Climate Change Could Tell Us Where the Zika Virus Will Spread Next
It sounds like the plot for a horror movie: A virus with subtle, easily overlooked symptoms (cough, fever, rash) that has the potential to cause serious birth defects and long-term neurological problems is carried by a tiny mosquito that’s rapidly spreading throughout the world.
Global travel and climate change are creating conditions perfectly designed to increase that mosquito’s breeding potential and lengthen its biting season, leading scientists to use the term “pandemic” to describe the Zika virus’ potential for destruction. On Thursday, the director of the World Health Organization warned that the virus is “spreading explosively” in the Americas.
“There are two things most surprising about Zika,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor University in Houston. “First is the rapid spread—it’s alarming how quickly it’s going across all the Caribbean into Central America and Mexico.” There’s also concern about the virus’ potential spread to Southern Europe.
Second, said Hotez, is the “vertical transmission from mother to child—we haven’t seen that before with one of these arboviruses.” While scientists are still trying to pinpoint causality, there has been a shocking spike in cases of microcephaly—a central nervous system deformity causing an abnormally small head and brain—among babies born to mothers in Brazil, where Zika has infected as many as 1.5 million people in the past year.
In just the week ending Jan. 16, Brazilian health officials announced that 363 babies had been born with Zika-linked microcephaly, and on Wednesday they said there were 287 new suspected cases in the week ending Jan. 23. There have been a total of 4,180 cases of Zika-associated microcephaly since October, when the country instituted mandatory reporting. A spike in microcephaly had previously been noted in French Polynesia, tied to a Zika outbreak there.
Both outbreaks have also been linked to an increase in the incidence of Guillain-Barré syndrome, a little-understood autoimmune disease in which the body attacks its own central nervous system, causing long-term neurological consequences, including paralysis.
Named for a forest in Uganda where researchers first identified the virus in monkeys in 1947, Zika has only in the past eight months been considered a threat beyond a narrow slice of equatorial Africa and Asia where it appeared to be safely contained for decades.
It wasn’t until 2014 that Zika made it to the western hemisphere, appearing in Brazil shortly after the World Cup—an event most experts now think was the nexus for the virus’ introduction.
“This virus has crept up very quickly—there’s no question about that. We didn’t know it was in the hemisphere until last year; we didn’t know it caused encephalitis until last summer—this is moving at lightning speed,” said Hotez.
Indeed, in the New England Journal of Medicine, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, termed the spread of the virus “explosive,” noting that the pace of the “pandemic reemergence” of the virus is unprecedented.
The Lancet published an article on Jan. 9 estimating that 200 million Americans, or upwards of 60 percent of the country’s population, including large swaths of the East and West Coasts and much of the Midwest, live in parts of the United States that might roll out the biological welcome mat for Zika.
“I think we’re going to see this all over the western hemisphere except for Canada, the northern part of the U.S., and Chile, and the question is, where will it go next?” said Hotez.
The Centers for Disease Control and Prevention and the National Institutes of Health have put out travel advisories warning pregnant women not to travel to infected areas and offering relocation services to pregnant Department of Defense employees stationed in high-risk regions.
The problem is, all it takes is one traveler with the virus in her bloodstream and a mosquito to pick up that virus, then pass it along by biting a new victim. With the Zika virus’ primary carrier, the Aedes aegypti mosquito, now happily ensconced in much of the Southwest and Hawaii, it seems likely that we’ll soon see the virus in those regions, many experts believe.
Add to that the sound evidence that a second mosquito variety, the rapidly invading Asian tiger mosquito, Aedes albopictus, can also transmit Zika, and you have a much larger geographic scope for the disease.
“We’ve seen these migrations before,” said Hotez, citing the 2013 outbreak of chikungunya virus in the Southwest and earlier outbreaks of dengue in Texas and along the Gulf Coast.
“The role of climate change is still unclear, but I think it has to be looked at very seriously,” he added. “Flooding following periods of drought leads to collecting pools of water, allowing mosquitoes to breed, and warmer temperatures allow them to emerge less seasonally, so they can be coming out and feeding all year.”
This is by no means the first time global warming has been implicated in outbreaks of such diseases; the rising incidence of Lyme disease and other tick-carried diseases appears to be climate-related as well.
Then there’s El Niño, which is heating up temperatures in the Pacific Ocean, especially along the coast of South and Central America. “It’s interesting that it coincides with spread of Zika. Whether they’re all causally related we don’t know, but it’s certainly important to follow up on,” Hotez said.
Warming temperatures and waters are not the only environmental factor experts are investigating when it comes to Zika. They also cite socioeconomic factors such as population growth, urbanization, and poverty, and the related environmental consequences.
“All you have to do is look back to the U.S. in the 19th century, with malaria, yellow fever, and dengue rampant in most of the Eastern states, especially along the seaboard,” said Ilia Rochlin, an entomologist with the Suffolk County, New York, Department of Public Works who researches mosquito migration. “The reason we don’t have these diseases now is mainly because of improved sanitation and vector control.”
Vector control—limiting insects’ ability to spread disease—means pesticides.
Without the “big gun” pesticides such as now-banned DDT, vector control programs throughout the world lost much of their muscle, Rochlin said. “Mosquito control programs worldwide became very heavily dependent on DDT, which was cheap and extremely effective against both adults and immature mosquitoes.” Though the U.S. government banned DDT in 1972 after Rachel Carson’s Silent Spring exposed the pesticide’s devastating impact on birds and other wildlife, it continued to be used throughout the developing world for decades.
“Community-based mosquito control programs pretty much crumbled after DDT was abandoned, and the reemergence of many of these pesky ‘tropical’ diseases can be traced to that period,” Rochlin said. The result: “The perfect storm of conditions we’re currently experiencing.”
So what does the future hold? Lots to worry about, it seems. In recent weeks, new cases of Zika have emerged in several European countries, and the first baby was born in the U.S. with microcephaly as a result of the virus, which her mother picked up while traveling in South America before returning to Hawaii. A vaccine is likely at least two years away, Hotez said. There’s also some evidence that Zika is transmissible through blood and semen.
With the virus’ rapid dissemination and mother-to-fetus transmission still largely a mystery, scientists are researching its ability to mutate and undergo changes that increase the likelihood of being picked up by humans.
What most alarms Hotez? The list is long. The potential for an epidemic in Haiti, with its Zika-perfect conditions; the risk in Hawaii, Texas, and other places where Aedes aegypti is prevalent; the vulnerability of many poor Gulf Coast towns, where people don’t have window screens and other protection against mosquitoes.
Then there’s the wild card of climate change. “Global warming is bringing us diseases we’re unprepared for, and we just don’t know all the factors involved,” he said. “The question is, what else do we need to worry about?”