Health officials closely monitoring tests for St. Louis encephalitis

In the fight against mosquito-transmitted diseases, the Southern Nevada Health District is Clark County’s lookout, ready to sound the alarm in case of danger.

The district has boosted its mosquito surveillance infrastructure this year in light of concerns over the Zika virus, and is actively searching for the two types of mosquitoes that commonly transmit Zika while remaining on the lookout for West Nile virus.

This season, the district is also keeping its eyes on the resurgence of St. Louis encephalitis — another virus that typically causes no or mild symptoms but can lead to severe neuroinvasive disease, especially in the elderly.

“We have not seen very much West Nile virus, which I think is much different than what we’ve seen in years past, so that’s an interesting ecological shift,” said Vivek Raman, an environmental health supervisor with the district.

St. Louis encephalitis, which is transmitted to humans by mosquitoes that bite infected birds in a similar fashion to West Nile, has been found in up to 18.2 percent of the 29,995 mosquitoes the health district has submitted for virus testing so far.

Because mosquitoes aren’t tested individually, an exact number of mosquitoes that tested positive is not available.

No human cases of St. Louis encephalitis have been reported in Clark County since 2007.

However, the number of mosquitoes possibly carrying St. Louis encephalitis in Clark County is significantly higher than at any point since the creation of the health district’s mosquito surveillance program in 2004.

In 2015, two groups totaling 100 mosquitoes tested positive for St. Louis encephalitis, compared with 174 mosquito groups totaling 5,458 mosquitoes so far this year.

It’s unclear why.

James Wilson, director of the Nevada State Infectious Disease Forecast Station at the University of Nevada, Reno, described the Southern Nevada numbers as interesting but not extremely troubling.

He said it’s not uncommon for St. Louis encephalitis to pop up in certain spots and linger after being away for a decade.

“Keep a balanced perspective,” he said. “Yes, the mosquitoes lit up, but how many cases have we had?”

Ramen said the health district is in the planning stages of possibly sending mosquito samples for testing to the Centers for Disease Control and Prevention as well as to the Davis Arbovirus Research and Training at the University of California, Davis.

St. Louis encephalitis has also been reported in the mosquito population of White Pine County in Northern Nevada this year.

Ramen pointed out that the Southern Nevada Health District has also submitted more than twice the number of mosquitoes for virus testing compared with last season.

Overall this year, the district has submitted 29,995 mosquitoes, compared with 13,244 at this time last year.

In June, the health district submitted 17,500 mosquitoes. Last June, about 5,500 mosquitoes were sent for testing.

Contact Pashtana Usufzy at pusufzy@reviewjournal.com or 702-380-4563. Follow@pashtana_u on Twitter.