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

Michael Cooley,Leta Crawford-Miksza,Jeff Farrar,Christopher Rose,Diana Carychao,Robert E Mandrell,Carol Myers,Christine Keys,Michele T Jay,Debbie Fox

doi:10.1371/journal.pone.0001159

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 pre-harvest 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.

Highlights

Escherichia coli O157:H7 (EcO157) is an enteric pathogen that can cause life threatening hemorrhagic colitis and, in very severe cases, hemolytic uremic syndrome [1]
We have described the incidence of EcO157 in an important leafy vegetable production region
Some molecular similarities among environmental strains raise questions regarding potential spatial connections and persistence of certain strains in the environment. These studies were stimulated by the isolation of a single strain of EcO157 from the sediment in a creek adjacent to a farm identified in three separate traceback investigations (Table S1, RM4403)

Summary

Introduction

Escherichia coli O157:H7 (EcO157) is an enteric pathogen that can cause life threatening hemorrhagic colitis and, in very severe cases, hemolytic uremic syndrome [1]. Between 1982 and 2002, 350 outbreaks caused by EcO157 were reported; 52% and 9% were caused by foodborne and waterborne sources, respectively [2]. Ground beef and produce were associated with 75 and 38 outbreaks, respectively, but the increased consumption of fresh fruits and vegetables in the U.S appears to correlate with increased produce-associated outbreaks [2]. Since 1995, there have been 22 outbreaks of EcO157 associated with fresh lettuce or spinach, and 9 of these outbreaks have been traced to, or near, the Salinas Valley region of California [3,4,5,6,7,8,9]. The Salinas Valley region on the central coast of California is the major leafy vegetable producer in the US [10]

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Conclusion