Arizona's Maricopa County Reporting Record West Nile Virus Activity

West Nile virus cases have been relatively low overall this summer season but, following a wetter than usual season, Arizona has already seen 6 times the normal number of seasonal cases.

Although the number of West Nile virus cases in the United States so far in 2019 has been low compared with recent years, 1 Arizona county has reported a 400% spike in the number of cases this season.

This season marks 20 years since West Nile virus was first detected in the United States, and the virus has since gone on to cause outbreaks in the summer months in each of the 48 contiguous states. Although the last several West Nile Virus seasons in the US have each seen more than 2000 cases across the country, the US Centers for Disease Control and Prevention (CDC) has reported only 206 cases of the virus this season as of August 20, 2019. Of those cases, though, 126 have been reported in Arizona, with Maricopa County hit the hardest so far.

According to Arizona Department of Health Services director Cara M. Christ, MD, the state is experiencing record high West Nile virus activity. “This summer a record high West Nile virus and Saint Louis encephalitis-positive mosquitoes have been reported in the state, particularly in Maricopa County. The increase is likely due to the wetter conditions that Arizona has experienced during the last winter,” she explained in a recent blog post. The state’s unusual West Nile activity has led to 7 deaths to date. “The number of human West Nile virus cases is also a record high so far, with 126 confirmed and probable cases reported in comparison to an average of about 20 cases for this time of the year.”

California, which documented 217 West Nile virus cases in the state last year—of which 11 were fatal, is reporting well below its average number of cases, with just 45 reported cases in 11 counties as of August 23, 2019. That total recently jumped by 31 cases, mostly due to 25 new cases reported in Fresno County, where news of a West Nile-related death recently came along with a report of a positive case of Saint Louis encephalitis. Ken Bird, MD, of the county’s Department of Public Health, is reminding the public that there is plenty of time left in the West Nile virus season. “August and September are peak periods of West Nile virus transmission in Fresno County. I urge everyone to take every possible precaution to protect themselves against mosquito bites,” he said in a press release.

In additional West Nile virus news, a recent study by investigators from the University of South Florida has found that light pollution from energy-saving LED street lights may have an impact on infectious disease risk. The study, published in the Proceedings of the Royal Society B, focused on house sparrows, which can carry West Nile virus and pass it to mosquitoes, although the animals don’t often die from the infection. Investigators found that house sparrows with the virus exposed to light at night remained sick and infectious 2 days longer than a control group kept in the dark at night, increasing the potential for West Nile virus outbreak by approximately 41%.

The study authors write that artificial light at night likely affects other vectors, and called for additional research into zoonotic disease transmission in areas with light pollution.