Noting that the United States is experiencing its earliest-ever start to the West Nile virus (WNV) season, the Centers for Disease Control and Prevention (CDC) is urging Americans to take precautions against mosquito bites over the Fourth of July holiday weekend.

Along with the early start to the season, the country has seen the most cases of West Nile disease in people by this time of year since 2004. Of the 48 infections reported by 12 states as of June 30, 38 (79%) have caused severe neuroinvasive illness. This is a marked rise from the average 10 cases usually reported to the CDC by June 30.

“This surge is driven by early circulation of the virus, with 23 states reporting West Nile virus activity—also the highest number recorded over the last 10 years,” the statement said.

WNV is the country’s leading cause of mosquito-borne illness, accounting for an average of several thousand infections and about 100 deaths each year. Most infected people have no symptoms, but about 20% develop a fever and symptoms such as headache, body aches, joint pain, vomiting, diarrhea, or a rash. 

Less than 1% of infected people experiences severe neurologic disease, including meningitis or encephalitis, which can lead to long-term disability or death. Risk factors for severe disease are older age (60 years and older) and having certain underlying medical conditions.

So far this year, Arizona has reported the most human cases (32), while Texas has logged four and Tennessee two. Arkansas, California, Colorado, Florida, Hawaii, Nebraska, Oklahoma, Pennsylvania, and South Dakota, have each documented one, according to the CDC’s ArboNET surveillance system.