Virus Often Mistaken for the Flu, Doctors Say

West Nile virus is appearing at unusually high levels and can resemble influenza when symptoms develop. So far this year, 45 states have identified at least one case. CDC data list 1,328 infections, including 1,004 neuroinvasive cases—the serious form that affects the central nervous system. Two deaths were recently announced by North Carolina health officials.

Thomas Russo, M.D., of the University at Buffalo in New York, says infections are substantially undercounted. Up to 80% of infected people experience no symptoms, while about 20% develop an illness that can feel like the flu. Possible signs include fever, headache and body aches, sometimes accompanied by rash, nausea or diarrhea. Weakness and fatigue may continue for weeks or even months.

The symptom pattern can help distinguish the illnesses. Amesh A. Adalja, M.D., says West Nile becomes more plausible when respiratory signs such as coughing, congestion and a sore throat are absent. Remembering a mosquito bite is less useful because many people do not notice one. William Schaffner, M.D., recommends ruling out flu and COVID-19 before discussing West Nile testing with a provider. A test exists, although Russo says access and turnaround times vary.

Cases ordinarily occur from June through October and tend to crest from late August into early September. By July 1, however, the country had recorded its largest case total for that point in the year since 2004. California, Arizona and Texas have had the greatest numbers, with notable activity also occurring in Colorado, North Dakota, South Dakota, Nebraska and Utah. There is no treatment that targets the virus; medical care focuses on easing symptoms.