Number of mosquitoes carrying encephalitis jumps in Las Vegas

A significant jump in the number of mosquito groups testing positive for St. Louis encephalitis in Clark County is a concerning pattern that should prompt further investigation, according to one infectious disease expert.

The Southern Nevada Health District’s Vector Surveillance Program reported 124 additional clusters of mosquitoes have tested positive for the virus since the district announced ealier this month that it found mosquitoes carrying St. Louis encephalitis.

While no human cases have been reported in Clark County since 2007, the

increased presence of the virus in collected mosquito populations “raises concern for the potential of disease transmission to humans,” according to the health district.

The number of mosquito groups testing positive this year is higher than at any point since the surveillance program was created in 2004.

James Wilson, director of the Nevada State Infectious Disease Forecast Station at the University of Nevada, Reno, said that the numbers present “a concerning signature pattern” and that physicians who see patients suffering from headaches and fevers in Southern Nevada should consider St. Louis encephalitis as a potential cause.

“This is an impressive degree of positivity in mosquito pools,” he said.

He also said that the increase is unusual, as the virus has shown up only sporadically in Nevada mosquitoes in the past, but he added that the data released so far allow for only a limited assessment.

“It is strange, and I’m curious as to what’s happening,” he said.

Vivek Raman, an environmental health supervisor with the district, said the testing process starts when a selection of mosquitoes from one area is gathered. The mosquitoes aren’t tested individually but are instead crushed and tested as a group.

A single mosquito that carries St. Louis encephalitis can cause an entire group to test positive.

During all of 2015, only two groups jointly totaling 100 mosquitoes tested positive.

Since May 23 this year, 130 mosquito pools have tested positive in Clark County. The size of each group was not available, but combined, the number of mosquitoes potentially affected totaled 4,468.

Claudia Muñoz, assistant professor of neurology at the University of California, Riverside, said it’s rare that St. Louis encephalitis infections lead to noticeable symptoms.

Symptoms of a mild infection include fever and headaches, according to the health district.

“It can go away, or the virus can enter the central nervous system and cause encephalitis,” Muñoz said.

Encephalitis is an inflammation of the brain.

A more severe infection can include symptoms such as dizziness, nausea, stiff neck, tremors, confusion, coma and in rare cases long-term disability or death, the health district said.

Dr. Joseph Iser, chief health officer for the health district, said it’s possible that there has been an increase in infected birds settling in Southern Nevada’s waterways, where mosquitoes would have bitten them.

The virus is spread when mosquitoes bite infected birds and then bite other birds and give them the virus. Mosquitoes can also spread the virus to humans.

Iser said that though the increase in pools testing positive is a concern, it is not a major concern, as no human cases have been reported.

Wilson said peak mosquito-to-human transmission of the virus in Nevada occurs for just a couple of weeks in August.

Iser said the positive mosquito presence will be addressed with abatement tactics. The health district and Clark County will work together to kill mosquito populations that test positive.

Raman said that the killing is targeted and that other measures include larval mosquito control and environmental management procedures such as draining areas with stagnant water.

Iser said the health district has asked for aerial photographs and the ability to access GIS data to assess where else mosquitoes might be breeding.

“This increase in St. Louis encephalitis-positive mosquitoes serves an important reminder to our community that we do indeed have mosquitoes in Southern Nevada, and it is important for people to take steps to eliminate breeding sources and to protect themselves from mosquito bites,” he said in a news release.

Meanwhile, no mosquitoes in Clark County have tested positive for West Nile virus this season, and the type of mosquitoes that commonly spread Zika virus has not been detected.

The health district and the Centers for Disease Control and Prevention advise Las Vegas residents to wear long-sleeved shirts and pants, to stay in places with air conditioning and window and door screens and to use approved insect repellents.

For information on eliminating breeding sources, visit the CDC’s Controlling Mosquitoes at Home page.

Contact Pashtana Usufzy at pusufzy@reviewjournal.com or 702-380-4563. Find @pashtana_u on Twitter. Contact Lawren Linehan at llinehan@reviewjournal.com or at 702-383-0381. Find @lawrenlinehan on Twitter.