Predicting the effects of global warming on health requires an examination of the current incidence and distribution of major vector-borne diseases. According to the report of the World Health Organization (WHO) Commission on Health and Environment (1992b) Our Planet, Our Health, "The conditions in which environmental factors are most prominent are tropical diseases, which to a large extent are caused by infection by parasites requiring one or more intermediate hosts and vectors for their development."
Vector-borne diseases are currently prevalent in the tropics and subtropics and relatively rare in temperate zones. These diseases contribute to a large proportion of the health problems in developing countries. WHO receives data on morbidity and mortality due to vector-borne diseases from a number of countries and regions. WHO's report Tropical Diseases provides the most up-to-date estimates of the number of people infected by the most serious vector-borne diseases: malaria, 270 million; schistosomiasis, 200 million; lymphatic filariases, over 90 million; onchocerciasis, nearly 18 million; leishmaniasis, 12 million; dracunculiasis, 1 million; and African trypanosomiasis, 25,000 new cases per year (1990b). WHO indicates the available data are generally considered to underestimate the prevalence of these diseases in human populations.
Other sources also present information on the demographic and geographic distributions of vector-borne diseases: an unpublished 1989 WHO document "Geographical Distribution of Arthropod-borne Diseases and their Principal Vectors"; the section "Infectious and Parasitic Diseases" of WHO's publication Global Health Situation and Projection--Estimates (1992a); the chapter "Human Health" of UNEP's book Saving Our Planet (1992b); Part 5, "Human Health," from UNEP's Environmental Data Report 1989/1990; and Chapter 6, "Population and Human Development," of the World Resources Institute's report World Resources 1992-1993. Given the inconsistencies in the way data are gathered and reported by countries and regions, however, international and interregional comparisons may not be valid, according to UNEP's Environmental Data Report.
Several summary tables from the sources listed above are of special interest. The table "Trends in Malaria Morbidity" in UNEP's Environmental Data Report 1989/1990 provides statistics on malaria from 1965 to 1987 for individual reporting countries and regions. The table "Onchocerciasis Morbidity" from the same UNEP report estimates the number of persons at risk, infected, and blind as a result of onchocerciasis around 1983 for countries where the disease is endemic in Africa, the Eastern Mediterranean, and the Americas. The table "Major Causes of Death in the World, 1990" in WHO's Global Health Situation and Projection--Estimates gives the estimated number of deaths in 1990 for 14 infectious and parasitic diseases (1992a).
In addition, WHO's book Our Planet, Our Health presents the table "Causes of Death in Developing Countries around 1985" that includes a breakdown of the deaths due to infectious and parasitic diseases for the population under 5 years old, 5 years and older, and all ages (1992b). Figure 7 and figure 8 in UNEP's 1992 book The World Environment 1972-1992 provide information useful for following changes in the distribution of schistosomiasis and malaria, respectively.
Although vector-borne diseases are currently considered a major health hazard only in tropical and subtropical regions, climate change could create conditions suitable for outbreaks in temperate regions as well. In "The Potential Impact of Climate Change on Patterns of Infectious Disease in the United States," part of the EPA's 1989 report to Congress The Potential Effects of Global Climate Change on the United States, Longstreth and Wiseman describe five vector-borne diseases--Lyme disease, Rocky Mountain spotted fever, malaria, dengue fever, and viral encephalitis--whose incidence in the United States might change due to global warming.