Every year, about one million Americans and tens of millions of people worldwide suffer the debilitating effects of salmonella poisoning, episodes sometimes serious enough for hospitalization. Cholera, while rare in the United States, has been increasing steadily in other countries for the last decade, with up to 5 million cases annually, and poses a real threat after natural disasters.
For the two bacteria that cause these gastrointestinal diseases, and possibly for other infectious agents like malaria, global warming presents a real opportunity.
They like it hot.
In recent years, atmospheric climate change has prompted a growing number of extreme weather events, including heat waves, prolonged drought, heavy precipitation, and superstorms, which, in turn, have encouraged shifts in the delicate balance of the planet’s ecosystems. These weather events, which already kill tens of thousands of people annually, also indirectly encourage the spread of infections when food and water become contaminated, a scenario that almost certainly will increase the prevalence of such infections as salmonella and cholera.
Moreover, human migration and the loss of health infrastructure, as well as malnutrition caused by food insecurity, could make humans more susceptible to infections. All of these could exacerbate the spread of infectious diseases if global warming continues unabated.
“Infectious diseases involve interactions with humans’ environment and microbes, and it’s a complicated interrelationship,” said Dr. Robert Hall, a program officer in the division of microbiology and infectious diseases at the National Institute of Allergy and Infectious Diseases (NIAID). “Climate has an impact on both the environment and on humans.”
Outbreaks of cholera, for example, typically stop in winter and reemerge in summer, making it a major global concern, since hot days are getting hotter and their seasons are lasting longer. Natural disasters can exacerbate cholera when drinking water and food become contaminated. Cholera kills more than 100,000 people globally every year, according to the Centers for Disease Control and Prevention.
“I would put cholera highest on my list to worry about with respect to climate change,” said Dr. David M. Morens, senior advisor to the director of NIAID. “Cholera likes warm weather, so the warmer the Earth gets and the warmer the water gets, the more it’s going to like it. Climate change will likely make cholera much worse.”
The impact of climate change on salmonella poisoning also is worrisome, experts say. Extreme heat and precipitation — which will intensify if global warming continues at its current pace — will increase the danger, particularly among those living on the coast. A recent study, in fact, published in the journal Environmental International, found that such events escalate the risk of salmonella infections among people living in coastal communities. These likely will rise if global warming worsens, according to the study.
Coastlines are especially vulnerable to the ravages of climate change, which include sea rise and flooding, and which, in turn, can result in contaminated water and the growth of salmonella. This could put millions at risk, as coastal communities are vastly more crowded than the United States as a whole, according to the National Oceanic and Atmospheric Administration (NOAA). More than 123 million people, or 39 percent of the nation’s population, live on the coast, according to 2010 figures.
Although the study focused on coastal communities in Maryland, the scientists plan to expand into more national coastal settings to see if the results are the same.
“Salmonella is food- and water-borne, and warmth helps it replicate,” said Amir Sapkota, study author and associate professor at the Maryland Institute for Applied Environmental Health at the University of Maryland. “Salmonella reproduces more in higher temperatures. Heat facilitates its growth. That’s why we see a lot more salmonella cases in the summer. Extreme temperature and extreme precipitation increases salmonella, and climate change is causing them to become more intense and frequent. Under those conditions, we can expect cases of salmonella to go up.”
Similarly, cholera “can have an impact on low-lying settings,” said the NIAID’s Hall. “Coastal areas in the world generally have poor sanitation — the intensification of the fecal/oral cycle — which runs the risk of increased exposure to cholera. Storm surges and increased rainfall can increase the risk.”
Perhaps paradoxically, prolonged drought also can result in cholera outbreaks, he said. “If you have an extremely dry cycle, and the sub-surface is dry and cracking up, water can channel from the latrines and pull from the same places as drinking water,” he said.
Malaria is a parasitic disease that is a major killer of children, especially in Africa. It is also a warm weather opportunist, afflicting 198 million people in 2013, with 500,000 deaths, according to the CDC. The connection between malaria and climate has long been studied in India, where epidemics occurred linked to excessive monsoon rainfall and high humidity. These conditions enhance mosquito breeding and survival, according to the World Health Organization.
Furthermore, WHO says, the risk of a malaria outbreak increases about five-fold in the year following an El Niño event. “Malaria is a disease of warm weather,” Morens says.
While other infectious pathogens also are sensitive to climate, the outlook for them in a heat-driven scenario is less certain. Mosquito-borne diseases — dengue, West Nile, and encephalitis, for example — and other vectors that carry diseases — ticks, tsetse flies, rodents — respond in various ways to temperature increases and to sea level rise. Climate affects the factors that influence transmission, reproduction and geographic range. But the dynamics can be complex, and not always predictable.
Most scientists agree that global warming will have an impact; they just are not sure how bad it will be. When climate causes one factor to change, other factors change too, they point out. Cold weather, for example, hurts mosquitoes, so they may do better as it gets warmer. But heat has the potential to kill viruses mosquitoes carry. At the same time, warmth may cause more plants to grow in certain regions, providing mosquitoes more places to hide from birds that eat them.
“When climate change occurs, you can’t isolate one thing and say: this is what it’s going to do to mosquitoes and birds,” Morens said. “If you move one thing, everything else moves too. It’s bound to be true that climate change over time will upset the apple cart and change the variables, but how it’s going to play out, we just don’t know.”
Also, they note, while some diseases — dengue, for example, a viral disease carried by mosquitoes — may be on the rise, “it may have more to do with population growth, rubber tires [where mosquitoes breed] moving around the world, and things humans do that change their environment, such as moving into crowded areas and letting garbage pile up,’’ Morens said.
“That’s not to say that climate change won’t be a player in all of this,” he added. “Certainly, as the world gets warmer, it’s bound to shake up the whole ecosystem in which that virus exists. But, again, there are many variables that interact with one another.”
The most worrisome effects of climate change on infectious diseases likely will be the ability to respond, experts say. “We have the answers of how to prevent and treat most infectious diseases” Hall said. “But in a storm situation, when you have flooding, for example, it could become difficult to access health care options and facilities.
“We have everything we need, we just have to ensure it is effective and responsive,” he added. “The problem with climate change is that it will make access more expensive, more difficult and sometimes impossible.”