California’s Central Coast produces a significant portion of the fresh vegetables consumed throughout the United States. At the heart of the region is the Salinas Valley, a 90-mile-long coastal valley south of Monterey widely known as the “Salad Bowl of the World.” This valley alone produces about half of the nation’s lettuce, including iceberg, leaf, and romaine, as well as broccoli, spinach, strawberries, and hundreds of other crops that feed the country year-round.
But this productivity doesn’t come without challenges. Several food poisoning outbreaks occurred between 2016 and 2020. connected to green leaves It has been traced to the central coast, and in many cases is caused by the same strain, E. coli O157:H7. In response, the U.S. Food and Drug Administration (FDA) is working with the Western Center for Food Safety at the University of California, Davis, the California Department of Food and Agriculture, and local stakeholders to California Longitudinal Study In 2020.
This study is designed to identify environmental factors that influence the introduction, persistence, and movement of foodborne pathogens in the region and provide actionable insights to help growers, regulators, and researchers improve produce safety practices. The initial results significantly enhance existing knowledge about how pathogens circulate in animals, waterways, soils and agricultural growing environments on California’s central coast.
Here’s what the research has revealed so far:
Large-scale environmental sampling efforts
Over five years, researchers conducted one of the most comprehensive environmental monitoring efforts ever undertaken in an agricultural growing region.
Between August 2020 and May 2025:
- The sampling event took place over 40 days.
- The study covered approximately 7,000 square miles.
- Sampling took place in 80 public locations, 14 cattle ranches, 5 produce farms, 2 composting facilities, and 2 vineyards.
Researchers collected 6,134 unique samples, including:
- Surface water (704 samples)
- Soil (1,425 samples)
- Sediments (1,057 samples)
- Air and dust (312 samples)
- Livestock manure (923 samples)
- Wildlife feces (942 samples)
- Biological soil conditioners such as manure (237 samples)
- Insects (32 composite samples)
Each sample was tested for dangerous bacterial species, including Shiga toxin-producing Escherichia coli (STEC), which includes the well-known pathogen E. coli O157:H7. Researchers also used whole-genome sequencing to compare bacterial strains and track how they move through landscapes, animals, and environmental sources.
Key Finding #1: Animals are a major source of pathogens.
One of the most clear conclusions from this study is that animals play an important role in introducing pathogens into the environment.
Researchers have found STEC in fecal samples from a variety of animals.
- cattle
- wild pig
- deer
- coyote
- Birds such as crows, ravens, pigeons, and hawks
- Other wildlife, including bobcats and elk
The highest prevalence of STEC occurred in wild animal and livestock fecal samples. Highly pathogenic STEC serotypes O157:H7 and O26:H11 have been isolated from wild pig, coyote, and bobcat feces. STEC O26:H11 has also been isolated from bird, elk, deer, squirrel, and rabbit feces samples. In total, 29 different STEC serotypes were recovered from wildlife fecal samples, including some considered highly pathogenic to humans.
Pastured beef cattle had particularly high levels of contamination compared to animals such as sheep and horses. Forty-six different STEC serotypes were recovered from livestock feces, including O157:H7, O26:H11, O103:H2, and O111:H8.
The study also found that some bacterial strains found in wildlife matched strains found in cattle, indicating that pathogens can circulate between domestic livestock and wildlife populations.
Key Discovery #2: Pathogens Can Remain in Animal Feces
The study confirmed that STEC bacteria can survive for long periods of time in animal waste, increasing the potential for environmental contamination. Researchers found STEC in both fresh and old, dried feces.
This indicates that the risk of contamination may persist long after the animals have left the area. Events such as heavy rain, flooding, or animal activity can disturb these droppings, potentially spreading bacteria into the surrounding soil or water.
Key Discovery #3: Waterways help move bacteria across the landscape.
Surface water has been shown to be one of the most important environmental pathways for pathogen movement. Researchers frequently find STEC in sediments within rivers, streams, surface waters, and waterways. In contrast, bacteria appeared less frequently in irrigation runoff and much less frequently in soil or air.
Areas near cattle pastures or riparian habitats tend to have higher levels of contamination in water and sediment samples. In one case, a drainage ditch located downhill from a livestock grazing area tested positive repeatedly for STEC over three years. These findings confirm that water systems can serve as reservoirs and transport routes for pathogens across regions.
Key Finding #4: Soil and air are less important vectors.
Although bacteria have occasionally been detected in agricultural soils, they are relatively rare overall.
- Only about 1% of soil samples contain STEC.
- Less than 1% of passive air samples showed viable bacteria.
This suggests that soil and air are not the primary drivers of pathogen movement in Central Coast agricultural systems. However, contamination rates were found to be slightly higher in soils closer to waterways or livestock grazing areas. Researchers also observed that field flooding events can temporarily increase pathogen detection in nearby soil and irrigation runoff.
Key Finding #5: Many variants exist, but no outbreak variants exist.
One of the most notable results of this study concerns the diversity of pathogens.
Scientists have confirmed that:
- A total of 68 STEC serotypes
- Includes 6 highly pathogenic types:
- E. coli O157:H7
- O26:H11
- O103:H2
- O111:H8
- O121:H19
- O145:H28
In total, 12 different E. coli O157:H7 strains were found in environmental samples. However, researchers have not identified the specific outbreak strain responsible for the resurgence of the disease between 2018 and 2020. Some strains have persisted in the environment for nearly two years, and the same strains have sometimes been found more than 70 miles, or 15 months, apart, suggesting that wildlife movements may be contributing to local spread.
What does this mean for food safety?
For agricultural produce growers and regulators, the findings largely confirm what researchers and industry experts have long understood. This means that foodborne pathogens can occur naturally in agricultural environments, and factors such as livestock, wildlife, surface water, soil, and weather conditions can contribute to their presence in agricultural production areas.
Results further support existing risk management practices to reduce contamination risks, including:
- Managing livestock and wildlife activity near agricultural fields
- Monitoring and evaluation of agricultural water sources
- Conduct pre-harvest environmental risk assessments
We thank California Livestock and Production Industries for their assistance with this study.
What comes next?
Analysis of the data set is ongoing, and researchers are currently testing samples for additional pathogens, including Salmonella and Campylobacter. To date, the study has identified 606 Salmonella and 428 Campylobacter strains, all of which are being genetically analyzed to better understand their distribution and potential risk.
More detailed findings are expected to be presented at upcoming scientific meetings, including the Western Food Safety Conference in Salinas, California, in May 2026, and the International Food Protection Association Annual Meeting in New Orleans, Louisiana, in July 2026.
