of as contextually providing suitable conditions for the persistence of infectious disease agents or more directly driving the variability of vector and host populations and interactions. Environmentally mediated mechanisms explaining vector and host dynamics focus on nonlinear changes to time before an infectious mosquito can retransmit a virus or extrinsic incubation period (EIP), vector population explosions, or changing host-seeking behavior (Jupp et al., 1986; Kilpatrick et al., 2006; Reisen et al., 2006).

For a full discussion of the topic, refer to Patz and Olson (2006) earlier in this chapter. Next, we provide an update on the two most prevalent vector-borne diseases in North America: Lyme disease and West Nile virus.

Lyme Disease

Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, and tick-borne encephalitis are the most common vector-borne diseases in temperate zones in the northern hemisphere. Climate affects tick habitat, host and reservoir species, the interval between blood meals, and pathogen transmission.

Lyme disease is the most prevalent tick-borne disease in North America for which there is new evidence of an association with temperature (Ogden et al., 2006) and precipitation (McCabe and Bunnell, 2004). In the field, temperature and vapor pressure contribute to maintaining populations of the tick Ixodes scapularis, which, in the United States, is the microorganism’s secondary host. A monthly average minimum temperature above −7°C is required for tick survival (Brownstein et al., 2003).

The northern boundary of tick-borne Lyme disease is limited by cold temperature effects on the tick, Ixodes scapularis. Linking to future projections of climate via global climate models (GCMs), the northern range limit for this tick could shift north by 200 km by the 2020s, and 1,000 km by the 2080s. Plausible tick geographic ranges were developed from the Coupled Global Climate Model version 2 (CGDM2) and Hadley Centre Coupled Model, version 3 (HadCM3) models using the A2 Intergovernmental Panel on Climate Change Special Report on Emissions Scenario (Ogden et al., 2006).

West Nile Virus

Climate variability has been shown to affect West Nile virus (WNV), a disease only recently introduced into the New World in 1999. The summer adult WNV vector Culex spp. monthly abundance across diverse U.S. and Canadian biomes is largely controlled by antecedent moisture and temperature conditions (Raddatz, 1986; Day and Curtis, 1989; Andreadis et al., 2004). Optimal temperatures increase the rates of juvenile mosquito maturation, adult females biting, virus replication (decreases the extrinsic incubation period), and the total amount of virus transmitted (Madder et al., 1983; Buth et al., 1990; Rueda et al.,



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