Sunday, October 26, 2014

California Drought Increases Incidence of West Nile

At the close of the "water year" on September 30th,  California's Department of Water Resources officially announced that this past year was California's third most dry of all 119 years on record.  The drought in California has been acutely felt by many counties across the state.  Some homes, such as those in Tulare County, have lost running water entirely.  The loss of running water has disrupted the daily lives of these Californians and even with water-saving measures being taken by many across the state, recovery seems a distant possibility.

The impacts of the drought are far-reaching.  It has increased the number of cases of West Nile virus in the state this year indirectly through its effects on the vector---mosquitoes---and reservoir---birds---of the virus.  Insects and birds breed and feed (respectively) in pools of water.  As a result, the exchange of virus among its reservoir and host often occurs at these pools of water.  Because of the drought, these water sources are small and more scarce.  Thus, the birds and mosquitos are concentrating their interactions, increasing the transmission of the West Nile virus they harbor.  Additionally, the pools of water that persist are more stagnant than usual due to the drought and they are therefore more conducive to mosquito breeding, creating a larger population of the vector and thereby increasing transmission.  When the mosquitoes that transmit the virus among birds bite humans, humans can become infected with West Nile.  Thus, increasing rates of transmission among birds and mosquitos precipitate increases in the number of human cases of the virus.  311 human cases of West Nile had been confirmed in the state by September 22nd according to NPR, which was twice the number of cases reported in California last year and made California the most West-Nile-riddled state in the US.  As of the California Department of Public Health's weekly report published Otcober 24th, the total number of human cases has risen to 608.  There were 46 new human cases reported in the preceding week alone and at least 2376 birds have been killed by the virus so far this year.

Fortunately, human mortality rates associated with West Nile virus are relatively low.  Most human cases show no symptoms at all, while only %1.5 develop severe disease.  There have been 20 human deaths so far in California.

The impacts of the California drought on the spread of infectious diseases like West Nile amplify the import of efforts to ameliorate the critical condition of the state's climate.

--Laurie Rumker

More on the drought at: AND
More about West Nile in California at: AND

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