+Environmental Factors and Risk Areas of West Nile Virusin 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 2007.

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 2006. The infection generally ended in late November or December.

2008 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. 2012. Environmental Factors and Risk Areas of West Nile Virus in Southern California, 2007-2009. Environmental Modeling and Assessmant 17:441-452.