(The Center Square) - Arizona leads the country in the number of West Nile virus cases.
The state has 62 cases as of July 31, said Irene Ruberto, mycotic diseases program manager for the Arizona Department of Health Services.
Ruberto told The Center Square that Arizona's current case count is two and a half times higher than what the state has “seen in the past five years for the same timeframe.”
It is difficult to pinpoint what is causing the increase in West Nile virus cases, Ruberto said.
Centers for Disease Control data shows the next closest state is Texas with 12.
Arizona experienced a total of 66 West Nile virus cases in 2025.
Ths year, the vast majority of West Nile virus cases are coming from Maricopa County, which has 52 confirmed cases. This represents 84% of the confirmed cases. Maricopa County is home to Phoenix and nearly two-thirds of Arizona’s total population.
So far in 2026, five people have died from the virus in Maricopa County. Last year, the number was seven. The total number of deaths was nine last year in Arizona, but a total number was not available so far this year in the state.
The fatality rate for West Nile virus is generally around 10%, Ruberto told The Center Square. She added that deaths typically occur in people 65 and older..
Compared to the same time last year, Maricopa County had 32 cases, according to the county's data. Maricopa County ended up having 56 cases in 2025, said Melissa Kretschmer, zoologist for the Maricopa County Department of Health.
Kretschmer told The Center Square that Maricopa County may have more cases this year, but the number depends on how the monsoon season in Arizona plays out. The season lasts from June 15 to Sept. 30, according to the state Department of Water Resources. Heavy rainfall creates standing water, which provides sites for mosquitoes to lay eggs and hence increase their population. Mosquitoes carry the virus.
Ruberto noted the virus is “naturally present in the environment.”
She said the virus came to Arizona in 2003, coming from birds migrating from the East Coast.
“Birds will get infected, then the mosquitoes feed off of them and then spread it onwards to other birds and people,” Kretschmer said.
According to Ruberto, symptoms for the virus include a headache, fever, swollen lymph nodes, body and muscle aches, as well as a rash.
Kretschmer said only 20% of people infected with the virus will experience these symptoms.
Only 1% of people who get the virus end up getting a neuroinvasive disease such as meningitis or encephalitis, Ruberto explained.
Meningitis occurs when protective membranes around a person’s brain and spinal cord swell, according to Cleveland Clinic. Encephalitis happens when a person has swelling around their brain tissue, the American Brain Foundation noted.
Kretschmer said about one in 150 people develop severe symptoms from the virus.
The disease can be severe in people who are immunocompromised, Ruberto noted.
From April to November, Arizonans should be aware of mosquitoes, Ruberto noted.
Rain and cool weather tend to attract more mosquitoes to an area, Kretschmer said.
Ruberto suggested Arizonans use insect repellant and place screens on their windows to fight off mosquitoes.
She also recommended Arizonans keep mosquitoes away by not have any standing water around their homes.
The Arizona Department of Health and Human Services conducts surveillance on the virus, which means the department works with “county health departments and local vector control agencies to investigate each case,” Ruberto explained.
County health departments with more resources may trap additional mosquitoes and examine surveillance data on them, Ruberto said
Maricopa County's environmental services vector control division does mosquito surveillance, Kretschmer noted.
The county’s Department of Public Health focuses on getting the message out about the virus to health care providers and the general public, she added.
For more information on the West Nile virus, visit the Arizona Department of Health Services’ website.
