A common virus spread by mosquitoes in the US may be deadlier than we thought

Zika isn’t the only virus carried by mosquitoes that should be on your mind this summer.

West Nile virus has been a problem in the US for 17 years now, after spreading from the Middle East and North Africa. It’s the most commonly spread mosquito-borne infection in the US, and for 80% of people infected with West Nile, there are no symptoms.

But the virus, which scientists estimate has infected millions of Americans, is not harmless. In about 1% of cases, West Nile virus can lead to serious inflammation in the brain and spinal cord. And a new study published Wednesday in the journal Emerging Infectious Diseases suggests that more cases of this potentially deadly inflammation are triggered by West Nile than we thought.

The researchers from the University of Texas Health Science Center and Baylor College of Medicine looked at nine hospitals in Houston, an area that’s hit particularly hard by West Nile. From there, they pinpointed 751 people who had meningitis or encephalitis, two serious conditions characterised by inflammation in the brain or spinal cord that’s caused by an infection.

They found that of that 751, only about a third had been tested for West Nile virus. Thirty-two (11%) of the patients tested had the virus in their system.

That led to the conclusion that the West Nile virus was significantly underestimated as a cause of meningitis and encephalitis. The researchers attributed that to a lack of testing that happened on the rest of the cases, where West Nile might have been missed as a cause, since it was never tested for.

In 2014, an analysis estimated that the West Nile virus epidemic had cost the US about $800 million over the first 14 years. If cases of West Nile causing these brain inflammatory conditions are underestimated, that cost may be even higher.

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