A pickup truck mounted with a fogging machine drives along an East Valley roadway, unleashing chemicals into the night air in an effort to control mosquitoes and the disease they carry.
Just this past July, Maricopa County Environmental Services sent out its trucks 17 days, mostly to the East Valley and largely to the Gilbert area, according to the county’s fogging calendar.
They could be making many more trips as the second wettest monsoon season in two decades increases the breeding areas for mosquitos.
Even without the heavy rains, mosquitoes take to Gilbert and the rest of the East Valley.
“The east side is comprised of areas with lots of older communities and drainage systems,” department spokesman Johnny Dilone explained. “Plus, it has floor irrigation and agriculture areas in closer proximity to housing developments.”
August’s rainfall so far is making this monsoon season the second wettest on record since 1990, according to meteorologist Marvin Percha with the National Weather Service in Phoenix.
“So far in the monsoon season we are definitely above average,” Percha said. “Now, of course, we still have the rest of it to go and if we dry out and don’t get anymore, we will fall behind.”
From June 15, the start of the monsoon season, to Aug. 12, the Phoenix metropolitan area got on average 2.3 inches of rain, Percha said.
“Right now, the outlook favors above-average for the remainder of the monsoon,” he said. “The odds are we will have an El Niño by late fall and winter and there is a tendency for those years to be wetter.”
El Niño is a warming in the tropical eastern Pacific Ocean that brings moisture.
Water and heat make prime breeding grounds for mosquitoes, Dilone said.
Over 40 species of mosquitoes exist in the state, but only a handful spread disease when they bite people, according to the Arizona Department of Public Health.
Mosquito-borne viruses – such as St. Louis encephalitis, chikungunya and dengue – are found every year in Arizona, but West Nile is the most common in Maricopa County, according to the state health department.
Mosquitoes transmit the West Nile virus by biting an infected bird and then biting a person. West Nile virus can be detected with a simple blood test.
Health officials reported 93 confirmed and probable West Nile virus cases in Maricopa County in 2017, an uptick from the 63 in 2016. So far this year, one probable case has been reported.
Because of privacy issues, the department does not break down the cases by communities, said spokeswoman Nicole Capone.
But in a Maricopa County Department of Public Health’s mosquito-borne disease report in 2016, East Valley communities consistently were among the top for their high rates of human West Nile virus cases.
Scottsdale and Chandler were far ahead of other Valley communities in 2013, with a rate of 4.41 per 100,000.
Scottsdale was on top again in 2014 with a rate of 5.72, followed by Gilbert at 4.34. Peoria was third with 3.69.
Gilbert had the highest rate at 4.18 in 2015, followed by Scottsdale at 3.04 and for 2016, Gilbert was first with a rate of 3.80, followed by Chandler at 3.23.
West Nile virus cases have been reported throughout the country, according to the U.S. Centers for Disease Control and Prevention.
The virus arrived in the United States from Africa in 1999 and within three years spread from a six-county area around New York City to 44 states, according to the National Institutes of Health.
Maricopa County reported its first West Nile outbreak in 2004, a year after the virus reached the state, with 391 cases, according to county health officials.
Most people infected with the virus, which has no vaccine or cure, experience no symptoms. But about one in 150 infected people develops a serious and sometimes fatal illness, the CDC reported.
In Maricopa County, six people died from the virus in 2017, five died in 2016 and two died in 2015. Since 2008, the highest number of reported county deaths from the virus was 12, in 2014, according to state health officials.
The virus can cause encephalitis (an inflammation of the brain) or meningitis (inflammation of the tissue surrounding the brain and spinal cord).
Although public attention focuses on mosquitoes during the monsoon season, they are a year-round focus for the county’s Vector Control Division.
“Our department has 35 people, and we have 25 who do nothing but mosquitoes year-round,” said Kirk Smith, the division’s laboratory supervisor, noting:
“Even though we are dry, we have created a lush desert with lots of water everywhere, and we have mosquito problems year-round.”
So far, Smith said, the mosquito season is shaping up to be average in terms of mosquitoes testing positive for the West Nile and St. Louis viruses.
Dilone said the county’s fogging schedule is typically March to November but picks up during the monsoon season, which is June 15-Sept. 30.
The department’s mosquito surveillance program includes monitoring sites identified as problem areas or mosquito-breeding sites within the 9,224-square-mile boundary of the county.
“We inspect and treat 3,746 routine sites on a monthly basis that are known mosquito-breeding hot spots,” Dilone said. “In addition, we respond to thousands of citizen complaints annually and survey the areas to find problems and inspectors always look for potential sites during their daily travels.”
A total of 801 routine traps are deployed countywide weekly to help detect mosquito populations and disease prevalence. Additional traps are set up based on complaints from the public.
Traps are set daily and picked up the daily and brought to the lab for identification and testing.
The threshold to fog includes a trap containing 30 or more Culex females, 300 or more floodwater females or 50 or more Aedes aegypti females, Dilone said. Culex species can carry the West Nile virus and Aedes aegypti carry Zika and dengue.
A trap that meets one of the threshold criteria puts the location on the county’s fogging calendar within 72 hours and a square-mile area around it is fogged, Dilone said, adding:
“Our program continues to expand as new housing developments grow throughout the county. From 2014 to 2018 we added an additional 200 routine traps.”
Smith said his department also works with cities to get them to clean out their storm-drain system, another popular breeding haven for mosquitoes. If mosquitoes are found in a drain, the area is hand-treated to eradicate the pests.
Man-made lakes both in public and private settings are not a problem, Smith said – but stagnant swimming pools are.
“Lots of folks don’t maintain their pools, and swimming pools are a great place to breed mosquitoes,” he said.
He said his department investigates 6,000 to 8,000 pools a year, all complaint-driven. Neglected or green pools were a big problem for the county during the Great Recession, but the numbers have dropped with fewer foreclosures and vacant properties, according to Dilone.
Officials reported 883 green pools in the county, with 210 of them in the East Valley to date. Last year there were 1,892 reported green pools, with 524 in the East Valley.
“It has been minimized,” Dilone said. “But it’s always a problem. A lot of folks are not aware the pool pump is not functioning properly, and sometimes they don’t have the resources to fix the problem.”
When it comes to treating green pools, vector control uses mosquito-eating fish that are bred in a tank in downtown Phoenix. Mosquito fishes are free to residents, but there are not a lot of takers, Smith said.
“Our No. 1 treatment is using mosquito fish, and they do a good job,” Smith said. “They like to live in that water. It’s hot, murky and doesn’t smell too good.”
The freshwater fish Gambusia affinis can devour huge quantities of mosquito larvae and also can be put in unused spas and on horse properties in watering troughs, Smith said.
It doesn’t take much water for a mosquito to breed. “Sometimes water stays in an open plastic bottle or even a cap of a plastic bottle where mosquitoes can breed,” said Krijn Paaijmans, assistant professor at ASU’s School of Life Sciences.
Typically, it takes nine days for a mosquito to go from egg to adult, but because of Arizona’s heat, that cycle may be even shorter, Paaijmans said.
“Mosquitoes lay hundreds of eggs,” he said. “Only the females bite you and every two days, they can lay eggs after a bite. They need blood to build their eggs. Their only purpose is to feed, lay eggs and get offsprings.”
Paaijmans said he fears people bitten by a mosquito carrying the West Nile virus may not seek medical attention and may end up spreading the infectious disease.
Symptoms of West Nile are like those of the flu: fever, headache and body aches.
Although there is no cure, the disease can be contained by keeping an infected person indoors so as not to get bitten again. The area around the residence is also treated.
With the West Nile and St. Louis encephalitis viruses endemic in Arizona, state health officials are keeping their eyes on an emerging threat from the mosquito-borne diseases chikungunya, dengue, and Zika viruses – all of which are expanding in the Caribbean and Latin America.
“Those diseases are on our border in Sonora, Mexico,” Smith said. “And I’m quite concerned that the diseases will eventually work their way to Tucson and eventually to Phoenix.”