Potential health impacts of climate change pathways and examples of infectious disease impacts

The various pathways by which climate change could affect patterns of human health, acting via its various manifestations in climatic conditions and weather patterns, are summarized in Figure 14.4. The range of risks to human health is extensive - and potentially catastrophic in some circumstances. The risk of infectious disease is affected mostly by climatic influences on the biology and behavior of pathogen, vector species, and intermediate or natural host species.

Climate change

Direct impact e.g. heatwaves, floods, fires

Mediating processes (indirect)

Changes to physical systems/processes e.g. urban-industrial air pollution freshwater supply

Biological changes: processes, timing e.g. mosquito numbers, range; photosynthesis ->crop yields

Changes to ecosystem structure and function e.g. fisheries; constraints on microbes; nutrient cycles; forest productivity e.g. heatwaves, floods, fires

V economics Health demographic impacts disruptions ^ •

Figure 14.4 Climate change and health: pathways, impacts.

The risks span readily understood risks to health and survival from extremes of ambient temperature and from other extreme weather events, disruptions to food production and availability, and changes in the range and activity of pathogens and vector organisms. Infectious disease risk may also be affected more indirectly via social disruption, poverty, and population displacement (environmental refugees) occurring in response to climate change.

All infectious diseases that have climate warming will affect host-pathogen interactions by:

• increasing pathogen development rates, transmission, and number of generations per year

• constraining over-wintering restrictions on pathogen lifecycles (Figure 14.5)

• modifying host susceptibility to infection.

Changes in these mechanisms could cause pathogen range expansions and host declines, or could release hosts from disease control by interfering with the precise conditions required by many parasites. Clearly, not all pathogens have

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Figure 14.5 The influence of an average 1.5°C rise in temperature on the growth rate (reproductive number, R0) of a typical pathogen. If R0 >1, pathogen multiplies. When R0 is more than 1, pathogen growth will increase. The lower dotted curve represents the average weekly temperature before climate change; the upper dotted curve represents average weekly temperature after a 1.5°C increase. The lower horizontal line corresponds to R0 = 1 (below this temperature the pathogen population declines). The rate of pathogen growth increases above this temperature. We assume the risk of disease outbreaks becomes severe when temperature reaches the middle horizontal line, and epidemic at the upper horizontal line. Temperature increases also lead to an extension in the duration of the season when the pathogen is a problem. Reprinted from Harvell et al. (2002), with permission; ©2002 AAAS.

equal potential to control host populations or to be affected by warming. Climate warming is expected to disproportionately affect pathogens with complex life-cycles, or those that infect mosquitoes during one or more lifecycle phases (Harvell et al., 2002).

Of climate-sensitive infectious diseases, vector-borne diseases are strong candidates for altered abundance and geographic range shifts, because rising temperatures will affect vector distribution, parasite development, and transmission rates (Kovats et al., 2001). Climate change will affect the potential geographic range, seasonal transmission, and incidence of various vector-borne diseases. These would include malaria, dengue fever, and yellow fever (all mosquito-borne); various types of viral encephalitis; schistosomiasis (water-snails); leishmaniasis (found on the South America and Mediterranean coasts, and spread by sand-flies); Lyme disease (ticks); and onchocerciasis (West African "river blindness," spread by black flies).

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