What is clear is the difficulty of surface water contamination events can be for farms.
A summarized collection of the items of note from that report:
The E. coli Outbreak and identification of source:
- The romaine lettuce that made people sick was likely harvested between late September and mid-November 2018. This is based upon "known production practices; the anticipated shelf-life of romaine lettuce; and the fact that reported illness-onset dates occurred from October 7 through December 4, 2018."
- "All E. coli O157:H7 isolates from ill consumers had a rare genetic fingerprint, as determined by whole genome sequencing, that was closely related to one previously seen in ill consumers in the U.S. and Canada in the Fall of 2016 and the Fall of 2017. This source of this outbreak was not definitively proven, but it was felt that romaine lettuce was the source.
- The traceback indicated that multiple farms may have been involved, however only one reservoir sample was found to be positive linking that one farm. "An alternative explanation for this lack of traceback convergence to a single farm may be due to ill consumers having multiple romaine lettuce exposures and limited recollection of exposures and brands they ate."
- With regard to the progression of identification of the growing area - "By November 26, 2018, tracebacks identified a specific California growing region where romaine lettuce contaminated with the outbreak strain likely originated and this was narrowed down by December 13, 2018, to three specific California counties.
On the sediment-positive sample in the farm's reservoir system:
- "A sediment sample taken from an on-farm water reservoir in Santa Maria in Santa Barbara County, California tested positive for the outbreak strain of E. coli O157:H7. "
- "This farm, identified in multiple legs of the Fall 2018 U.S. and Canadian traceback investigations, was also identified as one of the potential suppliers of leafy greens or romaine lettuce in the 2017 U.S. and Canadian traceback investigations.
- "Bacteria, such as E. coli O157:H7 can float freely in water but because they have a charged surface, they are predominantly associated with fine particulate matter, such as sediment in bodies of water, where they can persist for extended periods of time. The sediment in the bottom of reservoirs where E. coli O157:H7 can persist is likely to be stirred up into the water when pumps are used to convey water out of a reservoir. "
- "The investigation team was not able to determine how this on-farm water reservoir became contaminated with the outbreak strain.:
- "This reservoir was not interconnected to other water sources or distribution systems in the growing region. In the case of the Santa Maria farm, the precise route of contamination is uncertain. However, based on interviews with the grower, there are several plausible ways in which water from the on-farm water reservoir may have come into contact with the implicated romaine lettuce, including direct harvest/postharvest application to the crop and/or use of reservoir agricultural water on harvest equipment food contact surfaces."
- "The use of agricultural water from a reservoir open to the environment has higher food safety risk as compared to that associated with use of ground water, because of the increased potential for human pathogens to contaminate surface water."
On water treatment of reservoir water:
- "In this case, the farm did have a procedure in place to collect and test reservoir agricultural water for generic E. coli and treat the agricultural water with a sanitizer before use. However, the investigation team noted that verification procedure records did not document that the water sanitizer was present at levels that would assure that the water used to contact romaine lettuce at harvest, during postharvest handling, and to wash/rinse harvest equipment food contact surfaces was not contaminated with pathogenic bacteria."
- "Inspection of water tank sanitizer treatment systems used in harvest/postharvest handling revealed that some units had undissolved sanitizer cakes and that some tank systems were constructed in a manner that likely did not allow for optimal sanitizer treatment of the agricultural water before use. Additionally, untreated water from the contaminated reservoir was used to fill tank trucks which broadly sprayed water on roads for dust abatement and these roads were traveled on by harvest equipment prior to commencing harvest operations."
On persistence of E. coli in water and potential contamination sources:
- "The first illnesses in this outbreak occurred in early October 2018, and therefore the outbreak strain may have been present in the on-farm water reservoir for some months or even years before the investigation team collected the positive sample, or the outbreak strain may have been repeatedly introduced into the reservoir from an unknown source. Generic E. coli has been demonstrated to survive in sediments much longer than in the overlying water. The investigation team did not identify any obvious route for contamination of the on-farm water reservoir."
- "However, evidence of extensive wild animal activity, including waterfowl, rodents, coyotes, etc., and animal burrows near the contaminated reservoir sediment was noted by the investigation team and likely warrants consideration as a possible source(s) of the human pathogen found in the on-farm water reservoir. Additionally, adjacent land use including the use of soil amendments, or for animal grazing on nearby land by cattle and horses, may have had the potential to be reservoirs of E. coli O157:H7."
FDA Outbreak Investigation Report
https://www.fda.gov/Food/RecallsOutbreaksEmergencies/Outbreaks/ucm631243.htm
Investigation Summary: Factors Potentially Contributing to the Contamination of Romaine Lettuce Implicated in the Fall 2018 Multi-State Outbreak of E. coli O157:H7
PDF - 80KB
February 13, 2019
