Tracking the 'how' of salmonella infections

Tracking the 'how' of salmonella infections

70-percent of all infectious diseases originate in animals. Changes in the environment and global travel contribute to their spread.

We are all connected.

by Linda Weiford, WSU News

Davis, Margaret
Dr. Margaret Davis

This month’s Thanksgiving turkey might contain more than bread stuffing. It could also harbor salmonella, a bacterial pathogen that causes food-borne illness in 1.2 million Americans each year, according to the U.S. Centers for Disease Control and Prevention.

Molecular epidemiologist Margaret Davis of Washington State University’s Paul G. Allen School for Global Animal Health investigates the many ways this microbe gets transmitted. As co-author of seven salmonella studies in the past 15 years, she also examines why some strains have become resistant to antibiotics.

A good thing, too, since unlike other foodborne illnesses, salmonella infections have been edging upward in recent years, according to the CDC.

“Salmonella is an extremely robust organism,” said Davis, with the ability to survive in a wide range of environments ranging from water and soil to, literally, the kitchen sink.

While most salmonella infections inflict mild to moderate gastrointestinal distress lasting 4-7 days, others leave a dark and permanent legacy, killing an estimated 100,000 people worldwide annually. Children, the elderly and people with impaired immune systems are most vulnerable, said Davis.

Salmonella bacteria (Courtesy, CDC)

“Not only are the bacteria ubiquitous in the environment, but they’re hardy,” she said, thereby making their way into many types of foods. Poultry, meat and eggs are the biggest targets, but disease outbreaks have also been tied to spinach and even peanut butter.

The organisms can withstand freezing temperatures, dry conditions, zero oxygen and high acidity inside the human gut. They can survive for months in soil and water and for weeks on countertops and cutting boards, she explained.

 “High heat kills the bacteria, so the biggest risk at home comes from undercooking," said Davis. “People should also be aware of cross-contamination, which occurs when countertops, sinks and utensils aren’t adequately cleaned after the raw poultry or meat is handled.”

In many cases, salmonella’s journey originates in animals’ gastrointestinal tracts, a natural home for the bacteria.

“Many animal species harbor the bacteria without being sickened themselves, so there’s no way to tell by looking at them whether they are infected hosts,” said Davis.

Animals typically pick up the bacteria from contaminated feces in soil. Also, food-animal carcasses can become contaminated at processing plants when exposed to tainted equipment, storage bins or even a person’s hands.

Large disease outbreaks have been traced to fruits and vegetables as well as poultry, meat and eggs. How do salmonella find their way to a field of tomatoes or cucumbers?  

“Contaminated water used to irrigate fields can, in turn, contaminate the foods being grown there,” Davis explained.  

Salmonella infections -- whether caused by contaminated turkeys or cantaloupes --“represent something that went wrong somewhere along the pathway from farm to table,” said Dr. Christopher Braden, director of the CDC’s foodborne and environmental diseases division in Atlanta. “These bacteria cause more hospitalizations and deaths than any other pathogen found in food,” he said.

And that is why, at WSU’s Allen School, Davis continues her work to understand how salmonella bacteria get from point A to point B and beyond, to ultimately carry out their dirty work.