Researchers: El Niño Could Lead to the Spread of Infectious Diseases

The planet’s weather over the past three years has been dominated by a natural cycle called La Niña — an oceanic phenomenon that results in below-average sea-surface temperatures in the central and eastern tropical Pacific Ocean and lower average temperatures worldwide. But forecasters predict that, sometime between this summer and the end of the year, La Niña’s opposite extreme, El Niño, will take over.

That seismic shift could have major implications for human health, specifically the spread of disease. El Niño will increase temperatures and make precipitation more volatile, which could fuel the spread of pathogen-carrying mosquitoes, bacteria, and toxic algae. It’s a preview of the ways climate change will influence the spread of infectious diseases.

“The bottom line here is that there are a range of different health effects that might occur in the setting of an El Niño,” Neil Vora, a physician with the environmental nonprofit Conservation International, told Grist. “That means we have to monitor the situation closely and prepare ourselves.”

A boon for mosquitos

As with La Niña, the effects of an El Niño extend far beyond a patch of above-average warmth in the Pacific. Parched regions of the world — like Chile, Peru, Mexico, and the American Southwest — are often bombarded with rain and snow. Some other parts of the world, including the Northeastern U.S., the Amazon, and Southeast Asia’s tropical regions, on the other hand, don’t see much rain at all in an El Niño year.

The planet could temporarily become 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer, on average, than in preindustrial times — a threshold scientists have long warned marks the difference between a tolerable environment and one that causes intense human suffering.

These patterns are a boon for certain vector-borne illnesses — defined as infections transmitted by an organism (usually an arthropod, a category that includes insects and arachnids). Regions of the world that will experience longer wet seasons because of El Niño, many of which are in the tropics, may see an increase in mosquito-borne illnesses, according to Victoria Keener, a senior research fellow at the East-West Center in Honolulu, Hawaii, and a coauthor of the U.S.’s upcoming Fifth National Climate Assessment. “El Niño will mean a longer breeding season for a lot of vectors and increased malaria potential in a lot of the world,” she said.

A 2003 study on the intersection of El Niño and infectious disease showed spikes in malaria along the coasts of Venezuela and Brazil during and after El Niño years. The study looked at more than a dozen cycles between El Niño, La Niña, and the cycle’s “neutral” phase, which taken together are known as the El Niño-Southern Oscillation, or ENSO. The researchers, who analyzed data dating back to 1899, also found an increase in malaria during or post-El Niño in Colombia, India, Pakistan, and Peru. Cases of dengue, another mosquito-borne illness, increased in 10 Pacific islands.

The manner in which El Niño impacts mosquitos and the diseases they carry is varied and often difficult to accurately calculate, said Christopher Barker, an associate professor in the Department of Pathology, Microbiology, and Immunology of the University of California Davis School of Veterinary Medicine. Mosquitos breed in warm, wet conditions. But too much water in the form of flooding rains can wash away mosquito larvae and ultimately contribute to a decrease in mosquito populations.

As the planet shifts into an El Niño year, Barker said the areas to keep a close eye on are ones where moderate or heavy rains are followed by dry, warm months. If the past is any indication, countries like India and Pakistan are especially at risk.

So is California. After years of drought, recent storms in the Golden State have generated a lot of flooding and cooler-than-normal conditions. If that leads into a hotter-than-normal summer, “that may set things up for bad conditions for West Nile virus,” Barker said of the mosquito-borne illness that is becoming more prevalent in the U.S.

El Niño is projected to bring unusual warmth to the Pacific Northwest and the northern Great Plains. Kristie L. Ebi, a professor of global health at the University of Washington, said warmth is often the determining factor in how far north vectors of disease move. “We know that mosquitoes don’t control their internal temperature,” she said. “When it’s hotter they’re going to see opportunities to move into new ranges. If the El Niño lasts long enough they get established and find habitat, then you can see an expansion in geographic range.” A study on the link between infectious disease in the U.S. and El Niño, published in 2016, found a link between tick-borne illnesses such as rickettsiosis — an infection that can damage the brain, lungs, and skin — and El Niño in the Western U.S.

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