This summer has brought a flurry of warnings about cases of mosquito-borne illnesses, including malaria, dengue and Eastern equine encephalitis.
But West Nile virus, transmitted primarily through the bite of an infected mosquito, continues to be the leading cause of viral disease spread by insects, known as arboviral disease, in the continental United States, according to a report published Thursday by the US Centers for Disease Control and Prevention.
The report summarizes how many infections of West Nile virus and other arboviral diseases occurred across the United States in 2021.
Forty-nine states and the District of Columbia reported more than 3,000 cases of arboviral disease that occurred domestically – and 2,911 of those cases were West Nile virus. A total of 2,099 people with West Nile Virus were hospitalized, and 227 died.
Nationally, the incidence rate of neuroinvasive West Nile virus disease, meaning the infection affected the nervous system, was the highest since 2012 at 0.6 cases per 100,000 people, largely driven by an outbreak in Arizona.
In 2021, three Arizona counties — Maricopa, Pima and Pinal — reported more than half of all identified neuroinvasive cases in the country. Jurisdictions with the next highest numbers of West Nile Virus cases were Texas with 130, Colorado with 101, California with 96 and Nebraska with 69.
West Nile spreads as mosquitoes bite infected birds and then people, making both bird and mosquito populations important parts of public health surveillance for the disease. Mosquitoes are the world’s deadliest animal because they spread diseases such as malaria, dengue and West Nile, killing more people than any other creature.
Although West Nile Virus was the most common cause of neuroinvasive arboviral disease in the United States in 2021, as in previous years, another disease – caused by the La Crosse virus – continued to be the most common cause of neuroinvasive arboviral disease in children, according to the new CDC report.
In 2021, 40 cases of La Crosse virus disease were reported from eight jurisdictions – Illinois, Indiana, Michigan, North Carolina, Ohio, Tennessee, West Virginia and Wisconsin – and 35 of those cases were in children. All 40 people were hospitalized, but none died from the disease.
“Although case numbers vary by year, virus, and geographic area, arboviruses continue to cause substantial morbidity in the United States. Weather, zoonotic host and vector abundance, and human behavior are all factors that can influence when and where outbreaks occur,” the CDC researchers wrote in the new report. “This complexity makes it difficult to predict future locations and timing of cases and underscores the importance of surveillance to identify outbreaks quickly to direct public health prevention efforts.”
As of Thursday, a few West Nile virus deaths have been reported across the US, including in Illinois, Nebraska and Colorado. A total of 247 West Nile cases have been reported to the CDC this year, and that number is growing.
A historically wet winter and, now, hotter summer are leading to “pretty big” warning signs for West Nile virus in the Western US, according to public health and mosquito control experts, suggesting that residents should take care to avoid bites.
People can reduce their risk of West Nile virus and other mosquito-borne diseases by using insect repellent and wearing long-sleeved shirts and long pants to ward off mosquito bites.
“The number of mosquitoes that have hatched off after the spring snowmelt is pretty tremendous in many of the states, whether it be Colorado or Utah or California,” Daniel Markowski, technical adviser for the American Mosquito Control Association, said earlier this month.
Many districts are starting to see West Nile virus in mosquitoes, he said, meaning “you’re at the right temperature, the right mosquito population and the right time of year for localized outbreaks to occur.”
About 1 in 5 people with West Nile virus may develop a fever with other symptoms, but most people do not have symptoms. About 1 in 150 people who are infected develop a severe or sometimes fatal illness affecting the central nervous system such as encephalitis, causing inflammation of the brain, or meningitis, causing inflammation of the membranes that surround the brain and spinal cord.
CNN’s Meg Tirrell contributed to this report.
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