A new University of Georgia study suggests that health agencies investigating Salmonella illnesses should consider untreated surface water as a possible source of contamination.
Researchers, whose results appear in the March issue of the journal Applied and Environmental Microbiology, tested water over a one-year period in rivers and streams in a region of south Georgia known for its high rate of sporadic salmonella cases. 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 Erin Lipp, associate professor in the UGA College of Public Health. “We found our highest numbers in the summer months, and this is also the time when most people get sick.”
Lipp, who co-authored the study with former UGA graduate student Bradd Haley and Dana Cole in the Georgia Division of Public Health, said that although contaminated water used to irrigate or wash produce has been linked to several well-publicized outbreaks of salmonellosis in recent years, the environmental factors that influence Salmonella levels in natural waters are not well understood. She said understanding how Salmonella levels change in response to variables such as temperature and rainfall are critical to predicting-and ultimately preventing-the waterborne transmission of the bacteria.
The team studied streams in the upper reaches of the Suwannee River Basin, which begins in south Georgia and flows into central Florida. The study area contains a mix of forested lands, row crops, pasturelands, wetlands and small cities. The researchers chose sampling sites near a variety of those environments but found little variation in Salmonella concentrations by location. The diversity of Salmonella strains, however, was highest near a farm containing cattle and a pivot irrigation system, suggesting that close proximity to livestock and agriculture increase the risk of contamination. The researchers also found a strong and direct correlation between rainfall for the two days preceding sample collection and the concentration of Salmonella, suggesting that runoff contributes to the contamination.
“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, because in many cases contaminated irrigation water, and not the produce itself, may be the cause of the outbreak.”