Too Darn Hot

Global Warming Accelerates the Spread of Disease

Heat stress is probably the most obvious thing people think of when the idea of global warming comes up. A heat wave in Europe during the summer of 2003 killed more than 10,000 people in France alone. Many of the dead were elderly; the group most likely to live alone and most susceptible to heat-related health problems.

Other climatic effects are more subtle, but no less deadly. Higher rates of ground-level ozone are a major respiratory irritant, and vector-borne diseases thrive in warmer temperatures. And that’s the problem that’s keeping New York City’s public health officials up nights.

In the summer of 1999—the hottest and driest in a century—62 cases of West Nile encephalitis were reported in New York, and seven people died (see "Beyond the Bite," Your Health, November/December 2003). A general health warning was issued, and city residents began to get used to helicopters overhead spraying clouds of malathion and pyrethriod pesticides.

According to Dr. Dickson Despommier, a professor of public health at New York’s Columbia University, the disease is spread by Culex pipens mosquitoes, whose favorite prey is birds. But periods of high heat and drought send such common urban-dwelling species as crows, blue jays and robins out of the city in search of fresh water. City bird populations are further reduced as unlucky individuals are bitten and killed by West Nile infection.

"By reproductive imperative the mosquitoes are forced to feed on humans, and that’s what triggered the 1999 epidemic," Despommier says. "Higher temperatures also trigger increased mosquito biting frequency. The first big rains after the drought created new breeding sites."

Despommier says that this same pattern is also discernible in recent West Nile outbreaks in Israel, South Africa and Romania. In Bucharest, his investigation turned up abandoned buildings whose basements were full of water, a perfect Culex breeding ground.

Another prominent proponent of the West Nile global warming connection is Dr. Paul Epstein of Harvard University. "Droughts are more common and prolonged as the planet warms," he says. "Warm winters intensify drought because there’s a reduced spring runoff. The cycle seems to rev up in the spring, as catch basin water dries up and what is left becomes organically rich and a perfect mosquito-breeding place. The drought also reduces populations of mosquito predators."

In 2002, West Nile spread across the country, appearing in 44 states and the District of Columbia. Five provinces of Canada were also affected. It spread to 46 states in 2003, when there were 9,862 cases in the U.S. As the New York Times reported, the mosquitoes that carry the disease can breed effectively even in dry cities like Phoenix, where abandoned swimming pools serve as effective incubators. West Nile has killed more than 500 Americans. "Because West Nile is apparently carried by migrating birds, wildlife managers in North America have few options but to watch and wait for its arrival, and hope that stricken bird populations can recover in time," says Jeff Burgett of the U.S. Fish and Wildlife Service.

In a growing scientific consensus, public health officials believe the next drought will give this serious virus even a wider reach. Spraying certainly hasn’t stopped these infectious bugs. Researchers at France’s University of Montpellier said in mid-2003 that a mutation in the West Nile mosquitoes" genetic code resulted in their singular resistance to pesticides.

A Gathering Force

It’s plain that global warming will have a major impact on human health, and that new threats will emerge. "With vector-borne diseases, it’s pretty well established that increased temperatures cause higher and higher loads of viruses," says Peter Daszak, executive director of the Consortium for Conservation Medicine. "You also get an increase in biting rates of carriers like mosquitoes."

We have so far identified only one percent of the bacteria and four percent of the viruses on the planet, according to Harper"s. Some 29 previously unknown pathogens emerged between 1977 and 1994. In 2001, the American Public Health Association cited such probable effects as increased heat-related mortality, more rapid spread of vector-borne disease, greater incidence of water-borne diseases because of more intense precipitation and threatened agriculture to recommend that more attention be paid to climate change.

The Congressionally mandated "National Assessment of Climate Change and Health," issued in 2001, foresaw greater incidence of heat stroke, malaria, yellow fever and respiratory disease as a result of global warming. As countries like the U.S. become more "tropical," it said, insect- and rodent-borne diseases may be seen more often in the First World. Increased flooding will also breed more of these carriers. A 2004 Pentagon report on the possibility of abrupt climate change envisioned grave threats to national security arising from loss of agricultural productivity and widespread famine.

Some of the most enlightening studies arise from new technology. Rita Colwell, a professor at the University of Maryland-College Park, has studied public health in Bangladesh for 25 years, and recent work with satellite imaging and computer modeling has helped her make some new correlations between cholera outbreaks, waterborne pathogens and climate that wouldn’t have been possible without such modern tools. "It gives us the ability to actually predict the number of cases based on the historical record and climate variables," she says, adding that she is "hesitant to say flat out" that global warming is a major player. But she certainly sees it as a strong possibility.

Research published in the American Journal of Public Health in 2001 by Dr. Jonathan Patz and others reported that 68 percent of waterborne disease outbreaks studied came after major precipitation events (which are predicted to occur more frequently because of global warming). A growing pile of studies like these are making it clear that climate change will present a major challenge to our already overburdened global public health system.

—Jim Motavalli, with research assistance by Aaron Midler