The Morbidity and Mortality Weekly Report (MMWR) looked at the outbreak to see if reports from four patients were involved.
Initial and expanded recall in July
Wawona Packing Company of Cutler, California recalled all products packed in its facility from June 1 through July 17, 2014 due to Listeria monocytogenes.
The firm shipped products directly to retailers and wholesalers who resold or further distributed them.
An initial recall was expanded to cover fresh, whole peaches, plums, nectarines and pluots.
Because it did not know the locations of the companies or stores that received products from direct customers, it issued a nationwide recall.
Although exposure to the recalled product was likely widespread, disease was very rare so there is not sufficient evidence to recommend people at higher risk for listeriosis avoid fresh stone fruits, said the researchers.
However, it does support the need to understand risks associated with contaminated, ready-to-eat fresh fruit so that prevention strategies can be strengthened, they added.
The initial recall in July was based on internal company testing. The firm shut down packing lines, retrofitted equipment, sanitized the facility, and worked with experts on Listeria to make improvements to the food safety program.
After the recall, clinicians, state and local health departments, CDC, and the Food and Drug Administration (FDA) received inquiries about listeriosis from consumers, who had received automated telephone calls informing them that they had purchased recalled fruit.
During July 19–31, the CDC Listeria website received >500,000 page views, more than seven times the number received during the previous 52 weeks.
However, no molecular information from L. monocytogenes isolates was available to assess whether human illnesses might be linked to these products.
PFGE and WGS findings
In August 2014, a two-enzyme pulsed-field gel electrophoresis (PFGE) pattern shared by three L. monocytogenes isolates from stone fruit associated with the recall was uploaded to PulseNet, the national molecular subtyping network for foodborne disease surveillance.
Four human isolates with isolation dates during the period May 8–July 8, 2014 (Illinois, Massachusetts, and South Carolina) and August 28 (Minnesota) were identified that had PFGE patterns indistinguishable from isolates from the company’s stone fruit.
Samples from the firm collected after the recall yielded 31 L. monocytogenes isolates, 22 of which were indistinguishable from the initial isolates by PFGE; three other PFGE patterns were identified that did not match any isolates from clinical specimens collected during May 1–August 31.
Whole-genome sequencing (WGS) analysis by whole-genome multilocus sequence typing showed that isolates from the Massachusetts and Minnesota patients were highly related to those from recalled stone fruits, while the Illinois and South Carolina isolates were not.
Strong evidence linked the Massachusetts case to recalled stone fruit, including food exposure interviews, receipt and shopper card data, and WGS results showing very high genetic relatedness between the patients isolate and isolates from nectarines.
Consumption data and WGS results suggest stone fruit was also the likely source of L. monocytogenes infection in the Minnesota case; however, the later dates of illness onset and fruit purchase suggest the patient consumed stone fruit not included in the recall.
The South Carolina patient reportedly did not eat stone fruit before becoming ill and the family of the Illinois patient could not be reached for interview.