CDC Declares Outbreak Linked to Imported Cucumbers ”Over”
The protracted outbreak of Salmonella Poona associated with imported cucumbers is finally over, according a final outbreak update issued by the CDC. The cucumbers, distributed by Andrew & Williamson Fresh Produce, ultimately caused 907 confirmed outbreak cases of Salmonella Poona; however, though the actual number of outbreak cases is likely much higher due to the consistent and widespread under-reporting of Salmonella poisoning.
The infection rapidly spread across 40 states, killing six and hospitalizing at least 204. Only 720 cases provided detailed information, including whether or not the illness required hospitalization, so the outbreak involved an effective hospitalization rate of 28%, meaning more than 204 confirmed cases likely required hospitalization. Texas, Arizona, and Oklahoma each reported one fatality, while California reported three. The following ten states ultimately did not report a confirmed outbreak case: Delaware, Maine, Massachusetts, Michigan, Mississippi, New Jersey, North Carolina, Rhode Island, Vermont, and West Virginia. All forty states not on that list confirmed at least one confirmed case of the outbreak strain of Salmonella Poona, a vivid illustration of how expansive the outbreak proved to be.
Just as the pathogen did not discriminate on the basis of location, the infection showed no regard for the victim’s age: confirmed cases occurred in children as young as one and adults as old as 99. The outbreak did, however, disproportionately affect the younger population: 49% of outbreak cases were children younger than 18 years old, also the median age for the outbreak.
Outbreak Investigation and Response Revisited
Multiple state agencies, the Food and Drug Administration (FDA), and the CDC engaged in a collaborative investigation to determine the source of the illnesses. Health officials conducted epidemiologic, laboratory and traceback investigation, ultimately identifying “slicer” cucumbers – also known as “American” cucumbers – imported from Mexico and distributed by Andrew & Williamson Fresh Produce as the likely source of the outbreak. When sold in stores, consumers can typically locate the dark green, 7-10 inch vegetables in bulk displays, unwrapped and unpackaged. In this case, the imported cucumbers contaminated with Salmonella were also distributed to restaurants and other food establishments, and were included in dishes such as salads.
Ultimately, however, while the imported cucumbers were identified as the source of the outbreak, the source of the cucumbers’ contamination was never identified.
Regulatory and Traceback Investigation
Consultation with independent industry experts early in the investigation provided important information, helping investigators focus in on certain foods as the potential culprit. Four independent produce industry experts were consulted on August 26, 2015, as part of the effort to gather information about the harvesting and distribution of fresh produce in the areas where illnesses were being reported. Information regarding crop production and distribution practices enabled officials to more accurately assess how plausible it was that cucumbers or other produce items were the source of the outbreak.
Laboratory Investigation
Agriculture and health departments on the state level tested imported cucumbers collected from retail locations for the outbreak strain of Salmonella. The tests, which came back positive for the outbreak strain, were shared with Andrew & Williamson Fresh Produce. Imported cucumbers collected directly from an Andrew & Williamson facility by state officials subsequently tested positive for the outbreak strain.
Results of a process called whole genome sequencing (WGS) demonstrated that the strain of Salmonella contaminating Andrew & Williamson cucumbers and the strain found in outbreak cases were close genetic relatives, which provided investigators with additional evidence that people were being made ill by consuming cucumbers distributed by the company.
Epidemiologic Investigation
Efforts involved in the epidemiologic investigation included interviews by local and state public health officials. The officials interviewed outbreak victims to determine what foods they consumed in the week before falling ill and whether any other potential exposures could have occurred. 519 people were interviewed in connection with the investigation. Of those 519 people, 75% (391 people) reported eating cucumbers. A baseline survey of healthy people reflected a significantly lower consumption rate of 49%, suggesting that cucumbers were somehow involved in the outbreak illnesses.
Further, due to the size of the outbreak, there were a eleven distinct groups of two or more people who do not live together that reported attending a common event, eating at the same restaurant or shopping at the same grocery store during the week before the onset of illness. These groups are referred to as “illness clusters,” which can provide information critical to identifying the source of an outbreak by strongly suggesting that the contaminated product was sold or served at that location. All eleven clusters involved in the Salmonella Poona cucumber outbreak, investigators determined that cucumbers had been consumed by all ill individuals.
