Deborah Marlow, assistant deputy director for environmental health services at the Williamson County and Cities Health District, does not hesitate to say the local frequency of West Nile virus is higher than normal.

“We’ve actually been expecting it this year,” she said.

Birds are the most frequent carriers of West Nile virus, Marlow said. Data show that a bird’s average life span of about four years correlates to a spike in West Nile cases about every four years, she said. Baby birds lack immunity to the virus and are most often its carriers.

“Our last big year was 2012,” she said. “Here it is, four years later.”

In October and November, mosquito samples have tested positive for West Nile in Hutto, Georgetown, Cedar Park and Leander. Samples normally consist of 30-100 mosquitoes, according to WCCHD.

Tarrant County and the Dallas area also experienced a higher frequency of West Nile, and because birds migrate south, Williamson County saw West Nile cases slightly later in the year, Marlow said. She said Travis County is noticing a similar trend.

“They’ve seen … more numbers this year also,” Marlow said.

Human cases On Nov. 1, WCCHD Executive Director John Teel told Williamson County commissioners about two suspected area human cases of the virus. Marlow said one of the cases was in Bartlett, on the city’s Bell County side, so Williamson County officials did not investigate the case. The other human case was found in the Georgetown area, she said.

There are two forms of the virus in humans—one causes a fever and flu-like symptoms, and it often goes undiagnosed and goes away on its own. The other form of the virus is neuroinvasive and causes motor and memory dysfunction. The neuroinvasive form only occurs in about 1 percent of cases, but the Georgetown patient was one of them, she said.

“They’ll have symptoms and problems the rest of their life,” Marlow said.

People with compromised immune systems, children and the elderly are more susceptible to the neuroinvasive form of the disease.

“But this person was not any of those,” she said.

When health officials find a human case of a mosquito-borne illness, they perform extra surveillance in that area to check for more infected pools of mosquitoes. Marlow said officials only spray insecticide in the area as a last resort because mosquitoes can build resistance if the area is sprayed too often.

“It’s the last thing we want to rely on,” she said.

WCCHD mosquito program The previous human case of West Nile in Williamson County was found in 2012, before WCCHD established its mosquito program, so the agency is still working to collect data and statistics, Marlow said.

“We’re just beginning to build our theories,” she said.

In 2016, WCCHD has seen one human case of chikungunya and more than one case of the Zika virus, she said. Those cases were all related to travel, and none were locally acquired.

“Nobody knows what’s going to happen with Zika,” she said.

The county has not seen any cases of dengue fever this year, she said.

The WCCHD’s mosquito program has not been in existence long enough to predict what the mosquito situation will be next year, but the warmer temperatures in Central Texas have created an ideal environment for mosquitoes to thrive, Marlow said.

“We’re going to have a warmer winter,” she said. “The mosquitoes are not going to die unless we have a couple freezes.”

Prevention Marlow said residents should clean up their yards this winter, when the mosquitoes are dormant underground.

“Walk that yard. Clean out those gutters,” she said.

Holes in trees should be filled with sand or dirt, and fountains should be drained or residents can pour bleach or larvicide in them, she said. Standing water in a container as small as a bottle cap is enough to allow mosquitoes to breed, she said.

“Once you have the disease in your area, and the mosquitoes that harbor it, it’s not going to go away,” she said. “We’re going to see that for awhile.”

She also recommends wearing pants and long sleeves and keeping a can of mosquito spray near doors in one’s home that lead outside. Culex mosquitoes, the species that can carry West Nile, are active at dawn and dusk, whereas Aedes mosquitoes, which can carry Zika, bite all day, she said.