The fourth Salmonella outbreak linked to fresh cucumbers since 2013 sickened people in at least eight states and spurred a national grocery chain to pull mini cucumbers from shelves this past spring — but federal officials didn’t go public about it until this week.

In a report dated today, eight months after the outbreak, scientists from the Centers for Disease Control and Prevention and several states provide details on the Salmonella Oslo outbreak. At least 14 people were sickened, with three requiring hospitalization. Their illness onset dates ranged from March 21 to April 9.

The Food and Drug Administration was aware of the outbreak at the time and helped with the investigation, but neither CDC not the FDA went public with any information until this week.

Of the 14 sick people, who ranged in age from 3 to 68 years old, officials interviewed 13. A dozen of them reported eating cucumbers in the week before becoming sick.

“Among the 12 patients who consumed cucumbers, 11 specifically reported Persian or ‘mini’ cucumbers, which are small, seedless cucumbers with smooth skin. Eight of 13 respondents reported purchasing their cucumbers from chain A,” according to the CDC’s outbreak analysis in its Morbidity and Mortality Weekly Report.

Federal and state officials believe the implicated cucumbers were sold in the U.S. by at least two nationwide grocery chains, which have only been identified as “chain A and chain B.” No recall was initiated at the time of the outbreak and no public notice was provided.

“With regard to this outbreak, the FDA wasn’t able to find the source,” said an FDA spokesman who fielded questions Thursday.

“We narrowed it down to three possible countries of origin, but we were unable to fully traceback the pathogen. FDA also wasn’t able to confirm cucumbers as the vehicle. There were no positive samples.”

The three possible countries or origin, according to the CDC’s report, were Canada, Mexico and the Dominican Republic.

“The investigation identified two Canadian (mini) cucumber suppliers during the timeframe of interest, but a single grower was not identified. Growers who could have supplied these cucumbers were located in Canada, Mexico, and the Dominican Republic,” according to the CDC’s report.

“These Canadian-grown cucumbers would have also been distributed in Canada and no reported clinical cases matching the U.S. outbreak pattern were identified.”

Traceback revealed that the cucumbers victims purchased at both “chain A” and “chain B” were sourced from a common produce supplier. “Chain A voluntarily removed all Persian cucumber products from their shelves while the investigation and traceback efforts were ongoing,” the CDC reports.

The trouble with cucumbers is, … to be determined
This spring’s outbreak marks the fourth time since 2013 that fresh cucumbers have been implicated in a Salmonella outbreak in the United States. More than 1,200 people total were sickened by the outbreaks in 2013, 2014 and 2015. Two of the outbreaks involved cucumbers from Mexico and the other was traced to cucumbers from the Delmarva region (Delaware, Maryland and Virginia).

Those outbreaks were red flags for FDA, which launched a special testing program on cucumbers in an effort to find out more about foodborne pathogen risks from the vegetable that is most often consumed raw.

“Because the prevalence of Salmonella in cucumbers is unknown, FDA has initiated an enhanced sampling program for both domestic and imported whole, fresh, raw cucumbers within fiscal year 2016,” the CDC scientists noted in their report on this past spring’s outbreak.

“The data  — approximately 380 domestic cucumber samples and more than 1,200 imported cucumber samples — will suggest whether any common factors, such as season, region, and whether the product was produced domestically or imported, are associated with Salmonella contamination.”

Despite the lack of a smoking gun and the ongoing testing project, federal officials said in the CDC report that the discovery and investigation of the outbreak this past spring shows that high-tech tools are paying off.

Scientists turn into Sherlocks
State and local public health officials in Minnesota and Michigan were the first to begin looking into the Salmonella Oslo outbreak this past spring when they identified four people with unusual infections. Then the CDC’s PulseNet, a national molecular sub-typing network for foodborne disease surveillance, confirmed a multi-state cluster of illnesses from a previously unknown strain.

“This PFGE pattern (pulsed-field gel electrophoresis pattern) was new in the database; no previous infections or outbreaks have been identified,” CDC reports.

The disease detectives used a combination of high-tech whole genome sequencing and old-fashioned Q&A for the next phase of the investigation.

“Initial interviews of ill persons found that shopping at a national chain grocer — chain A — and purchasing produce was commonly reported. A structured, focused supplemental questionnaire was developed to collect detailed information on exposure to grocery stores and produce, including cucumbers and leafy greens, in the seven days before illness onset,” according to the CDC report.

“Cucumber samples were collected from the point of sale, from patients’ homes, and from one of the Canadian suppliers, approximately one month after the patients’ purchase date, but no cucumbers yielded Salmonella.

“… This report highlights some of the inherent difficulties associated with outbreak investigations in which relatively short shelf life produce items are suspected. Given that the typical shelf life of cucumbers is 10–14 days, suspected cucumbers were no longer available in homes at the time ill persons were interviewed.”

Cooperation, coordination crucial
Although science and technology are making it easier to detect and trace outbreaks, the CDC report on this year’s Salmonella Oslo outbreak shows more research is needed, according to the scientists behind it.

“Further research is needed to understand the mechanism and factors that contribute to contamination of cucumbers during growth, harvesting, and processing to prevent future outbreaks,” the CDC investigators wrote.

They are particularly interested in finding out more about how cucumbers are contaminated with Salmonella because the bacteria is the most common cause of foodborne disease in the United States. It annually causes the more hospitalizations and deaths than other foodborne pathogens.

“Recent outbreaks have used industry consultations to help provide clues to focus the investigation, so information about cucumber harvesting and distribution was readily available,” according to the CDC report.

“Early identification and prompt investigation of this outbreak while it was still occurring was important because it enabled investigators to present evidence to chain A, a national grocer. Chain A’s swift action to remove all Persian cucumber products, in addition to the short shelf life of cucumbers, likely contributed to the small size and short duration of this outbreak.

“Quick action by industry is essential to control future outbreaks. Continued communication between state and federal agencies and implicated retail locations and industry can also enhance the timeliness of response to effectively end outbreaks.”

Recent cucumber-related Salmonella outbreaks

• 2015 Cucumbers imported from Mexico and distributed by Andrew & Williamson Fresh Produce — 907 people infected with the outbreak strains of Salmonella Poona were reported from 40 states. A total of 204 ill people were hospitalized, and six deaths were reported, but Salmonella infection was not considered to be a contributing factor in two of the deaths.
• 2014 Cucumbers from an unnamed grower in the Delmarva region (Delaware, Maryland and Virginia) — 275 people from 29 states and the District of Columbia were reported in this outbreak of Salmonella Newport. Of 141 patients for whom complete information was available, 48 were hospitalized; one death was reported in an elderly man with bacteremia.
• 2013 Imported cucumbers supplied by Daniel Cardenas Izabal and Miracle Greenhouse of Culiacán, Mexico, and distributed by Tricar Sales Inc. of Rio Rico, AZ — 84 people infected with the outbreak strain of Salmonella Saintpaul were reported from 18 states. About 28 percent of the victims were hospitalized. No deaths were reported.

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