borne diseases (e.g., cholera, schistosomiasis) and vector-borne diseases (e.g., malaria, yellow fever, dengue, leishmaniasis) if not countered by changes in adaptation, public health, or treatment availability. These changes will be driven largely by increases in precipitation leading to favorable habitat availability for vectors, intermediate and reservoir hosts, and/ or warming that leads to expansion of ranges in low latitudes, oceans, or montane regions. The host-parasite dynamics for abrupt climate change have not been targeted specifically as yet, but Daszak et al. (2001) suggested three phenomena that indicate abrupt climate change may have had heightened impacts on key human diseases:

  1. There appears to be a strong link between El Niño-Southern Oscillation (ENSO) and outbreaks of Rift Valley fever, cholera, hantavirus, and a range of emergent diseases (Colwell, 1996; Bouma and Dye, 1997; Linthicum et al., 1999), and if ENSO cycles become more intense, these events may become more extensive and have greater impact;

  2. Malaria has reemerged in a number of upland tropical regions (Epstein, 1998) (although this is debated by Reiter, 1998); and

  3. Recent extreme weather events have precipitated a number of disease outbreaks (Epstein, 1998).

Criteria that define emerging infectious diseases of humans were recently used to also identify a range of emerging infectious diseases that affect wildlife (Daszak et al., 2000). They include a fungal disease that is responsible for mass mortality of amphibians on a global scale and linked to species extinctions (Berger et al., 1998), canine distemper virus in African wild dogs, American ferrets and a series of marine mammals, and brucellosis in bison as well as others. An ongoing reduction in biodiversity and increased threats of disease emergence in humans and livestock make the impacts of these changes potentially very large.

Emerging diseases are affected by anthropogenic environmental changes that increase transmission rates to certain populations and select for pathogens adapted to these new conditions. Daszak (2001) points to abrupt climate change as pushing environmental conditions past thresholds that allow diseases to become established following their introduction. For example, African horse sickness (a vector-borne disease of horses, dogs, and zebras) is endemic in sub-Saharan Africa. Although it usually dies out within 2 to 3 years of introduction to Europe, the latest event involving imported zebras to Spain resulted in a 5-year persistence, probably because



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