By Cecilia Chan, GSN Managing Editor

Vector control specialist Jason Gillette checked a funky-looking mosquito trap hanging from a tree in the Ashland Ranch neighborhood.

The gallon cooler is filled with dry ice and poked with holes that emit carbon dioxide — mimicking respiration and attracting mosquitoes that are then blown into a cylindrical net by a hand-size fan.

“We catch mosquitoes year-round,” said Gillette, gathering the trap — one of 29 he collected that morning in Gilbert and part of Chandler for the trip back to the Maricopa County vector lab. “This season seems like it’s a little busy.”

Not only is the mosquito season an active one but the county lab is seeing a significant number of traps testing positive for West Nile virus and St. Louis encephalitis, said Johnny Dilone, spokesman for the county Department of Environmental Services.  

As of July 1, the number of mosquito traps testing positive for West Nile in the county was 315 and 153 for encephalitis, Dilone said. For all of 2018, the county reported 138 positive traps for West Nile and 106 positive traps for encephalitis.

“If we keep on experiencing the same trend this week, we would most likely see those numbers go up,” Dilone said.

West Nile virus, which can lead to neuroinvasive disease in humans such as meningitis and encephalitis, is the most common mosquito-borne disease in Arizona.

So far this year there have been 18 confirmed cases of humans contacting West Nile virus from the bite of an infected mosquito, according to Jessica Rigler, assistant director for public health prevention at the state Department of Health Services. All of the cases were in Maricopa County. Eleven of those cases involved some kind of neurological complication, Rigler said.

Last year, 27 human cases in the state tested positive for the virus — 24 in Maricopa County — resulted in six deaths, according to the department.

St. Louis encephalitis, on the other hand, is rare in Arizona with no human case reported this year or last year. In 2015, Maricopa County saw an outbreak that sickened 22 people and resulted in two deaths.

Dilone attributed the increase in positive samples mostly to a higher number of infected birds.

“Obviously mosquitoes get the virus from birds and spread the virus to other birds and to humans,” he said.

Rigler said pools testing positive for mosquitoes also are on the increase this year statewide. When adult mosquitoes are collected from a surveillance trap, they are divided by species, with one pool counted for each species. 

“We’ve had a significant increase of positive pools so far this year for West Nile,” she said. “We had 184 positive pools compared to 20 the same time last year. St. Louis encephalitis so far we’ve had 107 positive pools compared with four the same time last year.”

She said most of the positive mosquito pools were in the eastern side of the county but could not explain the reason. She also could not explain the increase in mosquitoes testing positive for the two viruses.

As of May 31, over 11,700 mosquito traps have been set in Arizona, mostly in Maricopa and Pinal counties, the state agency reported — about a third of the total 41,755 traps set up for all of 2018.

Rigler said more surveillance traps will be added as the year progresses.

Dilone said over 800 of those traps are deployed weekly in different locations in Maricopa County. Traps are set up in the afternoon and taken down the next morning.

A number of traps placed in Gilbert included residential neighborhoods, the Riparian Preserve and Kokopelli Golf Club.

The traps that Gillette oversees are set in populated areas mostly near green belts and on land next to horse properties.

“We have thousands of areas identified as problem areas throughout the year that had issues with mosquito breeding,” Dilone said. “There are problem areas throughout the Valley. Obviously where there is water, there is a risk there. Lot of times it can be a property that has containers collecting water or a swimming pool not working properly or a pond.”

So far this year, the county has received 1,016 complaints about green swimming pools — another source of mosquitoes. In 2018, it received 728, according to Dilone.

Traps are brought to the county lab, where the captured mosquitoes are counted and sorted by species.

“We separate the female mosquitoes and then we test each trap individually to see if they test positively for West Nile virus or St. Louis encephalitis,” Dilone said.

Lab technicians also look out for Aedes aegypti mosquitoes, which are tested for Chikungunya, Dengue and Zika viruses.

While Dilone attributed this year’s significant number of mosquitoes to wetter weather during the spring, things are starting to slow down.

“We’ve not had the rain or accumulation of rain,” he said. “This situation of traps testing positive and having a large number of mosquitoes may change if we experience a dry summer. Weather is always a crucial one. Mosquitoes need water, need stagnant water for three to four days (to breed).”

So far, the monsoon season, which generally brings rain and mosquitoes, is off to a late start.

The season officially started June 15 but is delayed this year with the Phoenix area seeing abnormally dry weather for the next few weeks due to El Niño persisting longer than expected, according to meteorologist Sean Benedict with the National Weather Service in Phoenix. Monsoon season ends officially on Sept. 30.

“Just because it’s a dry start doesn’t mean a dry ending,” he said. “It could be like last year, when there was more rain toward the end of the season.”

What moisture that’s come so far has been to the periphery of Phoenix and southeastern Arizona, such as along the White Mountains, he said.

As for the upcoming winter, it’s trending toward a typical drier season, he said.

But even in dry weather mosquitoes can still breed. Female mosquitoes can lay their eggs in a single bottle cap filled with water.

And because mosquito eggs can remain dormant until activated by water, it’s important for people to dump out any standing water, Rigler said.

Besides year-round monitoring and education, the county’s protocol for combating mosquito-borne diseases includes fogging, the airborne spraying of pesticides from the back of a truck from midnight to 5 a.m.

Fogging is conducted in a 1-square-mile area of a trap provided it meets one of three conditions –  30-plus Culex mosquitoes present in the trap, 300-plus mosquitoes (primarily floodwater mosquitoes)  in a trap or the trap tests positive for West Nile or encephalitis, according to Dilone.

Asked if the department expected to increase fogging this year, Dilone said it depended on the weather.

“Fogging is conducted based on traps meeting our fogging protocol or not,” he said. “We have been fogging quite actively this year, since April 21.”

In Gilbert so far this year, areas surrounding traps meeting the county criteria have already been fogged once. The area that includes Zanjero Park and the area that includes Mercy Gilbert Medical Center have both been fogged three times already.

Fogging also occurred three times in an area that encompasses Roadrunner Park in Chandler and a portion of the Layton Lakes neighborhood in Gilbert.

“Usually the southeast is an area with lot of activity because we have lots of farms and lots of areas that are conducive for mosquitoes like the Riparian,” said Dilone, who lives in Gilbert. “I know there is still ‘ag land’ in Gilbert, more than in other areas.”

The county also offers free mosquito-eating fish, called gambusias, that can be placed into personal ponds or stagnant swimming pools.