"The influence of climate and the environment on infectious diseases has been a subject of debate, speculation, and serious study for centuries," according to Shope in the 1991 article "Global Climate Change and Infectious Diseases." Shope refers to Jacob Henle's 1840 treatise On Miasmata and Contagia in which Henle contends that "Heat and moisture favor the production and propagation of the infusoria and the molds, as well as the miasmata and contagia, therefore miasmatic-contagious diseases are most often endemic in warm moist regions and epidemic in the wet summer months." While the causes of many tropical infectious diseases are better understood today, the complexity of the disease processes makes it difficult to predict the precise effect that climate change will have on vector-borne diseases.
To understand how climate might affect the incidence of vector-borne diseases, one must first examine the life cycles of the diseases and the environmental parameters associated with each stage. The transmission of vector-borne diseases to human populations depends upon the attributes and requirements of at least three different living organisms: the pathologic agent, either a virus, protozoa, bacteria, or helminth (worm); the vector, usually arthropods such as ticks or mosquitoes; and the human host. In addition, intermediary hosts, such as domesticated and/or wild animals, often serve as a reservoir for the pathogen until susceptible human populations are exposed. The vector receives the agent from an infected individual and transmits it either to an intermediary host or directly to the human host. The different stages of the agent's life cycle occur during this process and are intimately dependent upon the availability of suitable vectors and hosts.
Weihe's and Mertens' chapter "Human Well-being, Diseases and Climate" in the 1991 Proceedings of the Second World Climate Conference, Climate Change: Science, Impacts, and Policy, describes how the occurrence of tropical diseases depends upon "the interplay of the host, the prevailing vector species, the parasite distribution, their mutual adaptations and resilience, densities and behavior of the people." Each of these factors in turn is influenced by climate.
Three of the key components that determine the occurrence of vector-borne diseases are presented in the World Health Organization Task Group's (1990a) report Potential Health Effects of Climatic Change: 1) the abundance of vectors and intermediate and reservoir hosts; 2) the prevalence of disease-causing parasites and pathogens suitably adapted to the vectors, the human or animal host, and the local environmental conditions, especially temperature and humidity; and 3) the resilience and behavior of the human population, which must be in dynamic equilibrium with the vector-borne parasites and pathogens. How climate change may affect each of these factors is summarized in the figure "Possible Effects of Climatic Change due to the Greenhouse Effect on Vector-borne Disease Epidemiology". In the chapter "Human Health" of the 1985 Department of Energy report Characterization of Information Requirements for Studies of CO2 Effects, White and Hertz-Picciotto illustrate how the influence of weather and climate on vectors and vectors' intermediary hosts often determines the prevalence of disease in human populations.
A more detailed discussion of the parasite-host population dynamics and the response to long-term climatic changes is provided by Dobson and Carper in the chapter "Global Warming and Potential Changes in Host-Parasite and Disease-Vector Relationships" of the 1992 book Global Warming and Biodiversity. They point out that the extent to which long-term climatic changes affect the distributions of different parasites and pathogens will depend upon their specific characteristics, such as place of reproduction, size, duration of infection, and the degree of immune response stimulated in human hosts, as well as other attributes of their life cycles. For example, increased temperature may reduce the number of larvae that survive at the infective stages of the parasite, but this may be offset by the increased infectivity of the larval stage. The heat may also hasten the parasite's development and cause rapid population growth. Each stage of a pathogen's life cycle can be linked to a range and optimum level of temperature and humidity.
"Infectious Diseases and Atmospheric Change," Shope's contribution to the 1990 book Global Atmospheric Change and Public Health, attempts to characterize those diseases that would spread if global warming occurred. Shope lists the ecological attributes of infectious diseases that would be necessary for an increased prevalence in North America under the anticipated conditions.
Any impacts of climate change on vector-borne diseases may first occur at the margins of their current distribution. A 1993 Lancet article, "Global Health Watch: Monitoring Impacts of Environmental Change," with contributions by Freier et al., suggests how specific diseases may shift poleward, rise to higher altitudes, and spread beyond areas where they are traditionally endemic. Others may be eliminated entirely due to extreme heat or dryness.
In the chapter "Weather, Vector Biology, and Arboviral Recrudescence" of the 1988 book The Arboviruses: Epidemiology and Ecology, Reiter discusses how climate has a significant effect on arthropods' behavior, development, and dispersion. He explores the weather-related aspects of the natural history of the vector that could trigger recrudescence, the sudden reactivation of an infectious disease. Nicholls also examines the effects of weather extremes on vector-borne diseases. In his 1993 Lancet article "El Nino-Southern Oscillation and Vector-borne Disease," Nicholls recounts several cases around the world where periods of extended, heavy rainfall preceded epidemics. Many of such extreme weather events accompany the El Nino-Southern Oscillation (ENSO). Nicholls contends that any changes in the behavior of ENSO, resulting in greater weather variability, could have more significant effects on the incidence of vector-borne diseases than changes in the mean climate.
More information is needed to understand both the direct and indirect effects of global warming on vector-borne diseases. 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 review the literature and point out the uncertainties. They recommend more surveillance of direct impacts, such as changes in the reproduction rate of the vector or the agent, the biting frequency of the vector, and the amount of time the host is exposed to the vector due to changes in temperature, rainfall, humidity, or storm patterns. Even less information is available to evaluate the impacts of societal and individual activities on the transmission of vector-borne diseases. Changes in hydrology, agriculture, forestry, and infrastructure in response to global warming may also indirectly affect the interrelationship among the disease agent, vectors, and hosts.