Searching for E. coli

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Field of spinach
Field of spinach

It is 2006 and a woman lies in a hospital room suffering from severe cramps and vomiting. Her doctor has just told her that she has an Escherichia coli O157:H7 (commonly called E. coli) infection, a bacterial infection that causes serious stomach and intestinal distress and is sometimes fatal, especially in children and the elderly. She learns that contaminated spinach has been linked to an E. coli outbreak. “How could I have E. coli?” she asks. “I shop so carefully for my vegetables.”

A farmer is alerted that his spinach crop could be contaminated by E. coli and officials want to test it. “I take precautions and use only well water to irrigate my crops,” he muses. “I have no animals on my property. How could this happen? My livelihood is at stake.”

Both the woman and the farmer could be victims of well water contamination, which can be caused by a number of factors or combinations of factors. These include rainfall, drought, some types of soil, groundwater and nearby surface waters, well structure and maintenance, and the presence of animals near wells and surface waters. Also, adding to the complexity of the problem, contaminants may be present only under certain conditions, perhaps for just part of the year.

Contaminated Spinach

CDC’s “disease detectives” are epidemiologists who track down the source of disease outbreaks. In 2006, a CDC team worked with officials from California and the Food and Drug Administration (FDA) to investigate an outbreak of E. coli O157:H7. The epidemiologists found that the source of the illness was contaminated spinach. They traced the spinach back to a California farm and matched the particular type of E. coli found in irrigation water at the farm to the E.coli in sick patients.

A systems-based approach takes a complete view of a system, organization, or process affected by a problem. Using systems thinking, problem solvers can evaluate how all the forces within a system interact. They can see how a decision that may have positive results for one aspect of the system can have unintended negative consequences in another.

What was unique and important about this investigation was that it did not stop there. Environmental health specialists from the National Center for Environmental Health, and food safety specialists from the FDA and the California Department of Public Health, joined together to investigate the source of contamination. Specifically they examined the potential effects of irrigation water quality on the spinach. The team conducted a “systems-based” health assessment of the watershed characteristics, irrigation wells, and movement of water both above and below ground in the area. Carol Selman, an environmental health expert who collaborated during the outbreak investigations, says, “A systems-based approach implies a complete view and analysis of the entire system affected by a problem. It assesses the elements of the system as well as the underlying interactions of those elements to determine what is creating the problem (e.g., the how and the why). Viewing a system as a whole from the beginning may save time and money in the long run.”

Contaminated Groundwater

Irrigation pump in spinach field
Irrigation pump in spinach field

The assessment found that winter runoff was stored in reservoirs (lakes) and then released during the dry summer season to refill aquifers (underground layers of water) used for irrigation. In those dry summers during the growing season, the groundwater table dropped below the level of the river. When that happened, the river water migrated into the groundwater on the farm. The river was likely contaminated by cow manure from surrounding areas.

The team found that the type of soil in the fields combined with frequently pumping irrigation wells during the dry season could have sent contaminated river water into the irrigation wells and from there into the fields and onto the crops. Max Zarate-Bermudez, a water contamination expert who conducts environmental assessments during outbreak investigations, says, “The environmental assessment focused on the entire irrigation system at the farm, from the water source to the irrigation pumps and pipes. It also assessed information about climate conditions during the time preceding the outbreak, soil types of the area, and ground- and surface-water levels. Several environmental factors may have contributed to the well water contamination such as surface water runoff from grazing areas onto cultivated fields, construction of irrigation wells, direct use of surface water for irrigation, and groundwater-surface water interaction.”

Systems-based Epidemiology

These investigations demonstrate how epidemiologists and environmental health specialists can help find

  • what was contaminated (spinach),
  • where the contaminated food came from (California farm), and
  • how and why it became contaminated (contaminated river water getting into the irrigation system; watershed management).

Using a systems-based health assessment helped scientists to see all parts of the picture and provide farmers and decision makers the information they needed to take action to correct the problem.

For more information, see Irrigation Water Issues Potentially Related to Multistate E. coli Outbreak on Spinach.


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Page last reviewed: July 9, 2015
Page last updated: July 9, 2015