Mills et al. density of the host or vector that would result in increased or decreased frequency of contact with humans or with other hosts and vectors; c) changes in the prevalence of infection by the pathogen in the host or vector popula- tion that would increase or decrease the fre- quency of human (or other host or vector) contact with an infected host or vector; and d) changes in pathogen load brought about by changes in rates of pathogen reproduction, replication, or development in hosts or vectors that would affect the likelihood that a human (or other host or vector) contact would result in pathogen transmission. The evidence: what do existing studies por- tend? Range shifts. Geographic changes in the distributions of wildlife species in response to climate change are difficult to document because of the absence of current and historic data on distributions of many species and populations (Jannett et al. 2007; Thomas et al. 2006). Nevertheless, range shifts have been observed for a variety of taxa, including impor- tant mammalian hosts and arthropod vectors. These range shifts have generally been pole- ward and upward (toward higher elevations) (Hickling et al. 2006; Rosenzweig et al. 2007), and they have resulted in overall expansions, contractions, or no change in the total area occupied by a population or species. Altitudinal and latitudinal range shifts have occurred for Ixodes ricinus, the vector of the agents of Lyme disease and tick-borne encephalitis (TBE) in Europe (reviewed by Gage et al. 2008), and these shifts are associated with (“association” as used throughout this review is not proof of causation) new foci of disease and an increase in incidence of TBE (Kerr 2001; Lindgren and Gustafson 2001). Northerly range shifts also have been observed for Ixodes scapularis, a vec- tor of Lyme disease, human granulocytic ana- plasmosis, and babesiosis in North America. Models indicate that this species will extend its range farther into Canada while contracting its southern range (Brownstein et al. 2005; Ogden et al. 2006, 2010). The hispid cotton rat (Sigmodon hispidus), which occurs in the southern United States, is the host of Black Creek Canal hantavirus (Table 1), a known cause of HPS (Rollin et al. 1995) as well as Muleshoe hantavirus (Rawlings et al. 1996), and Tamiami arena- virus (Calisher et al. 1970). The distribution of the hispid cotton rat is climate limited, and precise temperature minima for the spe- cies’ distributional limits have been described (Mohlhenrich 1961). Nevertheless, that distri- bution has undergone a northward expansion (Cameron and Spencer 1981; Schmidly 2002) in the last few decades, and a new altitudinal record was described for hispid cotton rats in New Mexico (Dunnum 2002). The white- footed deer mouse (Peromyscus leucopus), a host for the agents of Lyme disease, human granulo- cytic anaplasmosis, babesiosis, and HPS, has extended its range northward in Wisconsin (Long 1996) and Minnesota (Jannett et al. 2007). Hispid cotton rats and white-footed deer mice are common species in the United States and have been studied by generations of mammalogists. Little or no baseline data are available for the huge number of rodents in the Disease Table 1. Some of the important VBZDs mentioned in this review, with their etiological agents, vectors, and vertebrate hosts. Pathogen Vector Vertebrate host tropics, the area where hosts (especially rodents and bats), vectors, and pathogens have their greatest diversity, yet they are the least studied [for a discussion of tropical disease diversity, rodents, and bats, see Supplemental Material (doi:10.1289/ehp.0901389)]. Known and unknown tropical host and pathogen systems are likely to shift their ranges closer to sub- tropical and temperate population centers as climate changes at higher latitudes. Altitudinal temperature gradients are about 1,000 times steeper than latitudinal gradients (Colwell et al. 2008), which makes altitudinal transects more practical models for studying the effects of temperature change on plant and animal distributions. A recent range survey of small mammals in Yosemite National Park, California (USA), serves as a model (Moritz et al. 2008). Compared with similar surveys conducted approximately 100 years earlier (when minimum temperatures were 3°C lower), half of the 28 species moni- tored showed large (average = 500 m) upward changes in elevational limits (Figure 1). Low- altitude species shifted both their lower and upper range limits upslope (e.g., the piñon deer mouse, Peromyscus truei) or expanded their upper range upslope while maintaining a stable lower range limit (e.g., the California pocket mouse, Chaetodipus californicus). High-altitude species such as the alpine chip- munk (Tamias alpinus) showed overall range contractions because of upslope shifts in their lower limits. A continuation of this pattern suggests the ultimate extinction of some high- altitude species. White-footed deer mouse (Peromyscus leucopus) and eastern chipmunk (Tamias striatus)a Rodents Livestockb Rodents Rodents, lagomorphsc Birds, especially crows and jays Many wild and domestic mammals Humans Humans North American deer mouse (Peromyscus maniculatus) White-footed deer mouse (Peromyscus leucopus) Hispid cotton rat (Sigmodon hispidus) Rodents of subfamily Sigmodontinae Norway rat (Rattus norvegicus; for Seoul virus) and several other rodents of subfamily Murinae Rodents of subfamily Sigmodontinae Terrestrial rodent species Egyptian fruit bat (Rousettus aegyptiacus) and likely other bat species Fruit bats (Pteropus spp.) Vector-borne zoonotic diseases Lyme disease Tick-borne encephalitis (TBE) Rift Valley fever Plague Tularemia West Nile fever Leishmaniasis Chagas disease Vector-borne nonzoonotic diseases Malaria Dengue fever Nonvector-borne zoonotic diseases Hantavirus pulmonary syndrome (HPS) Hemorrhagic fever with renal syndrome Seoul virus (and other South American hemorrhagic fevers Monkeypox Filovirus hemorrhagic fevers Nipah virus encephalitis Nipah virus Borrelia burgdorferi (bacterium) TBE virus Rift Valley fever virus Yersinia pestis (bacterium) Francisella tularensis West Nile virus Leishmania spp. (protozoan) Trypanosoma cruzi (parasite) Dengue virus Sin Nombre virus New York virus Black Creek Canal virus Numerous viruses in South and Central America Eurasian hantaviruses) Arenaviruses Monkeypox virus Marburg virus and Ebola virus Ixodes spp. (tick) Ixodes spp. (tick) Aedes and Culex spp. (mosquito) Xenopsylla cheopis (flea) Ticks (several species)c Culex spp. (mosquito) Triatomine bugs Aedes aegypti None None None None None None None None None Phlebotomus and Lutzomyia spp. (sand fly) Many mammal species Plasmodium spp. (protozoan) Anopheles spp. (mosquito) aPrincipal hosts in eastern North America; other rodent species are hosts in western North America, Europe, and Asia. bVirus survives in eggs of infected mosquitoes; vertebrate reser- voir not required for viral maintenance. cVectors, hosts, and routes of transmission are highly variable. 1508 ovlume 118 | number 11 | November 2010 • Environmental Health Perspectives