individuals in this first study (in addition to avocado and guacamole).

In June 2008, the CDC and state and local health departments in 29 states conducted a case-control study to better identify sources. In particular, data were collected on patterns of eating at Mexican-style restaurants and the consumption of raw produce.

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.