UC Davis School of Veterinary Medicine
News & Events

Scientists to begin looking for the sources of E. coli in the Salinas Valley

October 16, 2006

Before the year's end, a team of scientists will begin collecting data for a long-planned four-year study on the sources of E. coli O157:H7 in the Salinas Valley. The study is funded with a $1.2 million research grant from the USDA.

"Exactly where the E. coli is coming from is a focus of the grant," said Edward (Rob) Atwill, a UC Davis School of Veterinary Medicine specialist in waterborne infectious diseases and co-principal investigator of the study. "There has been much speculation that livestock or wildlife are possible sources of the E. coli O157:H7 in these outbreaks in fresh produce from Salinas Valley region. We will examine livestock and wildlife that are out on the rangeland up above the farmland, and wildlife that live near canals and on the periphery of vegetable fields on the valley floor."

In the first two years of the study, crews will collect thousands of samples of domestic animal and wildlife droppings; creek, ditch and irrigation water; farm soil and lettuce growing on Salinas Valley farms. (This study was designed before the late summer 2006 outbreak of E. coli O157:H7 associated with spinach. Several previous outbreaks of the virulent bacteria were associated with contaminated Salinas Valley lettuce.)

The data collected in the field will be carefully analyzed to identify the vertebrates that are sources of E. coli O157:H7, assess the climate, landscape attributes and irrigation management practices that are correlated with increased risk of contamination, and determine whether contaminated lettuce is associated with certain farming practices or environmental factors.

"We need to deal with facts," said Robert Mandrell, research leader of the USDA-Agricultural Research Service Produce Safety and Microbiology Research Unit and the principal investigator of the study. "Right now, we can't say what's happening. We will get facts that will give us a basis to determine what more we need to investigate."

Facts will come also from the use of sophisticated tests called MultiLocus Variable tandem repeat Analysis (MLVA) and Pulsed Field Gel Electrophoresis (PFGE) to "fingerprint" and track bacteria found in various animals, waterways, farms and vegetables. PFGE is the same process used by the Centers for Disease Control to discover whether food-borne disease outbreaks are related to one another, as was found to be the case with spinach in September 2006, and MLVA is a recent method for assessing the relatedness of strains isolated.

"Most strains of E. coli O157:H7 bacteria are so similar, we need to look at the genome to accurately trace the source through the environment," Mandrell said.

The scientists hope their research will also help them understand the puzzling timing of recent E. coli outbreaks. E. coli bacteria are found more often in Salinas Valley waterways in the winter, when rainstorms can wash the bacteria from streets, farms, and rangeland into creeks, streams, sloughs and rivers. However, the outbreaks associated with fresh vegetables tend to happen during the summer and fall.

"Is there a hydrological link between what is occurring on rangeland above the fields and the E. coli contaminated vegetables that are reported in the summer and fall? That's what we are going to test," Atwill said.

Certain strains of E. coli bacteria normally live in the intestines of cattle, wildlife and humans causing no harm. The O157:H7 strain became known in 1982 as a result of an outbreak associated with hamburger meat. It causes severe diarrhea and can lead to kidney damage and even death. Young children and the elderly are particularly vulnerable. However, some humans with E. coli O157:H7 show no symptoms at all.

"There's a lot we don't know about why certain people don't get sick," Mandrell said. "Are certain strains more virulent? The particular strain in the spinach outbreak seems to be particularly virulent."

Compared to other strains of E. coli, the O157:H7 strain is unusually persistent in the environment. It survives in low temperatures, such as cold water and under refrigeration. Heat, in the pasteurization of milk and fruit juices or in cooking, destroys E. coli bacteria. E. coli on the outer surfaces of cuts of meat are easily destroyed during cooking. But E. coli in ground meat may be deep within the hamburger. Cooked hamburgers with pink meat in the middle can still have live E. coli O157:H7 cells.

Cooking vegetables also destroys bacteria, but contaminated vegetables meant to be eaten raw pose the problem. Washing rarely completely cleans all the bacteria off and very little bacteria can make a person ill. It is thought that only 10 to 100 bacteria can cause disease in humans; a large animal with E. coli O157:H7, such as a cow, sometimes can shed millions of bacteria per gram of feces.

The information from Mandrell, Atwill and their colleagues' E. coli study will be used to inform growers about specific strategies they can use to prevent contamination of vegetables in the field, to educate the public about the potential impacts of rangeland runoff, and to develop effective management practices for improving water quality.

The scientists involved in the study are Mandrell; Atwill; Michael Cooley, microbiologist, USDA-ARS-WRRC-PSMRU; Ken Tate, UC Cooperative Extension rangeland watershed specialist, UC Davis Department of Plant Sciences; Royce Larsen, UC Cooperative Extension area natural resource watershed advisor, San Luis Obispo County; Leta Crawford-Miksza, Chief, Food Microbiology Section, Food and Drug Laboratory Branch, California Department of Health Services; Janet Mohle-Boetani, Chief, Disease Investigations Section, Infectious Diseases Branch, California Department of Health Services; and Michelle Jay-Russell, Western Institute for Food Safety and Security, UC Davis and California Department of Health Services.


This press release was distributed by UC Agricultural and Natural Resources News

CONTACT: Jeannette Warnert, (559) 241-7514,
jwarnert@ucop.edu