Scientists are starting a four-year study on the sources of Escherichia coli O157:H7 in California's Salinas Valley, site of the recent contamination of spinach and previous contaminations of lettuce.

Some have speculated that livestock or wildlife are the sources of the bacteria in these cases of contamination, said Dr. Edward "Rob" Atwill, co-principal investigator and an expert in waterborne infectious diseases at the University of California-Davis School of Veterinary Medicine.

Dr. Atwill said the investigators will examine livestock and wildlife on the rangeland above the farmland and examine wildlife that live near canals and on the periphery of vegetable fields on the valley floor.

Crews will collect samples of livestock and wildlife droppings; creek, ditch, and irrigation water; farm soil; and lettuce growing on the farms. Scientists will analyze data to identify the vertebrates that are sources of E coli O157:H7; assess climate, landscape, and irrigation; and determine whether certain farming practices or environmental factors have any association with the contamination of lettuce.

Scientists hope the study also will help them understand the puzzling timing of recent E coli contamination. The bacteria appear more often in Salinas Valley waterways during the winter—when rainstorms wash the bacteria from streets, farms, and rangeland into creeks, streams, sloughs, and rivers. The contamination of fresh vegetables tends to occur during the summer and fall, though.

The U.S. Department of Agriculture is funding the study with a $1.2 million grant. Scientists from the USDA Agricultural Research Service, the University of California, and the California Department of Health Services are working on the project.