Defending against West Nile virus in Louisiana includes testing, trapping mosquitoes

Twice a week in the summer and fall, employees with West Baton Rouge’s mosquito control department — like workers in 31 other Louisiana parishes — set traps for the southern house mosquito, the species that carries the West Nile virus. 

The parishes are looking for information that will tell them where to send their mosquito-spraying trucks. They set up the traps in drainage ditches and near septic tanks, places where the mosquitoes like to lay their eggs. 

In fact, the traps are baited with what’s called “artificial stinkwater” and are designed to attract pregnant female mosquitoes — because only female mosquitoes bite people (and birds) for blood, said George Bragg, director of the West Baton Rouge Mosquito Control department. 

Both male and female mosquitoes feed on plant nectar, Bragg said, but “the females also take blood, because they need the protein to make eggs.” 

The mosquito control workers capture the trapped live insects in nets, then take the nets back to the office and put them in cold storage for 45 minutes, one way to kill a mosquito.

Then comes the important next step: preparing the mosquito samples for testing.

West Nile virus

About 80% to 90% of all West Nile virus cases have no symptoms, the Louisiana Department of Health said. That is one reason why it is important to monitor mosquito populations, according to a recent report. 

Other people with the virus may have mild to moderate flu-like symptoms and don’t feel bad enough to go a doctor or be tested for West Nile, the department said.

Then there is the small fraction of people with West Nile virus who can develop neuroinvasive diseases like meningitis and encephalitis. 

“Due to the severe nature of these cases, they are consistently detected and reported,” the Department of Health said in a report out this week on diseases transmitted to humans by insects this year through Sept. 14. 

According to the report, two deaths from a neuroinvasive disease have occurred in Louisiana during that time period.

Testing process

The testing of the trapped mosquitoes happens at LSU Diagnostics, an animal disease diagnostics lab housed in the LSU School of Veterinary Medicine.

But before that happens, parish mosquito control departments have more work to do. 

After the mosquitoes in the nets come out of cold storage, they are separated by species — there are more than 60 in Louisiana — and gender, Bragg said.

The mosquitoes are then put into vials, anywhere from five to 100 per vial, and sent to LSU Diagnostics.

“Surveillance is a vital part of mosquito control,” Bragg said. 

So far this year, 439 “mosquito pools” in Louisiana have tested positive for West Nile virus, out of 17,000 pools tested, said Alma Roy, Ph.D., a consultant with the diagnostics lab.

The state Department of Health describes a mosquito pool as aggregate samples of more than 50 mosquitoes from the same site.

Roy said the diagnostics lab at the LSU Vet School has been testing for West Nile virus since it first appeared in Louisiana in 2002. 

“We test up to 1,000 vials a week and up to 30,000 vials yearly,” she said. 

When the mosquitoes come in, they are ground up and tested for the genetic material of the virus.

“Not many people know about the lab, but we participate in the public health of the people of Louisiana,” Roy said.

Louisiana state epidemiologist Theresa Sokol said West Nile virus in humans is highly variable from one year to the next.

“The one thing you can count on is it’s unpredictable,” she said.

The West Nile virus season peaks in August, but will continue through the fall months, Sokol said. Testing ends in November.

“In Louisiana, I think, we’re particularly vulnerable, with our hot climate, lots of rain and humidity,” she said. 

“Everybody throughout Louisiana right now is at risk at being infected by West Nile virus.”

She encourages people to take measures to protect themselves against mosquitoes. 

The mosquito control department in West Baton Rouge is one of a handful in the state that regularly post their test findings, Bragg said. 

For instance, earlier this week, the department posted on the parish Facebook page that it had submitted 22 mosquito samples to LSU Diagostics for testing on Monday and two tested positive for West Nile virus. The post continued with the department’s plans for spraying and other measures. 

The numbers posted this week are small and no cause for worry, Bragg said.

The main reason the parish posts the information is that “we want people to be aware,” he said.

“We want to encourage people to do what they can around their yard” to protect against mosquitoes, Bragg said.

Some of those measures, according to Sokol and Bragg: 

• Empty water from all containers around the property at least once per week.
• Get rid of or treat standing water on the property.
• Repair any holes in window and door screens.
• Make sure any gaps around window air-conditioning units are sealed.
• Clean the gutters.
• Make sure swimming pools are well chlorinated. 
• Wear long, loosefitting clothing if possible.
• Wear mosquito repellent. If you wear sunscreen, put on the sunscreen first, then the mosquito repellent. Read the instructions for the repellent and reapply.
• Avoid activities at dusk and dawn when mosquitoes are most active.