West Nile virus more prevalent among mosquitoes in vineyards, orchards

By Annette Cary, Tri-City HeraldJanuary 31, 2013 

West Nile virus is more abundant among mosquitoes in vineyards and orchards in the Northwest than in other types of agricultural or natural areas here, according to new research.

The study was conducted at Washington State University in Pullman with the Washington State Department of Health and relied on information from mosquito control districts in the Mid-Columbia.

The findings could help focus how limited money is used to monitor for West Nile virus, prevent the disease and educate the public, said David Crowder, a WSU entomologist and the paper's lead author.

In 2012, two West Nile infections in people were reported that were believed to have been caused by exposure in Washington -- one in Benton County and the other in Yakima County. But many other cases likely were misdiagnosed or undiagnosed.

About one in five infected people develop symptoms such as fever, headache and sometimes a skin rash, and one in 150 infected people have more severe symptoms that can include disorientation and neurological problems.

Researchers knew from previous studies that there is a link between agricultural areas and the mosquito-born virus in the West.

But little work had been done to look at what specific western climate and agriculture habitats are linked to the virus, which is spread to birds, animals and people by infected mosquitoes.

The WSU research started with a broad look by county in Washington, Oregon and Idaho at West Nile data.

It found that the reported incidence of West Nile was reduced in counties with more precipitation, Crowder said. Scientists already knew that dry conditions may promote mosquito populations, possibly because of fewer predators, including dragonflies and spiders, and the difficulty delicate mosquitoes have flying in the rain.

It also looked at the three types of landscapes -- orchards and vineyards; farmland such as wheat, potatoes and alfalfa fields and pasture land; and natural landscapes, such as forests or shrub-steppe.

The research found no notable link between West Nile and agriculture fields or natural land, but it did find an increase in West Nile cases in counties with more orchards and vineyards.

Then researchers narrowed their look to eight Washington counties with good mosquito trapping data, and found the same results on a smaller scale, Crowder said. Counties included Benton, Franklin, Columbia, Yakima and Grant.

Orchards and vineyards were linked not only to more West Nile virus, but also had more mosquitoes and more robins, a key bird species that has been associated with West Nile infections, Crowder said.

However, researchers could only speculate about what aspects of orchards and vineyards made West Nile more likely there, he said.

They do provide habitat for birds and mosquitoes, which are focal points for the disease. For birds, orchards and vineyards provide nesting sites and fruit for food, and adult mosquitoes feed on flower nectar, he said.

The study results were published in the journal PLOS ONE.

-- Annette Cary: 582-1533; acary@tricityherald.com

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