Harvester was root cause of Listeria outbreak traced to Dole salads

A former Dole executive quietly reveled how company investigators found the root cause of a Listeria outbreak that sickened people across 13 states, killing two, in 2021.

The executive, Natalie Dyenson, Vice President for Food Safety and Quality at Dole Food Company, presented the company’s findings during a 2023 webinar presented to a small group of YouTube subscribers. The 300 subscribers were signed up with the Produce Safety Science group. 

A spokesman for Dole told Food Safety News that Dyenson has since left the company. The spokesman declined to further comment on the outbreak investigation. The company never issued a statement regarding the root source of the outbreak.

This past year Dole sold its fresh vegetable operation to organicgirl LLC a portfolio company of Arable Capital Partners. Dole sold the operation for $140 million. Dole retained its facilities in Huron, CA, and Yuma, AZ. The company has 110,000 acres of production. 

Outbreak details   
The outbreak in question was reported by the Centers for Disease Control and Prevention in late 2021. It was traced to Dole packaged salads. It sickened people from coast to coast.

Although the outbreak patients were spread across 13 states, lettuce in the contaminated bagged salads was traced to only two growing regions, California and Arizona. 

Whole genome sequencing showed that all of the patients were sickened by the same strain of Listeria. Investigators were mystified because such genetic test results mean that the pathogen originated from the same source.

The CDC never reported the root source of the pathogen responsible for the outbreak.

Investigation specifics
Listeria contamination most often occurs inside a food processing facility. Dole dismantled processing equipment at production facilities in Georgia and Michigan so that sample swabbing could be done. However extensive swabbing at the facilities, more than 13,000 tests, showed no evidence of Listeria contamination.

After inspection of the facilities, Dole investigators turned to other possible sources of contamination, such as recycled pallets and corrugated cardboard. The investigators did not find any such raw materials common to both production facilities. 

It was determined that lettuce from two different growing regions was processed at the two facilities. Traceback found that the lettuce processed at the Michigan and Georgia facilities was harvested by the same crew in two different regions of the country.

Harvest equipment investigated
Lettuce is grown in various regions in the United States and weather patterns shift harvest times from one area to the next.

Lettuce harvest equipment is moved across the country from one growing region to another as the growing seasons change. Investigators found that a bulk lettuce harvester used in California was later used in Arizona. They also determined that lettuce harvested with that machine was sent from California to Georgia and later from Arizona to Michigan. Lettuce from both places was found in the outbreak products. 

When the Dole investigators inspected the implicated harvester they swabbed various parts of it. They noted that dirt and debris was trapped in parts of the machine. 

The outbreak strain of the Listeria was found on the harvester. A  bacterial database had records of similar strains of Listeria that had been recovered from just 450 feet away from the harvest area. 

Actions
Following the investigation, Dole dismantled the contaminated harvester and sold it for scrap.

Automated lettuce harvesting equipment has remained much the same in the past 100 years. In her YouTube presentation the former Dole executive included historic photos that compared harvesters from current days to those from the 1920s and 1930s. They were almost identical. The equipment was designed to be relatively lightweight and small enough to be moved on highways.

Before the outbreak, Dole already knew that once bacterial pathogens are on leafy greens they are virtually impossible to wash off. With equipment designed to be efficient rather than hygienic the harvesting of lettuce was a problem waiting to happen.

With that in mind, Dole tasked its engineers with redesigning harvesting equipment. They came up new conveyer systems and replaced plastic parts with stainless steel. 

According to the former Dole official, the company also revised how equipment should be cleaned. They opted to change cleaning and sanitizing chemicals and developed a verification process to ensure the equipment was being properly cleaned.

Source: https://www.foodsafetynews.com/2026/01/harvester-was-root-cause-of-listeria-outbreak-traced-to-dole-salads/