Feb. 16, 2017 -- In one year, 2015, the Zika virus leapt out of relative isolation in small groups of islands in the Pacific and tore through the Americas, infecting an estimated 500,000 people in 40 countries.
Most had only mild symptoms. But for many pregnant women, the virus was devastating, inflicting grievous damage on the brain and nervous system of their developing babies.
Among the many unanswered questions about the Zika virus is this one: How did it suddenly spread so far, so fast?
New studies suggest that climate change may have been at least partly responsible for Zika’s rapid spread.
“The climate conditions were very high risk for having Zika transmission,” said Jonathan Patz, MD, director of the Global Health Institute at the University of Wisconsin-Madison. Patz is studying how climate may have impacted Zika’s sudden spread.
Zika was one of the case studies addressed at the Climate and Health Meeting at the Carter Center in Atlanta. The meeting became a political football last month after the CDC postponed it in January ahead of President Donald Trump’s inauguration. Former Vice President Al Gore salvaged the event and agreed to host it.
“We’ve had a few obstacles,” Gore said in a statement opening the event. “The experts who were looking forward to this felt it was valuable to go forward anyway.”
Speakers at the conference talked about the wide range of effects global warming and climate change are having on public health: making plants like poison ivy more poisonous, making allergy seasons longer and more intense, and increasing killer heat waves and smoggy air.
Organizers didn’t shy away from jabs at the Trump administration, which has been hostile to climate protections.
“As the climate changes, so will the infectious diseases that we confront. More outbreaks like Ebola and Zika. More pandemics like the bird flu. And here’s the catch: Walls will not keep these pathogens out,” said Ashish Jha, MD, director of the Harvard Global Health Institute, to appreciative murmurs and applause from the audience. “No borders are going to protect us. That’s what awaits us unless we act.”
His quote was an apparent reference to Trump’s pledge to build a wall along the U.S. border with Mexico.
“There is a clear warming trend, and it threatens our health,” said Kim Knowlton, DrPH, a senior staff scientist at the Natural Resources Defense Council in New York.
Zika's Connections to Climate
When Zika made its big move, 2015 was the warmest year on record, according to NASA, and not by a small amount. It broke the record by the largest margin ever recorded, boosted by a particularly strong "super" El Nino weather pattern. In an El Nino year, the Pacific Ocean warms near the equator, causing big shifts in weather around the globe. Some areas get far more rain than usual, while others have droughts.
El Ninos don’t just bring changes to the weather. Studies show they also bring spikes in disease.
For example, researchers studying nearly 2 decades of monthly reports of dengue fever in eight countries in Southeast Asia found that the disease, which is closely related to Zika, became far worse during El Nino years. During these warmer years, dengue fever spread farther, faster and infected more people than years when the average temperatures were cooler.
Patz said that makes sense, given what we know about the biology of the type of mosquitoes that spread not only dengue, but Zika and other kinds of infections, too.
“With any viruses carried by mosquitoes, warmer temperatures give you a faster development. Oftentimes mosquitoes are smaller when they hatch in warmer conditions, and they have more frequent feeding behavior to get enough blood for their blood meal to develop their eggs,” he said.
That means mosquitoes bite more often in warmer years, making them more likely to transmit disease.
Warmer temperatures also speed up the rate at which viruses move through the mosquito and become able to infect people.
“It was pretty striking, as far as how hot the temperatures were in Brazil 2 years ago,” Patz said. “I wouldn’t be surprised if there was as strong a parallel between temperature and Zika as there is with dengue.”
Since 2016 has since surpassed 2015 as the hottest, researchers say we should expect to see more outbreaks like Zika, and not just from diseases passed to humans from animals. Waterborne diseases like cholera and vibriosis, which people get from eating oysters contaminated with vibrio bacteria, are becoming more common as the world’s waters get warmer. Blooms of toxic algae, which poison both sea life and people, have become more of a threat. In his keynote address, Gore showed a slide depicting a massive algal bloom in the Pacific Ocean in 2015, which he noted closed fisheries from Alaska to Mexico.
“We’re seeing more emergence of new pathogens,” said Christine Johnson, PhD, DVM, head of global surveillance for the PREDICT center at the University of California at Davis. PREDICT is a government-funded program that keeps tabs on how new diseases pass from animals to people.
She pointed to re-emerging viruses like Rift Valley fever, which humans catch from herd animals like sheep and cattle and has caused recent outbreaks in Africa, and yellow fever, a disease carried by mosquitoes that’s recently had a resurgence in Africa and South America.
Another example is Middle East respiratory syndrome, or MERS, which was first reported in 2012. It's caused by a virus that passes from camels to people.
Johnson said there are several reasons behind the increase. For one thing, we’re better at finding new diseases. And humans keep encroaching on once-wild places, putting us in closer contact with the animals that host viruses and bacteria that are new to humans.
But those changes don’t explain the increase entirely, and climate change, she said, is certainly playing a role.
“There’s no question that the pace is increasing,” she said.