In the world of public health, climate change and antibiotic resistance are often included among the most serious and urgent threats to human health. Now, a new study finds the two phenomena may be more closely related than previously thought.
In the study, published earlier this week in Nature Climate Change, researchers found an association between hotter temperatures and higher degrees of antibiotic resistance in common bacterial pathogens. More specifically, they found that an increase in temperature of 10 degrees Celsius across U.S. regions was associated with increases in antibiotic resistance of 4.2%, 2.2% and 2.7% in Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae) and Staphylococcus aureus (S. aureus), respectively. All three pathogens are commonly acquired in community and hospital settings and all three can be potentially life-threatening.
The findings, researchers wrote, suggest that current projections on the future burden of antibiotic resistance could be a considerable undercount.
“We were surprised because the common knowledge before was that it was just overprescribing (that was driving antibiotic resistance),” said study co-author Mauricio Santillana, an assistant professor at Harvard Medical School and faculty member with the Computational Health Informatics Program at Boston Children’s Hospital. “But there seems to be more to the story.”
To conduct the study, Santillana and colleagues developed a database of U.S. antibiotic resistance patterns across the U.S. based on information from hospitals, laboratories and public health surveillance systems and zeroed in on the three bacterial pathogens mentioned above. The final dataset represented more than 1.6 million bacterial pathogens from 223 facilities across 41 states spanning 2013 to 2015. Santillana said researchers had a hunch they might find an association with temperature based on observations from previous research that suggested greater antibiotic resistance in southern European countries versus northern ones.
Study co-author Sarah McGough, a PhD student at Harvard T.H. Chan School of Public Health, added that scientists already know that certain temperatures are more conducive to an organism’s ability to replicate and transfer antibiotic-resistant genes, but there is little known about ways the external climate impacts resistance at a population level.
Not surprisingly, the study found that higher antibiotic prescribing rates were associated with increased resistance among all three pathogens studied. (Remember, overprescribing among humans and livestock is already known as a main driver of antibiotic resistance.) But they also found a connection to temperature, with researchers finding a strong association between higher local average minimum temperatures and degree of antibiotic resistance. Without adjusting for other variables, they found that an increase of 10 degrees Celsius across regions was associated with an increase in antibiotic resistance of 5.1%, 3.4% and 3.1% for E. coli, K. pneumoniae and S. aureus, respectively. And even after adjusting for variables such as prescribing rates, population density and laboratory standards, researchers still found that each 10-degree rise in minimum temperature was associated with an increase in resistance of 4.2%, 2.2% and 2.7% for the three pathogens.
The study also found a relationship between population density and antibiotic resistance. In particular, an increase in density of 10,000 people per square mile was associated with a 6% increase in antibiotic resistance for K. penumoniae and 3% increase for E. coli, but didn’t appear to impact S. aureus.
Santillana said because a relationship was observed across pathogens, instead of just for one particular pathogen, it suggests a larger pattern between hotter temps and degrees of antibiotic resistance. He and study co-authors McGough, Derek MacFadden, David Fisman and John Brownstein write:
The World Health Organization has identified climate change as a major driver of emerging infectious diseases globally, although vector-borne and enteric infections, such as cholera, have typically been considered as the most likely to be impacted. Our findings suggest that the spread of antibiotic resistance may be modified and potentially accelerated by regional temperature and future climate change. Based upon our findings, a 10°C increase in temperature, an extreme but conceivable scenario for some parts of the United States by the end of this century, could yield additional increases in resistance on the order of 10% for certain antibiotics.
Researchers offered several educated guesses as to why temperature might be having such an effect. For example, they suggested that temperature may foster the exchange of resistant genes between bacteria; that increased population-level transmission might facilitate the evolutionary selection of antibiotic-resistant strains; or that because temperature is a key factor in bacterial growth rates, hotter temperatures are increasing the amount of resistant bacteria in people and animals as well as transmission between the two groups.
“It was surprising that the relationship held even when controlling for known predictors of antibiotic resistance,” McGough told me. “The study doesn’t proclaim that temperature is the sole driver of the differences, but it was still a strong predictor.”
Santillana said a goal of the study was to open the door to further research on the topic, adding that an immediate next step should be investigating whether the temperature-resistance pattern is found in other parts of the world and ultimately, discovering the exact pathways through which hotter temperatures facilitate resistance to antibiotics.
“The health of our planet is now often seen through a ‘one health’ paradigm, and we feel that antibiotic resistance as a phenomenon falls very well into that framework because it affects multiple living organisms on Earth,” Santillana said, adding that efforts to better understand antibiotic resistance are “extremely urgent.”
The Centers for Disease Control and Prevention reports that each year in the U.S., at least 2 million people are infected with antibiotic-resistant bacteria and at least 23,000 people die from such infections. The resistant infections also cost billions in additional health care spending. McGough noted that global estimates already project that mortality from antibiotic resistance will exceed that of cancer by 2050. Of course, that estimate doesn’t take into account these new findings and the potential impact of a warming planet.
“If temperature can modulate or accelerate antibiotic resistance,” she told me, “we’d hit that target sooner than we think.”
For a copy of the new study, visit Nature Climate Change.