Global Warming and the Spread of Tropical Diseases

Commentary by H. Sterling Burnett

A recent study published in Science links global warming to a number of epidemics in plant, animal and human communities around the world. Its authors argue that among the deadly results of the earth's current warming trend are unusual weather patterns that favor "opportunistic pests" - rodents, parasitic insects, bacteria and fungi - which often carry and transmit tropical diseases, including cholera, dengue fever, yellow fever and malaria. They further argue that if these pests enter new regions such as the United States, the diseases they carry may spread dramatically.

With the multitude of other environmental and climate factors that affect the range and survival of tropical diseases, it is hard to conclusively tie the current outbreaks within plant and animal communities to human caused global warming.

What is clear, however, is that while a warm climate is a necessary condition for mosquitoes and many other pests that can carry diseases such as malaria and dengue fever, it is not sufficient for the diseases to become epidemic - at least not among human populations. Indeed, tropical diseases were fairly common in the United States during the last century, with widespread outbreaks of malaria and yellow fever in cities as far north as New York City and Baltimore and even in the state of Minnesota, at a time when temperatures were significantly cooler than they are today.

But despite the current warmer temperatures, these diseases are either rare or non-existent in the U.S. This is true even though right across the U.S. border, diseases may be raging. For example, while dengue fever wreaked havoc in Reynosa, Mexico (2,361 confirmed cases) in 1995, the entire United States had only 86 cases (all in Texas), all but eight of which were imported by immigrants arriving with the disease. In the U.S., while ground level temperature levels were increasing, malaria cases fell from 63,000 in 1945 to only 2,000 in 1950; and most new U.S. cases are the result of immigrants, foreign travelers and U.S. citizens returning from travel in the tropics.

Reinforcing the point that climate is not a primary determinant in the prevalence of tropical diseases is the fact that Singapore, a tropical country just 2 degrees from the equator, reported no malaria cases in 1992, while Malaysia, a nation that borders Singapore, reported 36,853 malaria cases, and the multi-island nation of Indonesia, which surrounds Singapore on three sides, reported more than 13,000 cases.

Indeed, malaria is not a rising cause of illness or death worldwide. From 1983 to 1992, the most recent year for which firm data exist for Africa - the most heavily infested region of the world - reported cases of malaria fell from more than 3.2 million to just over 420,000. The truth is, despite periodic regional outbreaks of communicable tropical diseases, malaria rates and those of other tropical diseases are decreasing on average globally, and at an even higher rate in developing countries.

Misguided public policies can reverse this positive trend, however, making outbreaks of communicable diseases more common. For example, Peru, due largely to chlorination of the drinking water supplies, had been cholera-free for many decades. But in 1991, based primarily on a study by the U.S. Environmental Protection Agency (EPA) showing that chlorine or its byproducts posed a hypothetical increased risk of cancer, Peruvian officials ended their policy of water chlorination. As a result, more than 100,000 Peruvians contracted cholera in the following year. The epidemic spread across South America, making more than 1 million people ill and taking more than 11,000 lives. In 1992, new research by the EPA determined that there was no link between cancer and chlorinated drinking water, but by then the damage was done.

In the U.S., by contrast, cholera and yellow fever have been virtually eradicated by the advent of filtered, chlorinated water and basic sanitation. In addition, malaria has been almost entirely exterminated through a combination of the judicious application of pesticides (most notably DDT), the draining of many of the nation's swamps, the use of screens on windows and doors, the rise of air-conditioners, and the discovery and widespread use of effective anti-malarial drugs. In every other country where this combination of factors has been used, malaria rates have drastically declined.

Misguided government policies kill more than people; they also kill economic growth. All health experts recognize that the prevalence of tropical diseases in the developing world stems from poverty and the conditions it entails, including lack of access to medical care, basic sanitation and various technological interventions. Yet politicians in these countries often refuse to adopt policies that would promote economic growth, thereby reducing disease and death.

Periodic outbreaks of tropical diseases are not a result of global warming; they are caused by bad policies. Eliminate the policies that discourage basic sanitation, the prudent use of pesticides and economic growth, and incomes will rise and tropical diseases will decline - regardless of the temperature.