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• Movement and replication of vectors and abundance of animal hosts. These include reservoir species for infectious diseases, such as migratory birds that carry avian influenza.

Climate also influences the distribution and transmission of infectious diseases through indirect effects on local ecosystems and human behavior. For example, abundant precipitation provides more and better breeding sites for vector species such as mosquitoes, ticks, and snails, while increasing the density of vegetation beneficial to these organisms (Githeko et al., 2000). Drought, on the other hand, may prompt people to store water in open containers, which also provide ideal breeding environments for mosquitoes.

Climate influences each component of the epidemiological triad of host-vector (see Figure SA-3), pathogen, and environment, which intersect to produce infectious disease. The complex ecologies of vector-borne diseases render them particularly sensitive to variations in temperature, which can alter patterns of disease incidence, seasonal transmission, and geographic range (McMichael et al., 2006; Sutherst, 2004). Some scientists predict that the effects of climate change and variability on vector-borne diseases are likely to be expressed in the form of short-term epidemics, as well as through gradual changes in disease trends (Githeko et al., 2000).

Climate's Role in Context

Climate interacts with a range of factors that shape the course of infectious disease emergence, including host, vector, and pathogen population dynamics; land use, trade, and transportation; social, political, and economic systems; human and animal migration; and interventions that control or prevent disease. These interdependent influences—or web of causation—can act together, resulting in outbreaks or epidemics of infectious disease; for example, people and animals (both domesticated and wild), if forced by climate disasters to migrate,

SUMMARY AND ASSESSMENT 13

SUMMARY AND ASSESSMENT 13

FIGURE SA-3 The epidemiological triad.

SOURCE: Reprinted from Snieszko (1974) with permission from Blackwell Publishing Ltd. Copyright 1974.

FIGURE SA-3 The epidemiological triad.

SOURCE: Reprinted from Snieszko (1974) with permission from Blackwell Publishing Ltd. Copyright 1974.

may introduce pathogens, parasites, and disease vectors into novel environments. The intersection of human, livestock, andwildlife movements and migration with climate change is discussed in greater detail later in this summary (see "Policy Implications") and in Chapter 4. An even broader view of disease emergence, the "Convergence Model" (see Figure SA-4), places climate among other physical environmental factors in disease emergence that intersect with biological and socioeconomic factors, as well as with host (human) and microbe (IOM, 2003).

Observed Effects of Climate Variation on Infectious Disease Range and Transmission Dynamics

The many factors confounding the interrelationships between climate change and infectious disease emergence vastly complicate attempts to investigate causality. As Haines and coauthors note, "Empirical observation of the health consequences of recent climate change, followed by formulation, testing, and then modification of hypotheses would require long time-series (probably several decades) of careful monitoring" (Haines et al., 2006). To inform health policy in the immediate future, risk assessments will need to be developed from short-

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