California drought leads to 10-year peak in West Nile Virus

10 Apr 2015


The incidence of West Nile infection in California in 2014 was the highest since 2005. The number of fatalities was also higher than in any previous year, the state Department of Public Health announced on Wednesday.

The state confirmed 801 human cases of the potentially fatal disease last year, second only to the 880 cases in 2005, with 31 of those cases resulting in deaths. Further 561 produced West Nile neuro-invasive disease, the more serious neurological form of West Nile that often resulted in encephalitis or meningitis.

But there were other records California set last year.

The proportion of mosquitoes infected with West Nile Virus stood at the highest level ever detected in California, as also the prevalence of West Nile infection in tested dead birds.

It was quite possible that the ongoing drought had contributed to the increases, due to limited water sources for birds and mosquitoes, according to Dr Karen Smith, director of the California Department of Public Health.

According to Smith as birds and mosquitoes sought water, they came into close contact amplifying the virus, particularly in urban areas. She added, the lack of water could have caused some sources of water to stagnate, making the water sources more attractive for mosquitoes to lay eggs.

With the weather warming, mosquitoes became more abundant and unseasonably warm weather this year could lead to increased mosquito abundance and promote an early start to the WNV disease season.

The WNV season typically began in the summer and tapered off in towards the end of the year, with the highest risk for disease occurring in mid-July through September.

The transmission of West Nile Virus to humans and animals happened through the bite of an infected mosquito. For most people, the risk of serious illness was low.

However, some individuals – less than 1 per cent - could develop a serious neurologic illness, such as encephalitis or meningitis.

People 50 years of age and older and people with diabetes and / or high blood pressure had the greatest risk of developing serious complications.

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