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The mosquito-borne West Nile virus has infected more than 326 people and killed 15 in the U.S. this year, and the numbers are still climbing, according to the U.S. Centers for Disease Control (CDC).
Fortunately, however, outbreaks are on track to be far smaller this summer compared with last year's 2,647 cases and 167 deaths.
Arizona is host to the worst outbreak of the disease, thought to incubate in birds and transfer to humans through bites of mosquitos that have previously bitten infected birds. Maricopa County, home to state capital Phoenix, reported nearly all of the 135 Arizona cases so far this year (two cases were in a neighboring county).
California followed with 57 cases and then Nevada with 25.
Researchers first discovered the mosquito-borne West Nile virus in the U.S. in 1999, and since then, it's been a concern for county health departments and the CDC every summer.
Though West Nile is not a serious disease for most healthy adults, it can be catastrophic for infants, the elderly, and, rarely, the fetuses of pregnant women. More than 200 of the 326 cases so far in 2019 were neuroinvasive, meaning the virus crossed from the bloodstream into the brain to cause West Nile encephalitis, according to the CDC.
"Among all people who become infected with West Nile virus, most have mild symptoms that do not get reported," writes Sandra Gonzalez Gompf, MD, FACP, an internist and medical author for MedicineNet. "Typically, less than 1% will actually develop severe neuroinvasive disease."
How Do You Know if You Have West Nile?
Mild or symptom-free infections are common with the West Nile virus. Among all people who become infected, only two out of 10 develop any symptoms, Dr. Gompf says. Of those, most only have mild symptoms similar to those of the flu, such as headache, body aches, joint pain, swollen lymph nodes, vomiting, diarrhea, or rash. The symptoms are not severe enough for most people to seek medical care, but tiredness and weakness can last for several weeks.
Neuroinvasive disease is more likely to occur in those over age 50. Meningitis is marked by headache, high fever, and neck stiffness, according to Dr. Gompf. Encephalitis causes these symptoms but may progress to stupor (sleepiness), disorientation, hallucinations, paralysis, coma, tremors, convulsions, and rarely death. Sometimes general weakness progressing to complete paralysis occurs, similar to polio; this is called acute flaccid paralysis.
How Can You Avoid Getting West Nile?
According to Dr. Gompf:
First, local and state health departments may monitor the bird population for this virus; this includes surveillance of birds that are sick or have died of disease. The CDC has guidance documents for setting up these surveillance programs.
Second, the community can watch for and remove sources of stagnant water, particularly around housing, where mosquitoes tend to breed.
Rigorous surveillance and mosquito control programs help to greatly reduce the likelihood that the virus may infect people.
What if You Already Have West Nile?
Because 80% of people who get infected never have any symptoms or signs, the overall prognosis (or likelihood of full recovery) is excellent, Dr. Gompf says. Of the 20% who develop symptoms and signs, most are mild and may last for a week, but they may be left with some level of weakness, fatigue, and difficulty concentrating for weeks to months.
These residual symptoms are most likely in those over age 50. A questionnaire study of people infected during the 1999 outbreak in New York found that only 37% reported complete return to normal by one year after infection. Interestingly, the likelihood of full recovery does not differ in those who have mild symptoms and signs versus severe disease.