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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Produce Safety and Microbiology Research » Research » Publications at this Location » Publication #218295

Title: Incidence and tracking of Escherichia coli O157:H7 in a major produce production region in California

item Cooley, Michael - Mike
item Carychao, Diana
item Mandrell, Robert

Submitted to: PLoS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2007
Publication Date: 11/14/2007
Citation: Cooley, M.B., Carychao, D.K., Crawford-Miksza, L., Jay, M.T., Myers, C., Rose, C., Key, C., Farrar, J., Mandrell, R.E. 2007. Incidence and tracking of Escherichia coli O157:H7 in a major produce production region in California. PLoS One. 0001159,

Interpretive Summary: The pathogen E. coli O157:H7 has been shown to infect produce. Previously there was little evidence to support the possibility of pre-harvest contamination. Environmental sampling over the last 2 years has established that E. coli O157:H7 could be found in streams at many locations, especially during periods of heavy rain, and was also present in the vicinity of farms implicated in outbreaks. Depending on conditions, the pathogen may be transported long distances in streams. Analysis of E. coli O157:H7 isolated from these environments showed that they sometimes were identical or closely related to human clinical strains, especially isolates from samples taken in the vicinity of one implicated farm during the baby spinach outbreak investigation in 2006. In this instance, E. coli O157:H7 was found to contaminate several sources, including wild pigs. These results support the likelihood of pre-harvest contamination of produce and highlight potential risks for human illness from the farm environment.

Technical Abstract: Fresh vegetables have become associated with outbreaks caused by Escherichia coli O157:H7 (EcO157). Between 1995-2006, 22 produce outbreaks were documented in the United States, with nearly half traced to lettuce or spinach grown in California. Outbreaks between 2002 and 2006 induced investigations of possible sources of preharvest contamination on implicated farms in the Salinas and San Juan valleys of California, and a survey of the Salinas watershed. EcO157 was isolated at least once from 15 of 22 different watershed sites over a 19 month period. The incidence of EcO157 increased significantly when heavy rain caused an increased flow rate in the rivers. Approximately 1000 EcO157 isolates obtained from cultures of >100 individual samples were typed using Multi-Locus Variable-number-tandem-repeat Analysis (MLVA) to assist in identifying potential fate and transport of EcO157 in this region. A subset of these environmental isolates were typed by Pulse Field Gel Electrophoresis (PFGE) in order to make comparisons with human clinical isolates associated with outbreak and sporadic illness. Recurrence of identical and closely related EcO157 strains from specific locations in the Salinas and San Juan valleys suggests that transport of the pathogen is usually restricted. In a preliminary study, EcO157 was detected in water at multiple locations in a low-flow creek only within 135 meters of a point source. However, possible transport up to 32 km was detected during periods of higher water flow associated with flooding. During the 2006 baby spinach outbreak investigation, transport was also detected where water was unlikely to be involved. These results indicate that contamination of the environment is a dynamic process involving multiple sources and methods of transport. Intensive studies of the sources, incidence, fate and transport of EcO157 near produce production are required to determine the mechanisms of pre-harvest contamination and potential risks for human illness.