Global Climate Changes and Infectious Diseases

John Last, MD
Director
Department of Epidemiology and Community Medicine
Institute of Infectious Diseases
University of Ottawa
Ottawa, Ontario, Canada

Available evidence and the majority opinion of atmospheric scientists suggest that greenhouse gas accumulation will cause (perhaps already 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 that some at least will get wetter.

A rise in ambient temperature enhances the activity and reproductive capacity of many insect vectors of disease, and in some cases, also of the pathogenic organisms that are carried by these vectors. The wider distribution and abundance of both vectors and pathogens increases the range and severity of certain epidemic and endemic infectious diseases.

A related change in stratospheric ozone layer attentuation leading to increased surface level ultraviolet radiation is also predicted. The consequences of this include impaired immune responses both of humans and domesticated animals, enhancing their vulnerability to infectious diseases. The outcome of these phenomena is a considerable increase in the risks both to humans and to animal herds of severe epidemics of infectious disease.

Other aspects of global change have to be taken into account. Climate change will likely be accompanied by declining soil moisture levels in the world's principal grain-growing areas leading to reduced agricultural output. Food shortages are already serious in several regions of the developing world: chronic under-nutrition aggravates the impact of infection, so there can be high fatality rates from otherwise trivial outbreaks of infectious disease.

Rich industrial nations have not experienced food shortages because they can afford to buy from food-producing countries, but if climate change seriously impairs agricultural output, this might change; and food shortages in the rich nations would have the same consequences as in the poorer nations of the developing world, including enhanced susceptibility to infections.

Underlying all the human-induced global changes is the relentless demographic force of population growth, currently at an annual rate of about 90 million. This is accompanied by unprecedented population movements, notably rural-to-urban shifts, especially into periurban slums in many parts of the developing world, and also movement from poor to rich nations of large numbers often described as "environmental" refugees. This set of factors also increases the risk of epidemics, people on the move take their diseases with them, and are often exposed to new diseases when they reach their destinations.