Blood in the water : wetlands, wet spells, and drought regulate mosquito-borne disease transmission at multiple scales

Outbreaks of mosquito-borne diseases pose a significant global public health challenge and are difficult to predict over space and time. In the United States, two of the most important mosquito-borne pathogens, West Nile virus (WNV) and Eastern equine encephalitis virus (EEEV), have irregular outbreak patterns that are potentially linked to variation in climate and land cover. In particular, spatial heterogeneity in WNV and EEEV transmission may be associated with wetlands because they provide suitable habitat for mosquito vectors and avian hosts. Additionally, drought and wet spells may affect multiple aspects of these disease systems, including their arthropod vectors and pathogen hosts. However, wetlands are not homogeneous and the effects of wetlands on WNV and EEEV transmission may vary due to vegetation, connectivity, size, and inundation characteristics. Also, drought and wet spells can lead to increases in disease transmission in some contexts and decreases in others. Finally, wetland characteristics and climate conditions likely interact in complex ways resulting in both spatial and temporal heterogeneity in disease transmission. In this dissertation, I address the influence of wetlands and climate on WNV and EEEV transmission at a range of temporal and spatial scales. In Chapter 1, I examined the effects of drought conditions and several wetland characteristics on county-level human WNV incidence in the northeastern and midwestern USA. I found that drought and wetland characteristics had regionally variable effects: counties west of the Mississippi River with small wetlands and counties undergoing drought with high levels of semi-permanent wetlands had 100% and 300% higher human WNV incidence, respectively, and counties east of the Mississippi River with high levels of connected wetlands had 50% higher human WNV incidence than counties without these wetland characteristics. In Chapter 2, I investigated the influence of wetland characteristics and drought and wet spells on EEEV vector infection rates and abundance in Connecticut, USA. I found that evergreen and deciduous forested wetlands were associated with high EEEV vector abundance and that emergent and shrub wetlands were associated with low vector abundance, but that the effects of wetlands on EEEV vector infection rates were weak. Wet conditions during the transmission season and during the fall/winter preceding the transmission season were also favorable for EEEV transmission. In Chapter 3, I examined the influence of drought and wet spells on WNV vector infection rates and abundance in Chicago, Illinois; Ft. Collins, Colorado; and Coachella Valley, California. I detected significant regional differences in the influence of drought and wet spells, likely due to variation in regional aridity and WNV ecology. I also detected local-scale dissimilarities in the influence of drought and wet spells, which were likely caused by surrounding cover of wetlands, impervious surfaces, croplands and forest. These findings demonstrate that the effects of wetland and climate on WNV and EEEV transmission are context dependent, and likely mediated by regional aridity, vector natural history, and wetland characteristics. This underscores the importance of avoiding sweeping generalizations about the influence of wetlands or climate on mosquito-borne disease transmission in the United States.