Arizona is responsible for nearly half of the country’s deaths from West Nile virus in 2019, and public health officials are trying to figure out why.

Of the 35 people who have died from West Nile in the U.S. so far in 2019, 16 of them have been Arizonans, according to recent data from the Centers for Disease Control and Prevention.  

That number is up from only six West Nile-related deaths in Arizona in all of 2018, according to the state health department

Arizona has also had the most cases of West Nile (162) of any state in the nation this year, outpacing even California, the largest state in the nation. 

Jessica Rigler, an assistant director at the Arizona Department of Health Services, told KJZZ that the real number of cases was likely even higher, since most people who contract the virus don’t show any symptoms. “Most people who get infected don’t even know that they have the disease,” she said.

Fifteen of the 16 deaths occurred in Maricopa County and the county is also responsible for 153 of the state’s total cases, a startling 750% increase over the same time in 2018, according to county data.

“For whatever reason, Maricopa County seems to be the West Nile virus hot spot this season, and we honestly don’t know why,” Dr. Rebecca Sunenshine, the Maricopa County Public Health Department’s medical director for disease control, told the Arizona Republic.

“It is by far the most we’ve seen in the last decade,” Sunenshine said. “We’re so dry and you don’t think of us having a heavy mosquito population. … Something is going on, and we’re working with the CDC to figure it out.”

The virus is mostly spread by mosquitoes, who become infected after feeding on infected birds. These mosquitoes can then spread the virus to humans and other animals when they bite them, according to the CDC. 

Arizona officials have yet to pinpoint a specific cause for the dramatic rise in cases and deaths, but they are studying bird and mosquito populations, as well as rainfall and disease patterns and local ecology, to determine what’s behind the increase.

West Nile season traditionally peaks in the summer, but Sunenshine told the Republic that risk still remains. “There is still definitely risk of West Nile virus out there. … We always worry that there will be a second peak (in infections) and that’s what we are on the lookout for,” she said. “The mosquitoes that carry West Nile are still breeding and circulating.”

Seniors are the most at risk population — the average age of those who died from West Nile in Maricopa County in the past five years was 74 — and those who have weak immune systems or pre-existing medical conditions such as cancer or diabetes are also particularly vulnerable. But Sunenshine told the Republic that “Everybody is at risk … We have definitely had individuals die from West Nile who are in their 30s and 40s.”

While most people who contract West Nile do not experience symptoms, roughly one in five do, according to the CDC. These symptoms include fever, headaches, body aches, and diarrhea. 

There is no vaccine or antiviral treatment to prevent the virus, but over-the-counter pain relievers can be used to reduce fever and alleviate symptoms. Most people who’ve contracted the disease make full recoveries, though fatigue and weakness can linger for weeks or months. 

About one in 150 people who are infected develop a serious illness that affects the central nervous system, such as meningitis or encephalitis, which often requires hospitalization. 

To learn more about the disease, visit the CDC’s website and the Arizona Department of Health Services’ website.