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Why is West Nile cyclical?

Modified: August 10, 2012 at 8:40 pm •  Published: August 10, 2012
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Why West Nile is cyclical

West Nile virus spikes every three years to four years. The rate of West Nile depends on a variety of things, including spring rainfall, hot and dry summers and also what the level of immunity of the virus is in the wild bird population.

If there is a large number of birds that have not encountered West Nile virus, when they are bitten, the virus multiples and amplifies in their bodies and tissues. This means that when another mosquito bites an infected bird, the mosquito will then become a carrier of West Nile virus.

“And as more mosquitoes pick up the virus from birds, that cycle repeats itself and amplifies in nature,” said Kristy Bradley, the state epidemiologist at the state Health Department.

This process creates a higher rate of mosquitoes that might decide to bite people or horses.

After 2007, a substantial proportion of the wild bird population that encountered West Nile virus survived and developed antibodies. The next year, when birds were bitten by infected mosquitoes, the antibodies in the birds' bodies suppressed the production of the virus, Bradley said.

But small- to medium-size wild birds have a life span of about two to four years. This means that once new birds hatch, they must again build up immunity to West Nile, Bradley said.


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