The Southern Nevada Health District is reporting its first human case of St. Louis Encephalitis in Clark County since 2007. The individual is male over the age of 50 who had the more serious neuroinvasive form of the illness. He was hospitalized and has been released.


Most people who are infected with St. Louis Encephalitis virus have mild symptoms or might never become sick. Symptoms typically develop between five and 15 days after being bitten by an infected mosquito. Mild infections are characterized by fever and headache without other apparent symptoms. Some people will experience severe illness, especially older adults. Symptoms of the more serious form of the illness include headache, high fever, neck stiffness, confusion, shaking, seizures and/or paralysis, and even coma.

The illness is diagnosed through a blood test and laboratory confirmation. There is no specific treatment for St. Louis Encephalitis virus disease, and mild cases resolve on their own. People with the more serious form of the illness are often hospitalized.

“While most people who get infected with St. Louis Encephalitis virus may not know it, or may only get mild symptoms, some people may develop the more serious forms of the disease. Children and especially the elderly are more at risk for complications,” said Dr. Joe Iser, Southern Nevada Health District Chief Health Officer. “With the increased number of St. Louis Encephalitis-positive mosquitoes in Southern Nevada this season, we are not surprised to receive confirmation of a human case of the illness. It is another reminder to the community to protect themselves from bites and check their homes and yards for mosquito breeding sources.”

The Southern Nevada Health District’s Vector Surveillance Program identified a sharp increase in St. Louis Encephalitis-positive mosquitoes in more than 30 ZIP codes in Clark County this year.