Salmonella Bacteria Thrives in Untreated Surface Water

Over the course of a year, researchers tested rivers and streams in a region of south Georgia known for its high rate of salmonella cases - the upper reaches of the Suwannee River Basin, which begins in south Georgia and flows into central Florida.

The team found salmonella in 79 percent of water samples, with the highest concentrations and the greatest diversity of strains in the summer and after rainfall.

"Streams are not routinely tested for salmonella, and our finding is an indication that many more could be contaminated than people realize," said microbiologist Dr. Erin Lipp, associate professor in the University of Georgia-Athens College of Public Health. "We found our highest numbers in the summer months, and this is also the time when most people get sick."

She said understanding how salmonella levels change in response to temperature and rainfall are critical to predicting, and ultimately preventing, the waterborne transmission of the bacteria.

Lipp said that although contaminated water used to irrigate or wash produce has been linked to several outbreaks of salmonellosis in recent years, the environmental factors that influence salmonella levels in natural waters are not well understood.

Co-authors Dr. Erin Lipp, seated, and Dana Cole (Photo by Cat Holmes courtesy UGA)

The study area contains forested lands, row crops, pasturelands, wetlands and small cities. The researchers found the diversity of salmonella strains was highest near a farm containing cattle and a pivot irrigation system, suggesting that close proximity of livestock and agriculture increases the risk of contamination.

Rainfall for the two days preceding sample collection was related to high concentrations of salmonella, indicating that runoff contributes to the contamination.

The highest concentrations and greatest diversity of strains were found in August, the warmest month of the year. Lipp says her findings support to the idea that salmonella illnesses could increase as a result of global warming.

The study area had 58 cases of salmonella illness per 100,000 people in 2007, the last year for which figures are available, compared to a state average of 22 cases per 100,000 people and a national average of 15 cases per 100,000 people.

Salmonella is one of the top three causes of diarrhea in the United States. Georgia ranks No. 1 in reported cases among the 10 states taking part in FoodNet, a surveillance system by which the Centers for Disease Control and Prevention monitor major sources of U.S. foodborne illnesses.

"Understanding the environmental factors that contribute to salmonella illnesses can guide our efforts to educate people about how they can avoid being sickened through the proper construction and maintenance of wells, basic hygiene such as hand washing and good food safety practices," Lipp said.

"We also have the potential to decrease the likelihood of larger outbreaks related to produce," she said, "because in many cases contaminated irrigation water, and not the produce itself, may be the cause of the outbreak."

Lipp co-authored the study with former UGA graduate student Bradd Haley and Dana Cole in the Georgia Division of Public Health. The research was funded by the National Oceanic and Atmospheric Administration Joint Program on Climate Variability and Human Health, and the findings appear in the March issue of the journal "Applied and Environmental Microbiology."

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