Mosquitoes carrying encephalitis strain found in 4 Clark County ZIP codes

Groups of mosquitoes across Clark County have tested positive for St. Louis encephalitis, a virus that largely causes limited or no symptoms in humans but can occasionally lead to serious health issues, according to the Southern Nevada Health District.

Mosquito pools in the 89104, 89120, 89146 and 89191 ZIP codes have tested positive for the virus, the district said in a news release.

No human cases of the rare disease have been reported in Clark County since 2007.

“We’re not very worried about it, but we wanted to let people know,” said Dr. Joseph Iser, the health district’s chief health officer.

Mosquitoes found in the areas where positive results were recorded will be killed as a precaution, he said.

Though most infected individuals experience mild fever and headache or don’t get sick at all, St. Louis encephalitis can also cause nausea, dizziness and vomiting, according to the health district.

The illness can lead to severe neuroinvasive disease, which includes encephalitis, or brain inflammation, according to the Centers for Disease Control and Prevention.

“Elderly persons are at increased risk of severe disease if they are infected,” the CDC said.

Long-term disability or death are rare, and there is no targeted treatment for St. Louis encephalitis, which is spread to humans by infected mosquitoes.

Clark County mosquitoes have rarely tested positive for St. Louis encephalitis in the past, according to the district’s 2014 mosquito surveillance report.

Between the creation of the health district’s vector surveillance program in 2004 and summer 2013, one infected mosquito group was reported in 2005.

Two pools of mosquitoes tested positive in 2015, health district spokeswoman Stephanie Bethel said.

Iser said the increase in positive mosquito pools indicates more birds carrying the virus are visiting the valley. Mosquitoes become infected by biting afflicted birds.

James Wilson, director of the Nevada State Infectious Disease Forecast Station at the University of Nevada, Reno, said peak transmission of St. Louis encephalitis from mosquitoes to humans in Nevada occurs in August.

He said it’s unlikely Clark County would see even one case this season, adding that the presence of positive mosquitoes is common around the country this time of year.

“It’s an early warning indicator telling people to watch out around areas where there are mosquitoes,” he said.

The Southern Nevada Health District has already ramped up mosquito surveillance in response to concerns over Zika virus, a typically mild illness for adults but blamed for birth defects in the babies of some infected women.

The type of mosquito largely responsible for Zika’s spread has not been found in Southern Nevada, but five travel-associated Zika cases have been reported in Clark County, according to the health district.

No mosquitoes have tested positive this season for West Nile virus.

To avoid illnesses spread by infected mosquitoes, the CDC and health district suggest wearing protective clothing and using insect repellent for individuals older than 2 months of age.

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