It starts with flu-like symptoms. Then come the headaches and groggy feeling as West Nile virus multiplies and starts to kill brain cells.

Most people at this point end up going to their doctor or the hospital and find they’ve been infected with a mosquito-borne virus that has been haunting Louisiana parishes since the first case was identified in 2001.

So far this summer, 18 of the 28 residents of East Baton Rouge Parish who are known to have been infected have contracted the neuroinvasive form of the disease, the most serious form of West Nile. That’s the highest number of neuroinvasive cases of any parish in the state to date this year, despite aggressive mosquito control operations that have reduced the number of mosquitoes found in traps across the parish.

According to the state Department of Health and Hospitals, East Baton Rouge saw three more cases of this serious type of the virus during the week of Aug. 31 through Sept. 6.

Statewide there were 12 new cases of confirmed West Nile that week, of which half were found among East Baton Rouge Parish residents.

The updated numbers bring the state total for the year to 92. That’s higher than last year’s total of 58 cases, but still far below 2012, when there were 397 cases reported.

However, the number of human cases reported in East Baton Rouge Parish has been frustrating for mosquito control professionals.

“We feel very good that we kept the number of mosquitoes down, but we still had a large number of them coming back positive (for the virus),” said Randy Vaeth, assistant director of East Baton Rouge Parish Mosquito Abatement and Rodent Control.

People get West Nile virus after being bitten by a mosquito that has picked up the virus from an infected bird.

Most people who get the virus don’t experience any symptoms and many may never be diagnosed as having been infected.

It’s estimated that up to 20 percent of infected people get a flu-like illness.

Fewer than 1 percent of people infected by the virus develop a very serious neuroinvasive form of the disease that affects their nervous systems. As brain cells are killed by the virus, the brain begins to swell and press against the skull.

Although there is no cure for West Nile virus, it is possible to minimize the damage caused from the swelling of the brain, said Dr. Raoult Ratard, state epidemiologist. Of those people who get the neuroinvasive form of the disease, about 10 percent will die in a week or two while another 10 percent will sustain permanent damage. The exact nature of the damage will depend on what parts of the brain were affected but these patients could experience tremors, partial paralysis or trouble speaking. About 80 percent of these people will recover.

“It’s a long, slow recovery, but they’ll do OK,” Ratard said.

The virus is the same in every case. It’s an individual’s reaction to the virus that presents the different outcome, with most of the serious neuroinvasive cases occurring in people ages 65 and older, Ratard said.

The potential seriousness of the disease means everyone, but especially older people, needs to protect themselves from mosquito bites by wearing repellent, long sleeves and emptying water containers around their house where mosquitoes can breed.

Most recent mosquito pool testing has shown that the number of infected mosquitoes has fallen below the threshold of 5 infected mosquitoes per 1,000 that East Baton Rouge Mosquito Abatement and Rodent Control uses to project a more likely chance of seeing human cases.

The tests from Aug. 24 through Aug. 30 show that the rates dropped to 3.5 per 1,000 in the parish, but people still need to be cautious, Vaeth said, because the lower rates don’t always mean fewer human infections.

Since June, the mosquito control office has been aggressively treating those areas that turned up positive for the virus and Vaeth said they’ve been spraying 30,000 acres a week.

In addition, he said, the staff has been trying to raise public awareness about the importance of protecting yourself from mosquito bites.

“We’re pretty frustrated we haven’t been able to keep (human infection) numbers down,” Vaeth said. “We’re not out of the season yet.”

Follow Amy Wold on Twitter, @awold10.