Possible Direct and Indirect Effects of Climate Change on Specific Vector-borne Diseases

Several researchers and panel groups have used current information and modeling techniques to predict the potential effects of climate change on the distribution of specific vector-borne diseases. The lack of sufficient data about the life cycles of the d iseases and the uncertainties surrounding the specific occurrence of climatic effects make these efforts difficult. In most cases, the studies suggest general shifts in the distribution of the diseases rather than specific changes for any particular locat ion.

The World Health Organization's (WHO) (1990a) task group report "Potential Health Effects of Climatic Change" considers potential changes in the distribution of the most prevalent vector-borne diseases: malaria, ly mphatic filariasis, onchocerciasis, schistosomiasis, African trypanosomiasis, leishmaniasis, dracunculiasis, and arboviral diseases such as St. Louis encephalitis and Rift Valley fever. The table " Global Status of Major Vector-borne Diseases" summarizes the population at risk, the prevalence of infection, the present distribution, and the possible change of distribution as a result of climatic change for each of these key diseases. Estimation o f changes are based solely on changes in abiotic factors, such as temperature, precipitation, relative humidity, wind, solar radiation, topography, or water systems. Changes in biotic factors, such as vegetation, hosts, natural predators, parasites, and p athogens of the vector are not reflected in the table's estimates. The report, however, does provide information on possible indirect effects that result from human behavioral responses, such as variations in agricultural practices and demographic movemen ts. The report also refers to the United Nations Food and Agriculture Organization's (FAO) 1987 working papers "Effects of Agricultural Development on Vector-borne Diseases," from the Seventh Annual Meeting of the Joint WHO/FAO/United Nations Environment Programme (UNEP) Panel of Experts on Environmental Management for Vector Control held in Rome, 7-11 September 1987. According to WHO, the working papers provide a great deal of information on "the impact of changes in agricultural practices on the epidemiology of vector-borne diseases."

Many authors provide an overview of the potential changes in distribution of several of the key vector-borne diseases due to global warming. In the chapter "Human Well-being, Diseases, and Climate" of Climate Ch ange: Science, Impacts, and Policy, Weihe and Mertens (1991) emphasize the importance of predicted climate changes on the distribution of malaria, trypanosomiasis, lymphatic filariasis, onchocerciasis, schistosomiasis, and arbovirus-related encephalit is. In the chapter "Infectious Diseases and Atmospheric Change" from the 1990 book Global Atmospheric Change and Public Health, Shope (1990) focuses primarily on those vector-borne diseases that could reloca te in temperate and Arctic regions given global warming: dengue and yellow fever, St. Louis encephalitis, leishmaniasis, and rabies. In the chapter "Impacts of Global Climate Change on Human Health: Spread of Infectiou s Disease" from Global Climate Change, Shope (1992) describes how dengue fever could spread to populated areas of North America given the different environmental parameters of its multiple mosquito vectors. He then suggests how schistosomiasis and Rift Valley fever may also extend their geographic range in Africa and Asia given global climate change.

In-depth studies are also available regarding potential changes in the distribution and abundance of specific vector-borne diseases and their vectors for particular regions of the world. In "Distribution and Abundance of Tsetse Flies (Glossina spp.)," Rogers and Randolph (1986) use both an empirical and an analytical approach to examine this link for trypanosomiasis, also known as sleeping sickness, and its vector, the tsetse fly in Africa. Insufficient data makes it necessary to extrapolate their results from local field studies to larger areas of Pan-Africa. The authors recommend the use of satellite-derived data to eventually predict the changing risks from trypanosomiasis. In the chapter "Global Warming and Potential Changes in Host-Parasite and Disease-Vector Relationships" of the 1992 book Global Warming and Biodiversity, Dobson and Carper also examine the response of the trypanosoma pathogen to the predi cted climate changes for the next century. In addition, the brief but interesting 1993 paper by Freier "Eastern Equine Encephalomyelitis" describes the natural history, transmission, and impact of climate change on this virus.

The Environmental Protection Agency (EPA) commissioned two papers on vector-borne diseases for its 1989 report to Congress The Potential Effects of Global Climate Change on the United States, Appendix G Health. Haile's paper "Computer Simulation of the Effects of Weather Patterns on Vector-Borne Disease Transmission" presents two weather-based models to simulate the population dynamics of two potentially critical vectors in the United States: the American do g tick, primary vector of Rocky Mountain Spotted Fever, and a species of Anopheline mosquitoes, one of several vectors for malaria. Longstreth's and Wiseman's paper "The Potential Impact of Climate Change on Pat terns of Infectious Disease in the United States" addresses Congress' concern that climate change could increase morbidity and mortality due to infectious diseases in the United States. The authors focus on the changes in vector-borne diseases rather than on infectious diseases in general, because their incidence is most likely to be changed due to variations in climate and the least likely to be eradicated by improvements in sanitation, nutrition, immunization, and treatment. Longstreth and Wiseman p rovide a detailed account of what is known about the impact of climatic variables on the five vector-borne diseases most likely to be affected in the United States: Lyme disease, Rocky Mountain spotted fever, malaria, dengue fever, and viral encephalitis. Maps of the geographic distribution of these diseases are also included.

Another regional study looks at potential variations due to climatic change in seasonal and geographical abundance of major vector species and vertebrate hosts for vector-borne diseases in Australia. Liehne's 1988 paper "Climatic Influences on Mosquito-borne Diseases in Australia" describes the underlying factors that could trigger outbreaks of diseases such as malaria, polyarthritis, Murray Valley encephalitis, Australian encephalitis, and dengue in tropical Austr alia, and polyarthritis and Australian encephalitis in temperate Australia. Liehne also considers the impact of possible changes to the El Nino/Southern Oscillation due to the greenhouse effect.

Certain regions of the world may be particularly sensitive to climate change and instability. In the 1993 Lancet article "Critical Regions, a Profile of Honduras," Almendares et al. focus on Honduras as a ca se study to illustrate how the spread of several vector-borne diseases (malaria, Chagas' disease, leishmaniasis, dengue fever) could threaten local populations. Other endangered regions whose basic life-support systems are threatened by climate change inc lude Haiti, the Philippines, the Basin of Mexico, south-eastern Kenya (Ukamani), eastern Borneo, Nepal's middle mountains, China's Ordos Plateau, Amazonia, and the Aral Sea (former USSR). Although many consider the introduction or re-establishment of vect or-borne diseases in temperate zones to be of primary concern, the authors emphasize that tropical ecosystems that have experienced repeated stress may be less resilient and more vulnerable to the effects of climate change.