Climate Tips Balance toward Disease, Pests
April 21, 2017
Major new journalism on climate and disease vectors.
Climate change is turning abnormal weather into a common occurrence: Last year was the warmest year on record, the third in a row, and there were more heat waves, freezes and storms in the United States that caused $1 billion or more in damage just in 2016 than in the years 1980 to 1984 combined. Anything that improves conditions for mosquitoes tips the scales for the diseases they carry as well: the West Nile virus that flattened Dallas, the dengue that returned to Florida in 2009 after 63 years and the newest arrival, Zika, which gained a toehold in the United States last year and is expected to surge this summer. “These aberrant years are becoming more common,” Haley told me. “Climate change is clearly altering the environment in ways that increase the potential for these diseases.”
When the health effects of climate change are discussed, the planet-scale impacts get the attention: rising temperatures, which can cause death from overheating; earlier springs, which pump more pollen toward the allergic; runoff from violent storms, which washes fecal bacteria out of sewer pipes; changing airflows that trap ozone near the ground, stressing the systems of people living with heart disease.
The unpredictable weather patterns stimulated by climate change affect infectious diseases, as well as chronic ones. Warmer weather encourages food-borne organisms like salmonella to multiply more rapidly, and warmer seas foster the growth of bacteria like Vibrio that make oysters unsafe to eat. Spikes in heat and humidity have less visible effects, too, changing the numbers and distribution of the insect intermediaries that carry diseases to people.
When former Vice President Al Gore spoke at a meeting on climate and health in Atlanta in February, he chose to start his talk not with a starving polar bear or a glacier falling into the sea, but with images of mosquitoes and ticks. “Climate change is tilting the balance, disrupting natural ecosystems and giving more of an advantage to microbes,” Gore said, standing in front of a giant image of Aedes aegypti, the mosquito species that transmits yellow fever and dengue, and now the Zika virus as well. “Changing climate conditions change the areas in which these diseases can take root and become endemic.”
Right now, yellow fever is causing an epidemic in South America, and dengue has been increasing in Central America. But in the United States, the most alarming disease linked to mosquitoes is Zika, which can cause devastating birth defects.
Zika has been a persistent concern since January 2016, when a Houston man became the mainland United States’ first case, arriving back from a trip to El Salvador with the fever, rash and red eyes of full-blown infection. Now more than 5,200 U.S. residents have come down with the virus, at least one in every state except Alaska. The vast majority were infected by being bitten outside the country, and a small number by having sex with someone who was infected that way. But more than 220 people have caught Zika from local mosquitoes carrying the virus. Almost all of those victims live near Miami, and six live in Brownsville, Tex., along the Mexican border. No one can say yet whether those clusters are random blips or early indications of a pattern of transmission that will blow up into an epidemic when the weather warms this year.
Aedes aegypti are present in more than half the states, from California to Florida and as far-flung as San Francisco, Kansas City and New Haven; entomologists have found that they regularly survive through the winter in sheltered spots in Washington, D.C. Unlike the salt-marsh mosquitoes that whine through beach towns at twilight or the night-biting Culex that carry West Nile between birds and humans, aegypti prefer proximity to people; we are their favorite meal. To get to us, they fly into houses and conceal themselves in closets and under beds and furniture. They have evolved to breed in the tiny pools of water we carelessly create around us: in an abandoned tire, the saucer under a houseplant, even an upturned bottle cap.
Like West Nile, Zika can cause high fevers and paralysis — but unlike West Nile, it can also trigger catastrophic birth defects in the children of women infected while they are pregnant. It appears to destroy brain tissue while a fetus is growing, causing the skull to collapse. It also seems to cause brain damage, and eye, ear and joint abnormalities later on — though what will happen to babies as they grow is uncertain, because all the children born to Zika-infected mothers are still toddlers. In the United States, the C.D.C. has identified that 1,311 women who were pregnant in the past year were possibly infected with Zika; 56 of their children were born with Zika-related birth defects. In seven cases, the pregnancies ended early, and the fetuses were shown to have been affected. The C.D.C. recently announced that nearly 10 percent of women infected while they were pregnant had a child with a birth defect — 15 percent if they were infected in their first trimester.
The combination of an ugly virus and a stealthy predator is unnerving — especially because in the year since that first Houston case, it has become clear that the United States is more vulnerable to Zika than anyone thought. Like generals basing their strategy on the last war they fought, public-health experts have set up their defenses based on what worked for previous threats. The traps that health departments bought to catch Culex mosquitoes are not attractive to Aedes. The spraying with pesticide by trucks and airplanes that knocks down nuisance mosquitoes cannot reach ones that have sneaked into buildings. The best defense against Aedes mosquitoes turns out to be not big municipal gestures but small individual actions: destroying their habitat by emptying the pools of water where they reproduce, and keeping them from eating by repairing windows screens and wearing bug repellent.