+ Environmental Factors and Risk Areas of West Nile Virus in Southern California, 2007–2009 Hua Liu & Qihao Weng Ivonna Reda

+ Background WNV – West Nile Virus West Nile Virus is a mosquito-borne virus, which normally transmits from bird to mosquito to bird Humans can become infected when bitten by mosquitoes that carry virus Most recently, the epidemic was widespread in Southern California, USA, setting for this study

+ WNV transmission The transmission of WNV is dependent on the presence of the bird host and the mosquito vector Environmental conditions and social–economic factors, or urbanization, may impact the presence of these species Mosquitoes reproduce and survive in hot, humid environments (urban heat island) Birds are commonly found in highly developed, urbanized areas

+ What they want to test The idea was to implement a disease surveillance method of WNV They used knowledge of environmental conditions and landscapes of ten counties to make a map, separating them instead into ecological zones based on environmental similarities to better understand why WNV spreads where it does Using a clustering method

+ Methods Cluster analysis is the grouping of space into a specific ecological zones. Since the ten counties contains diverse landscape and environmental conditions, it is necessary to generate homogeneous ecological zones for those counties Factors include surface temperature, elevation, average precipitation, landscape patterns (forests, open) Derived from remote sensing images, historical weather observation, and real-time remotely sensed land surface temperature and vegetation water content on WNV invasion

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+ Methods Data collected from: Centers for Disease Control and Prevention California Vector-borne Disease Surveillance System Geographic Information Systems (GIS)

+ Areas of study The following ten counties in California constitute the study area: San Luis Obispo, Kern, San Bernardino, Santa Barbara, Ventura, Los Angeles, Orange, Riverside, San Diego, and Imperial. Temperature difference between coastal counties and inland counties is about 4°F in winter and approximately 23° F in summer

+ Timeframe Studies took place: May - June (weeks 18–26) July - August (weeks 27–35) September - October (weeks 36–44) in each year of 2007–2009 Consists of late spring/early summer, summer, and late summer/early fall.

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+ Results South San Joaquin Valley in Kern County and south Los Angeles County were the most vulnerable locations for WNV outbreak. Main factors contributing to the WNV propagation included summer mean temperature, annual mean deviation from the mean temperature, land surface temperature, elevation, landscape complexity, landscape diversity

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+ Droughts The moderate drought conditions in 2006 and warmer than normal temperature contributed to the amplification of virus across southern California, for ex. Kern and Los Angeles in The severe to extreme drought conditions in 2007 and warm spring may contribute to the outbreak of WNV in summer 2008

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+ Results 2006 was not included in the study, but it is necessary to show how the drought may affect the outbreak of mosquito population in the following year Mosquito infection usually started to expand in May, reaching a peak in early or middle August (summer) except in 2004 and The infection generally ended in late November or December received the greatest total number of positive mosquito pools, most likely because of the drought of 2007 while year 2006 received the fewest mosquito infections. Temperature must be above the minimum temperature (58°C) required for virus replication in mosquitoes for WNV to be disseminated throughout the year, especially in the cold months, and this was the case in Kern and Los Angeles Lower elevations means higher temperatures, in which mosquitoes thrive Higher landscape diversity is usually associated with multiple land cover types such as urban, grass, and water, which attracts birds

+ Sources Liu, H., Qh Weng Environmental Factors and Risk Areas of West Nile Virus in Southern California, Environmental Modeling and Assessmant 17: