What El Niño can teach us about the way climate change drives infectious disease

A driver climbs out of a window of his car after driving onto a flooded road in Van Nuys, Calif. on Jan.5 after an El Nino-strengthened storm swept through the area. (Reuters/Gene Blevins TPX)

Many scientists agree that climate change will likely influence the spread of infectious diseases, from mosquito-borne illnesses like Zika to the bacteria found in contaminated food and water. It’s a relationship that’s largely been studied in developing nations, where the cumulative effects of global warming are likely to be more severe. Research on illnesses such as cholera and malaria, for instance, suggest that rising temperatures may boost their transmission.

But new research, published Monday in Proceedings of the National Academy of Sciences, suggests that higher-income nations like the United States shouldn’t write off the problem either. The study finds short-term shifts in climate and weather patterns, brought on by El Niño, have a history of influencing disease outbreaks in the United States — and these temporary changes may give us an important glimpse into what the effects of climate change may bring in the future.

“There’s this idea that the vulnerabilities are really only in low-income countries,” said David Fisman, the study’s lead author and a professor of epidemiology and medicine at the University of Toronto’s Dalla Lana School of Public Health. He and his colleagues — co-authors Ashleigh Tuite of Harvard University and Kevin Brown, also of the University of Toronto — were interested in investigating how infectious disease might respond in other parts of the world as well.

For the new study, the researchers analyzed national data on hospitalizations between 1970 and 2010 to see how illness rates might change under conditions related to El Niño. They found that El Nino conditions were associated with a higher risk of vector-borne diseases — diseases carried by organisms like mosquitoes and ticks — in the western U.S., although these illnesses didn’t change much in other parts of the country. At the same time, El Niño conditions were associated with a lower risk of diarrhea and intestinal illnesses in the West, but a higher risk in other parts of the country.

The change in vector-borne illnesses in the West seemed to be largely concentrated on tick-borne illnesses, such as Lyme Disease. The effect is not too surprising, according to Fisman. The warmer, wetter conditions El Niño tends to bring to the West Coast can influence the larval development of insects, as well as their biting rates. And they can also contribute to increases in mouse populations, which serve as reservoirs for the disease-carrying ticks.

The changes in intestinal disease rates were slightly more difficult to explain, Fisman said. Previous studies have suggested that both unusually wet and unusually dry conditions can lead to spikes in these types of illnesses, which tend to be transmitted through contaminated water sources. The scientists have noted that it’s difficult to determine the exact mechanism driving the changes in this study.

Because this study relied on hospitalization records, it likely captures only a fraction of the illnesses that actually occurred during the study period, many of which probably went unreported. This means that the effects reported in this study may actually be underestimated. It’s worth noting that previous research has suggested that climate and weather changes can influence the transmission of other diseases, such as pneumonia and influenza, while this study found no changes in those illnesses.

That said, the scientists are also not suggesting that the effects of future climate change are going to exactly mirror the conditions observed in this study. Rather, they say the important message here is that climatic shifts — whatever they may look like — can influence disease outbreaks even in highly developed nations like the U.S., and these changes may differ from one region to the next.

“There’s good reason to think that the seasonal surges in infectious diseases in different regions…may be driven by different environmental phenomena,” Fisman noted, adding the study speaks to the importance of conducting “more granular regional and local analyses” on climate and disease transmission in the future.

And on a broader level, the study reinforces the idea that high-income nations still have a personal stake in the fight against climate change. While developing countries are still likely to be much more vulnerable to the overall impact of global warming — and scientists have long argued for a moral, global obligation to halt its progress — evidence heavily suggests that no part of the world will escape untouched.

“I think it’s also really important for people in high-income countries to get a handle on this,” Fisman said. “There’s this tendency to assume, ‘Oh, we have sewage systems, if we have something like Zika or Chikungunya we’ll just kill all the mosquitoes.’ The problems are a lot bigger than that.”

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