People who live along the coast may have more to fear from climate change than rising waters. A team of Maryland researchers has found evidence suggesting that the odds of getting sick from a salmonella infection go up, especially for coastal residents, as the shifting climate produces more extreme weather conditions.
Drawing on a decade's worth of health and weather data, scientists with the University of Maryland and the state Department of Health and Mental Hygiene found that the risks of getting a salmonella infection increased when temperatures soared far above normal, or when torrential downpours occurred. What's more, they found the risks even greater in coastal areas than inland.
"When people talk about coastal areas and climate change, we think of sea-level rise," said Amir Sapkota, a senior author of the study and an associate professor at UM's School of Public Health in College Park. "We know coastal areas are vulnerable, but there hasn't been a characterization of the risk in terms of health."
One of the most common causes of food poisoning in the United States, Salmonella enterica bacteria sicken about 1.2 million people a year, according to the Centers for Disease Control and Prevention.
People tend to get infected by eating food contaminated with fecal material during processing, or by eating food prepared by handlers who failed to wash their hands. Most of those infected get diarrhea, fever and severe abdominal cramps, which tend to go away by themselves after four to seven days. But in about 19,000 cases annually, the symptoms are severe enough to require hospitalization, and 380 die each year, the CDC estimates.
Other studies have found salmonella infections track with temperature, occurring more often in summer than winter. The UM authors say their study, published recently in the journal Environment International, is the first to make a geographic connection between extreme heat or precipitation and the frequency of salmonella infections.
"People [here] don't think climate change is going to impact them, like it is in the Arctic," Sapkota said. "This shows we have impacts in our backyard — human health impacts — and it affects people differentially."
For years, scientists have been studying and debating the health impacts of climate change, with some warning that serious vector-borne diseases like malaria could spread to temperate regions as their transmitters — mosquitoes, fleas and ticks — follow warming temperatures. Others have painted a more complicated picture, suggesting that better physical and health infrastructure in more developed countries could prevent any epidemics.
Researchers are in more agreement that climate change could contribute to a variety of food-borne illnesses.
"We're not just talking about salmonella," said Carmen Cordova, a microbiologist with the Natural Resources Defense Council. "We're also talking about listeria, staph. They like warmer temperatures, they grow in our bodies. ... In simple terms, the warmer the better for these organisms that are looking to cause diseases in humans."
The Maryland team found empirical support for a salmonella-climate connection, based on where people live. They looked at 9,529 confirmed salmonella cases that were logged by the Maryland Foodborne Diseases Active Surveillance Network from 2002 to 2012. They then compared the number of cases each month with the number of "extreme" weather events, determined by analyzing 30 years' worth of weather data to identify the hottest temperatures and heaviest rainfalls recorded.
The researchers found a 4.1 percent increase in the risk of a salmonella infection for every additional day of super-high temperatures, and a 5.1 percent increased risk for every day of extreme downpours.
But their findings varied more depending on location. For extremely hot days, the increased risk in coastal areas reached 5.1 percent, compared with 1.5 percent in non-coastal areas. For extreme precipitation, coastal residents faced a 7.1 percent increased risk, versus 3.6 percent for those living far from the shore.
It's not clear why coastal residents would be at greater risk of getting salmonella, Sapkota said, but researchers have a few theories. One possibility, he said, is that precipitation may be greater in coastal areas. Other studies have shown that coastal areas are highly susceptible to flooding, which could contaminate drinking-water wells and surface waters with salmonella bacteria from overwhelmed sewage plants and septic systems. There may also be more wells in coastal areas, and more surface water bodies as well.
Another pathway to more salmonella may be proximity to large numbers of farm animals, the authors suggest. On Maryland's Eastern Shore, farmers raise hundreds of millions of chickens a year, which generate hundreds of thousands of tons of manure mixed with wood shavings and feathers. Much of it is applied as fertilizer on croplands. But when it rains, especially when it pours, that manure and attendant bacteria could run off into nearby streams or seep into wells.
While the increases in risk found by the UM researchers were not huge, they were "epidemiologically relevant," according to Richard S. Ostfeld, a disease ecologist with the Cary Institute of Ecosystem Studies in Millbrook, N.Y.
Ostfeld has researched the spread of Lyme disease, which relies on ticks. He said salmonella transmission seems simple by comparison, making it a good disease to examine for climate-related effects. He called the UM paper "convincing" and its statistical analysis "sound."
"So, I think this is an important study that should help health departments and providers anticipate salmonellosis outbreaks during heat waves and flooding events in specific areas," Ostfeld said."The number of disease systems in which risk goes up with climate change seems to be ever-increasing."