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close this bookWIT's World Ecology Report - Vol. 08, No. 3 - Critical Issues in Health and the Environment (WIT, 1996, 16 pages)
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Infectious Diseases and Climate Change

Robert Shape, M.D., Department of Pathology, University of Texas, Galveston, Texas.

Available evidence and the majority opinion of atmospheric scientists suggest that greenhouse gas accumulation is causing the biosphere to get warmer. Global climate models diverge in their forecasts of the extent of warming, the regional variation, and accompanying changes in precipitation. The majority opinion is that temperate zones will get warmer (perhaps 1-4 C) over the next 50-100 years, and wetter. Increase in ambient temperature and changes in rainfall will have direct effect on the vectors of infectious diseases and indirect effect on the interactions among vectors, vertebrate hosts, and the infectious agents that cause diseases. A rise in ambient temperature raises the body temperature of the invertebrate vector, and thus increases the temperature to which the infectious agent is exposed. The wider distribution and abundance of both vectors and pathogens increases the range and severity of certain epidemic and endemic infectious diseases. In most cases, this increase will hasten the rates of replication, evolution, and transmission of the infectious agents. In some cases, the vectors will not tolerate the temperature change and will either move to a more hospitable climate or die.

A related change in stratospheric ozone layer attenuation leading to increased surface level untraviolet radiation is also predicted. The consequences of this include impaired immune responses both of humans and domesticated animals, enhancing their vulnerability to infectious diseases.

Not all infectious diseases will be affected by global climate changes. Those that depend on vectors and wild vertebrate reservoirs for their survival and maintenance in nature are more likely to change their geographic distribution, to be modified in their virulence and/or to affect new human populations groups.

Dengue causes a severe tropical fever and sometimes hemorrhagic disease. The Dengue virus is transmitted by Aedes aegypti, a mosquito that thrives in warm climates, lives with people, and spreads readily with the tropics. As the globe warms, this mosquito will spread north and south from the tropics further into the current temperature zones. The potential for severe epidemics will be accentuated if the human population continues to increase and as cities grow in the newly warmed areas.

Other vector-borne agents of disease such as malaria, schistosomiasis, leishmaniasis, and yellow fever can be expected to become more prevalent and to appear in newly-infected geographical areas. Change in rainfall levels will also affect the prevalence of these diseases, and because of the special ecological adaptations of each, their frequency will be changed differently. Some vector-borne diseases have very specific ecological requirements; if the warming is of a degree that does not permit adaptation of a specific vector or a specific verterbrate host, then the disease will become less prevalent or even extinct. We do not always sufficiently understand the ecology to predict consequences.

Agents of disease that do not have an arthropod vector are also affected by ecological change. Cholera and vampire bat rabies are examples. These depend for their transmission on warm temperature, although in case of cholera we do not understand completely why.

Finally, there are undoubtedly diseases as yet undiscovered in sparsely populated regions. These diseases are ecology dependent and will develop into full-fledged human illnesses only when people migrate into their ecospheres. Regions such as northern Canada, Alaska, and parts of Siberia deserve careful study to find new agents and to characterize the growth potential of these microbes, especially under warmer conditions. Such studies are important to prevent surprises in the future and lessen the impact of future outbreaks of disease.

Major Tropical Vector-Borne Diseases and the Likelihood of Change of Their Distribution with Climate Change

Disease

Vector

Population at Risk (million)

No. of People Currently Infected or New Cases Per Year

Present Distribution

Likelihood of Altered Distribution with Climate Change

Malaria

Mosquito

2,400

300-500 million

Tropics/Subtropics

+++

Schistosomiasis

Water Snail

600

200 million

Tropics/Subtropics

++

Lymphatic Filariasis

Mosquito

1,094

117 million

Tropics/Subtropics

+

African Trypanosomiasis (Sleeping Sickness)

Tsetse Fly

55

250,000-300,000 cases/yr

Tropical Africa

+

Dracunculiasis (Guinea Worm)

Crustacean (Copepod)

100

100,000/yr

South Asia/Arabian Peninsula/Central-West Africa

?

Leishmaniasis

Phlebotomine Sand Fly

350

12 million infected, 500,000 new cases/yr

Asia/Southern Europe/Africa/

+

Onchocerciasis (River Blindness)

Black Fly

123

17.5 million

Africa/Latin America

++

American Trypanosomiasis Bug (Chagas' disease)

Triatomine

100

18 million

Central and South America

+

Dengue

Mosquito

1,800

10-30 million/yr

All Tropical Countries

++

Yellow Fever

Mosquito

450

<5,000 cases/yr

Tropical South America and Africa

++

NOTE: + = likely; ++ = very likely; +++ = highly likely; and ? = unknown.

SOURCE: IPCC, 1995.