Food Safety Hazards of Livestock and Leafy Green Contamination

Introduction: E. coli Outbreaks and Public Health Impact

The U.S. Centers for Disease Control and Prevention (CDC) tracks ongoing Escherichia coli outbreaks across the nation, including those arising from leafy green vegetables. For example, in 2012, 58 people were sickened by an E. coli O157:H7 outbreak in the Midwest that was eventually tracked to a Missouri farm producing romaine lettuce (CDC). In 2006, close to 200 people became ill when exposed to O157:H7-contaminated baby spinach harvested from the Central Valley of California (Warnert). The 2006 outbreak resulted in the death of two elderly women and a 2-year-old boy. Making matters worse, nearly 80 percent of the nation's leafy greens are grown in the Central Valley. Needless to say, the impact on public trust concerning the safety of farm produce was considerable.

The U.S. Food and Drug Administration (FDA) also tracked the behavior of consumers following the 2006 outbreak (Arnade, Calvin, and Kuchler 734). What was unique about this outbreak was the FDA's warning the public as the outbreak was occurring—for nearly two weeks—whereas in the past the FDA was typically behind the curve and notifying the public after the outbreaks had already ceased. Consumers in 26 states and a Canadian province became ill, requiring 104 hospitalizations.

Consumer Response and Market Consequences

As a result of the media coverage, nearly 87 percent of consumers were aware of the outbreak as it was occurring. By week 3 following the FDA announcement, bagged spinach sales had bottomed out at 63 percent of normal sales and remained 10 percent below predicted sales levels by week 68 (744). Bulk spinach bottomed out one week after the FDA announcement at 32 percent below expected sales volumes, but had recovered by week 26. During this shock to the spinach market, consumers ate more lettuce to compensate.

A more long-term consequence of the 2006 E. coli spinach contamination was increased research into the sources of contamination and changes in farming procedures. The U.S. Department of Agriculture (USDA) and California university researchers investigated the source of the 2006 outbreaks using the latest biotechnology tools (Warnert). Once the contaminated spinach field had been located, researchers first identified O157:H7 in cattle feces about a mile from the field (Jay et al. 1908).

Investigation of the 2006 Spinach Outbreak

During this investigation, feral swine were observed in the area, and an effort was begun to trap and test local wildlife, in addition to all forms of local livestock, pets, sources of water, and soil. The O157:H7 strain was detected in cattle and feral swine feces, in addition to water and soil samples (Jay et al. 1909). Although the contaminated spinach field was fenced to prevent cattle from entering, researchers found evidence for wildlife intrusion, including the presence of feral swine tracks.

The ultimate source of the O157:H7 strain was believed to be cattle, which transmitted the bacteria to the feral swine through contaminated groundwater (Benjamin et al. 81–82). In order to quantify the risk cattle pose to fields of leafy green vegetables, researchers surveyed coastal ranches in California for O157 contamination of livestock, water sources, and soil. O157 was detected in 2.6 percent of cattle fecal matter, 1.5 percent of water sources, and 1.1 percent of soil samples on 8 cattle ranches, although there was high variability in contamination levels between ranches (84–85).

Livestock as a Source of O157 Contamination

For example, 44 of 49 cattle tested on a single ranch on one date tested positive for O157-contaminated fecal matter, but the water sources were clean. Because this was a longitudinal study, researchers were able to determine that warmer weather, increased humidity, and larger herds increased the chances of O157 contamination. With the use of sequencing technology, researchers were also able to show that the different strains of O157 were rarely transmitted from ranch to ranch.

One interesting result from the study by Benjamin and colleagues is that higher wind speeds were associated with less contamination of soil samples, suggesting windy days helped to cleanse ranch soil of O157 contamination.

Airborne Transmission and Distance Risk Assessment

A more recent study examined the effect of airborne contamination at sites where cattle feedlots were located adjacent to fields of leafy green vegetables in Nebraska (Berry et al.). The water used for crop irrigation was negative for O157 during the entire study period (1104). Although one soil sample tested positive, the strain of E. coli in the soil was distinct from the strains found in the feedlots and on the leafy green vegetable crops, which suggests the soil was not a source for crop contamination.

The distances from the feedlots tested for contamination were 60, 120, and 180 meters, and as expected, the further the distance from the feedlots, the less likely the crops were contaminated with the same strains of E. coli contaminating the cattle. Interestingly, E. coli contamination was detected in the air above the feedlots regardless of wet or dry conditions, but never downwind, until the researchers used much more sensitive methods of detection (1108). Overall, airborne E. coli was believed to be the primary source of crop contamination.

Berry and colleagues concluded that the greatest threat of E. coli contamination to leafy green vegetable crops was from nearby cattle operations (1108). Dry conditions, cattle movement, and wind were suggested to increase the risk of nearby crop contamination. The data also revealed considerable crop and air contamination within 180 meters of feedlots in Nebraska, which is a significant concern given that current guidelines suggest a buffer of only 120 meters is sufficient to protect crops from feedlots. In other words, further research will be needed to determine the minimum safe distance to prevent airborne E. coli contamination of leafy green vegetable crops by feedlot activity and proximity.