E. coli O157:H7 in Leafy Greens: How Does it Get There?
By Fred Pritzker
It’s relatively easy for the general public to understand the sequence of an E. coli O157:H7 outbreak involving ground beef, but scientists are still studying how the pathogen finds its way from the stomachs of cattle and other hooved animals into fields of vegetables, causing widespread E. coli outbreaks involving lettuce, spinach, tomatoes and other fresh produce.
A study of E. coli O157:H7 contamination in the watershed of the Salinas Valley — otherwise known as America’s salad bowl — offers some interesting insights. It was submitted in October 2007 by nine researchers from the USDA’s Agricultural Research Service, California’s departments of environmental protection and health services and the U.S. Food and Drug Administration with help from the Centers for Disease Control and Prevention.
For background, more E. coli O157:H7 outbreaks are linked to ground beef than they are to vegetables. In the case of beef, the bacteria enter packing plants on the hides of cattle or they are accidentally released during slaughter, landing on the surface of primal cuts. If the cuts are ground into hamburger, the pathogens can survive all the way to a person’s dinner plate if an individual patty isn’t cooked at its center to at least 160 degrees.
Between 1995 and 2006, 22 produce outbreaks E. coli O157:H7 were documented in the United States, with nearly half traced to lettuce or spinach grown in California — mostly in the Salinas Valley. The researchers in the October 2007 study spent 19 months sampling water, soil, plants, feces and sediment throughout the Salinas Valley watershed. They found E. coli O157:H7 at least once in 15 of 22 watershed sites. They found that the incidence of E. coli O157:H7 “increased significantly” when heavy rains caused an increase flow rate in the rivers.
But the study didn’t decipher new risk factors. For instance, researchers weren’t sure if the higher prevalence of E. coli after rains was related to flow rates stirring up E. coli O157:H7 in the sediment of rivers and creeks or if the increased number of positive test results was caused by run-off from grazing areas adjacent to streams and rivers. Even then the researchers were surprised to find that certain strains of E. coli O157:H7 wouldn’t migrate more than 100 yards from a known hot-spot of E. coli. But other matching strains of E. coli O157:H7 were found more than 30 kilometers away from each other.
Overall, the researchers concluded that the incidence of E. coli O157:H7 in the environment in the Salinas Valley is dynamic and not reasonably predictable.. For instance, while the highest incidence of E. coli O157:H7 was detected after rains, the researchers also found evidence of the pathogen making appearances during drought periods. The researchers said it was possible that the micro-organism could travel in the valley in clouds of dust from areas were cattle graze.
The writer, Fred Pritzker, is founder and president of Pritzker Olsen attorneys, a national food safety law firm representing E. coli victims and victims of hemolytic uremic syndrome, or HUS – a severe complication of E. coli O157:H7 infection that can cause permenant health damage or death in young children, the elderly and people who have weakened immune systems. For more information or to reach an E. coli attorney at the firm, call 1-888-377-8900 (Toll Free).
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