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Linkages Between Climate, Ecosystems, and Infectious Diseases
Pages 20-44

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From page 20...
... The third section provides an overview of the different ways that climate can influence the emergence and transmission of infectious disease agents, and the fourth section reviews other factors that must be considered as part of a comprehensive understanding of disease dynamics. WEATHER AND CLIMATE: BACKGROUND CONCEPTS Climate Variability The distinction between weather and climate is not always appreciated but is important to understand in the context of the topics discussed in this report.
From page 21...
... The amplitude of seasonal climate variability is generally larger than that of the diurnal cycle at high latitudes and smaller at low latitudes. Normal annual cycles are modulated by interannual variations in average seasonal conditions.
From page 22...
... Most climate models project that global temperatures will continue to increase throughout the twenty-first century, with warming of several degrees
From page 24...
... Numerical weather models start with observations of the current atmosphere and integrate forward in time. These forecasts are often defined as solutions to "initial value" problems.
From page 25...
... Not until the 1980s did climate scientists begin to capitalize on the premise that the physical basis for seasonal climate modeling does not rest solely in the atmosphere. Seasonal climate variability is now understood to be a manifestation of complex interactions between the atmosphere and underlying surface, primarily the world oceans.
From page 26...
... The successful incorporation of coupled land surface models into seasonal climate forecast models is expected to enhance prediction skill.
From page 27...
... Thus, there is the conundrum that the tools for day-to-day weather prediction are constrained to less than two weeks, while the tools for climate prediction generally only work well on seasonal or longer timescales. Coupled ocean/atmosphere/land models may improve the ability of climate models to make predictions on shorter than seasonal timescales, and more sophisticated statistical techniques may improve the use of atmospheric weather models beyond 10 to 14 days.
From page 28...
... Thus, the number of cases that constitute an epidemic or outbreak will vary with each disease, location, and season. A further complication is the fact that numerous methods are used to estimate disease incidence, not all of which are highly accurate.
From page 29...
... Other frequently identified factors in disease emergence (especially in outbreaks of previously unrecognized diseases) are climatic or ecological changes that place people in contact with a natural reservoir or host of an infection, by either increasing proximity or creating conditions that favor an increased population of the microbe or its natural host (NRC, 1992; Wilson et al., 1994; Morse, 1995~.
From page 30...
... infect humans via water, soil, feces, or arthropod vectors. There are numerous examples of these types of transmission cycles, including aerosol-borne hantaviruses, water-borne cryptosporidiosis, sandfly-transmitted leishmaniasis, flea-vectored plague, tick-associated Lyme disease, and mosquito-borne Rift Valley fever.
From page 31...
... The SEIR Framework At the heart of many studies of infectious disease transmission dynamics is an SEIR modeling framework, which depicts the different states in the progression of a disease through a population: the proportions of individuals susceptible to infection (S) ; the proportion of people exposed to an infectious agent but not yet infectious (E)
From page 32...
... infected but not infectious FIGURE 3-4 Diagram of the SEIR framework used in modeling the transmission of disease agents. Definition of parameters: a = host per capital reproductive rate; A= host per capita death rate; ~ = transmission coefficient; or = host per capita death rate from being infected; 1/~= average duration of immunity; 1/d = average duration of latent period; 1/v = average duration of infection.
From page 33...
... WEATHER/CLIMATE INFLUENCES ON INFECTIOUS DISEASES: AN OVERVIEW Disease agents and their vectors each have particular environments that are optimal for growth, survival, transport, and dissemination. Factors such as precipitation, temperature, humidity, and ultraviolet radiation intensity are part of that environment.
From page 34...
... For instance, some respiratory viruses grow preferentially in the upper airways where the cells are a bit cooler than core body temperatures, and bacteria such as leprosy grow preferentially in the cooler tissues of the extremities (Robard, 1981~. Waterborne diseases such as cholera also are temperaturesensitive, with a minimum temperature required for replication in the environment.
From page 35...
... caused by a parasite common in tropical West Africa and South America, where the breeding of the disease vector, the Simulid fly, is governed by river water flow (WHO, 1985~. The ecosystem instabilities brought about by climatic changes can give rise to new interactions among hosts and infectious disease agents, possibly accelerating the problem of emerging infectious diseases.
From page 36...
... Over the long term, a wide array of other largely unforeseeable changes could greatly affect the type and global distribution of infectious diseases. For instance, it is possible that 50 years hence the potential to control diseases such as malaria and dengue could be greatly strengthened by vector eradication programs, improved sanitation/public health systems, or the development of drugs or vaccines.
From page 37...
... Long-term climate change may increase the risks of extreme events such as floods and droughts occurring in particular regions and could possibly affect the frequency and intensity of mid-latitude storms and hurricanes; however, such projections are highly uncertain at this time. While the primary risks associated with extreme weather events are injury and death that result directly from the event itself, there is also concern that weather-related disasters can lead to infectious disease outbreaks.
From page 38...
... It is important to characterize accurately the disease risks associated with different types of extreme weather events, so that limited resources are not needlessly diverted from other, perhaps more critical, facets of a disaster relief effort. The risks associated with waterborne disease are fairly well understood, and there are numerous examples of enteric disease outbreaks following floods, largely due to disruption in public health and sanitation services (i.e., water treatment)
From page 39...
... OTHER FACTORS THAT AFFECT INFECTIOUS DISEASE DYNAMICS The primary focus of this report is on the relationships between climate and infectious diseases, but in order to understand this relationship it is also necessary to appreciate the many other factors that contribute to the distribution and dynamics of infectious diseases. These include factors such as land-use patterns; social, demographic, and geographical considerations; transportation and migration patterns; and public health interventions.
From page 40...
... The development of more highly interconnected areas as regions grow economically is a long-term factor in the transmission of infectious diseases. For instance, the spread of cholera through the United States in the nineteenth century directly reflected the changing connectivity of the urban system.
From page 41...
... For example, the "epidemiological transition" is a commonly observed pattern in the development of most societies, wherein most mortality is initially due to infectious causes, but with improved sanitation and nutrition, mortality due to infectious diseases decreases while mortality due to non-infectious causes such as heart disease, stroke, cancers, and accidents increases. Along with this transition comes an increase in life expectancy, and the age structure of the population thus shifts to one where there are proportionately more middle-aged and older individuals.
From page 42...
... In tropical areas where previously endemic infectious diseases have remained highly localized, greater global interconnectivity is being reflected in the redistribution and spread of these diseases. Household design and architecture can also influence patterns of vectored disease transmission.
From page 43...
... Public health measures such as improved sanitation, better nutrition, and environmental modification have been of great significance in altering patterns of disease transmission. Indeed, some diseases have been eliminated in specific regions because of these measures.
From page 44...
... Some of the reasons for this include aging infrastructure of sewage and water treatment facilities in the face of increasing water demands; inability to identify the microorganisms associated with many diseases; newly recognized microorganisms that are more resistant to disinfection (e.g., Legionella, Cryptosporidium, Giardia) ; contamination of water supplies with sewage overflow or concentrated fecal sources from animal farming operations; and increasing non-point sources of pollution, including septic tanks.


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