West Nile virus in Maricopa County: What you need to know about mosquito-borne illness

Scientists hope that by finding the source of the West Nile virus, they can fuel efforts to stop the spread of carrier mosquitoes.
Claire Spinner
Arizona Republic

Following a 10th West Nile virus related death in Arizona and record-setting rainfall, Maricopa County officials are urging the public to take preventive action against mosquito-borne diseases.

With 138 confirmed cases across Arizona's most populous county as of Friday afternoon, health and environmental officials are looking to curb the trend of increasing cases.

The first step in preventing infection comes with understanding West Nile virus. Here are five things to know about mosquito season:

Phoenix has seen record-high levels of rain during this monsoon season

This means more mosquitoes and more West Nile virus.

West Nile virus was first detected in Arizona in 2004. According to Johnny Diloné, a Maricopa County Environmental Services Department spokesperson, the presence of the virus in Arizona is largely dependent on environmental conditions that are conducive to attracting mosquitoes.

“Last year, it was unusually and unseasonably dry. There was not a lot of water, which meant less mosquito activity." Diloné said. "This year, the monsoon season has been particularly intense, and we have been hit with heavy rainfall. The accumulation of water in addition to the warm temperatures allow mosquitoes to breed and their eggs to hatch."

Because mosquito eggs can lay dormant for up to a year before hatching, those that did not hatch because of the dry conditions in 2020 are hatching as new eggs are being laid, he said.

Diloné said that the Environmental Services Department has already seen an increase of more than 400% for West Nile positive mosquito samples compared with last year.

As of Friday afternoon, 690 mosquito samples have come back positive for West Nile virus, and nine have tested positive for St. Louis encephalitis, which is another mosquito-borne disease.

The number of mosquito-related complaints from Maricopa County residents has risen dramatically, as even areas previously not known for heavy activity are experiencing the effects of the rain on the mosquito population, he said.

Chuck Wells, president and owner of The Mosquito Squad of Greater Scottsdale, a mosquito-control company, has noticed similar trends for his business.

“It’s been absolutely insane with the number of calls and emails we have been getting. The mosquitoes are going crazy right now, especially after it rains.”

Wells is reporting similar numbers to the Environmental Services Department, estimating that the mosquito population has increased more than 400%, an estimate he bases on the rising West Nile case numbers and calls for service to his business.

"We do treatments in three rounds, usually spread out two weeks apart. We have had to cut this timeframe nearly in half to deal with the increasing problem,” Wells said.

West Nile virus spreads through the bite of a female Culex mosquito

It cannot be transmitted person to person.

Melissa Kretschmer, epizoologist for the Maricopa County Department of Public Health, explained that while the virus spreads among the mosquito population, people who contract the illness are not putting their households or close contacts in danger.

West Nile virus actually starts with birds, which can become infected. When a mosquito feeds off the birds, it will pick up the virus. From there, the mosquito can pass the virus to their offspring, maintaining it in their circles and eventually infecting people, she said.

Female Culex mosquitoes are responsible for the transmission of the virus, passing it to humans through their bites. Those mosquitoes are most active at dawn and dusk, and rarely fly in groups.

Mosquitoes need standing water and warm temperatures to breed; those conditions allow their eggs to hatch. Monsoon season in Arizona creates the ideal environment for mosquitoes to breed, which is why most West Nile virus cases are reported in the summer months.

RELATED DEATHS:10 people have died from the virus in Arizona since September

Aside from West Nile virus, a small number of Culex mosquitoes in Arizona have been found to carry St. Louis encephalitis. Cases of other mosquito-borne illnesses, including Zika, Chikungunya and Dengue, have been found in Arizona residents, but these cases are exclusively travel-related and there is no indication that mosquitoes in the state carry them.

Maricopa County Environmental Services works all year to combat West Nile virus

"We have implemented a year-round program, which we call Fight the Bite. Throughout the year, we deploy over 850 mosquito traps every week to hundreds of areas. We really want to be ahead of the game,” Diloné said.

Most consider “mosquito season” to be April through October, but because it doesn’t typically get cold enough in Maricopa County to kill mosquitoes, they still are found in smaller numbers throughout the year.

“I like to describe our program as aggressively proactive. We do all of the testing ourselves, and we monitor as many areas as possible, both those identified as problem areas and those we receive complaints about.”

Diloné explained that the mosquito vector control program created for the surveillance and abatement of mosquitoes uses a variety of criteria to determine whether or not a given area will need their services.

Wear lightweight clothing that covers your arms and legs and use an insect repellent if you must be outdoors when mosquitoes are active. Always follow the directions on the label.

Each trap set by the department represents one square mile of an area. The day after setting up a trap, the sample is collected by the department for testing at their laboratory.

An area is scheduled to be sprayed for mosquitoes if it meets one or more of several criteria. If a trap comes back with 30 or more female Culex mosquitoes or more than 300 mosquitoes of any species, the area will get a fogging. The samples are separated by species, regardless of the number or type of mosquitoes found. Any female Culex mosquitoes are tested for illness, and if the sample comes back positive for West Nile virus or St. Louis encephalitis, the Environmental Services Department will spray that area, too.

While West Nile virus has the potential to cause severe illness or even death

Only one out of every five people infected will experience symptoms.

Twenty percent of those infected will experience symptoms and most will present as flu-like illness, Kretschmer said.

Sometimes called West Nile fever, common symptoms include fever, headaches, body aches, fatigue, gastrointestinal issues or a rash.

A majority of people with mild symptoms will recover, though sometimes symptoms can be prolonged for weeks to months. Most symptoms appear two to six days after the bite of an infected mosquito. 

A small percentage of individuals, typically about one in every 150 people infected, can develop more severe symptoms that affect the brain and spinal cord.

Those infections can lead to encephalitis or meningitis and can sometimes even cause paralysis or death. Signs of a more severe infection include a high fever or severe headache; confusion and disorientation; vision loss or double vision; seizures; coma; and in some cases severe neurological issues that affect mental processing, which can be permanent. The severe neurological symptoms and diseases are called neuroinvasive.

Out of more than 200 confirmed West Nile virus cases, almost 65% have resulted in neurological disease, according to the CDC. Experts are using the number as a rough estimate, as many West Nile virus cases are undiagnosed. 

So far this season, 10 deaths have been reported in Arizona. One of the most recent West Nile-related deaths was a retired police officer, Nathan Ryberg, who was pronounced dead Thursday. The cause of death has been confirmed to be related to neuroinvasive symptoms of the disease, which are being reported in a majority of severe cases.

Those who are most at risk for severe infection are people over 60, immunocompromised individuals and children. While it is unlikely that someone in good health will experience intense complications because of West Nile virus, it is important that anyone experiencing severe symptoms seek medical attention.

Take these steps to stop mosquito breeding and prevent mosquito bites

Preventing mosquitoes from breeding is the first step in minimizing the spread of West Nile virus.

Diloné, Wells and Kretschmer each emphasized the importance of monitoring an individual’s property for standing water.

“Especially after it rains, it is important to go around your property and make sure there is nothing collecting water. Whether you are in a large house or small apartment, you will almost always find something,” Diloné said.

Some ways that an individual can make sure their property does not become a breeding ground are:

  • Avoiding over-watering lawns and keeping grass short.
  • Draining water from toys, potted plants, pet dishes and even roofs and gutters after it rains.
  • Turning over canoes and other boats when not in use.
  • Properly maintaining any bodies of water present on the property. This includes bird baths, ponds and swimming pools, all of which should have circulating water because they pose the highest risk of becoming breeding grounds.
  • Scrubbing and cleaning any containers that contain standing water in order to prevent any eggs laid from hatching.

“We all have the personal responsibility to ‘defend and drain:’ keeping yourself protected from bites and making sure that you are not creating conditions for mosquito breeding in your own neighborhood,” Kretschmer said.

Diloné recommends that anyone who does notice standing water on their property or in their community should contact Environmental Services.

The department will assess the situation free of charge and can provide fogging services for large areas, or mosquito-eating fish called gambusia, which can be placed in a pool or pond to control the population and destroy eggs.

Chuck Wells said that if one chooses to hire a pesticide service, make sure that there is consistent treatment with multiple rounds. Pesticide services can help control the issue through non-toxic chemical treatments.

Even after taking the precautions to prevent breeding, it is important that individuals take steps to  avoid bites. Diloné, Kretschmer and Wells recommend:

  • Wearing light-colored, loose fitting clothing when outside.
  • Using a mosquito repellent containing Deet or other EPA approved natural options such as picaridin or oil of lemon eucalyptus. Always read labels carefully and apply as instructed.
  • Make sure door and window screens are keeping bugs out and are free of holes.
  • Avoid mosquito traps that attract mosquitoes, because they can cause mosquitoes to migrate from other areas.

“At the end of the day, all we are really asking folks to do is do their part in preventing mosquito breeding and avoiding mosquito bites. West Nile virus is not a cause for panic, but it is up to us to prevent its spread,” Diloné said.

For more information or to submit an online complaint regarding mosquitoes in your area, visit fightthebitemaricopa.org or call the Environmental Services mosquito hotline at 602-506-0700.