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13 states report cases of West Nile virus as mosquito season begins

The mosquito-borne illness has no treatment or vaccine, but only about 20% of those infected with the disease develop symptoms.
Image: Aedes aegypti mosquito
This 2006 photo provided by the Centers for Disease Control and Prevention shows a female Aedes aegypti mosquito in the process of acquiring a blood meal from a human host. On Friday, Feb. 26, 2015, the U.S. government said Zika infections have been confirmed in nine pregnant women in the United States. All got the virus overseas. Three babies have been born, one with a brain defect. (James Gathany/Centers for Disease Control and Prevention via AP)AP
/ Source: TODAY

More than a dozen states are reporting West Nile virus activity and infections. The virus is primarily transmitted through mosquito bites and cannot be spread by person-to-person contact.

According to the Centers for Disease Control and Prevention (CDC), eight cases of the virus in people across five states have been reported to the organization, with no deaths reported as of June 30. Eight other states have reported non-human cases in mosquitos and birds.

Individual state health departments are reporting higher numbers. In Florida's Miami-Dade County, 10 human cases were reported as of June 25. According to NBC News affiliate, one case has been reported in California, which is not reflected in the CDC data.

What is the West Nile virus?

West Nile virus is a disease primarily transmitted to people by mosquitos during mosquito season, which starts in the summer and continues through the fall. West Nile virus is the leading case of mosquito-borne disease in the continental United States, according to the CDC, and while there are no preventative vaccines or treatments available for the disease, about 80% of those infected do not experience symptoms.

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security in Baltimore, Maryland and an infectious disease physician, said that West Nile virus cases are typically under-diagnosed, since so few people who are infected with the disease show symptoms. In 2019, 958 cases of West Nile virus in humans were reported to the CDC.

"Many people get infected and you have no symptoms at all; other people get get infected and they have flu-like symptoms," said Adalja. "You only see more neuro-invasive (cases) getting diagnosed."

What are symptoms of the West Nile virus?

Only 1 in 5 people who are infected develop a fever and other symptoms, with just 1 in 150 people experiencing a "serious, sometimes fatal illness." Severe illnesses associated with West Nile virus include encephalitis (inflammation of the brain) and meningitis. According to the CDC, people over the age of 60 are at greater risk for developing a serious illness. Others with medical conditions, including cancer, diabetes, hypertension, kidney disease, and organ transplant patients may be at greater risk.

Woman using anti mosquito spray outdoors at hiking trip.
Mosquito repellant is one key tool people can use to avoid getting bitten and contracting the virus. Getty Images

How to reduce the risk of mosquito-born diseases:

The best way to prevent the disease, according to Adalja, is for health departments to proactively try to mitigate mosquitos.

"They have to go after the breeding sites, they have to have a lot of public awareness ... It's an aggressive community mitigation effort when trying to control West Nile," he said. "People don't really aggressively support it until there is an outbreak, so it's often very reactive."

To prevent mosquito bites yourself, wear long-sleeve clothing if possible and mosquito repellant when spending time outdoors. To keep mosquitos out of your yard, the CDC recommends emptying items that hold water, like toy buckets, planters, birdbaths, flowerpots or trash containers. This can help to stop mosquitos from laying eggs in water.

Adalja said that COVID-19 guidance, which encourages spending time outdoors instead of inside, may lead to higher cases than previous years.

"Obviously, most mosquito bites occur outdoors, and we are encouraging people to spend as much time outdoors as possible, because of COVID, and that may be something that may increase someone's chance of getting bitten by a mosquito," he said. "(People) need to take care when they are outdoors, and remember that there are other threats as well."