Large Number of Mosquitoes Test Positive for West Nile

Mosquitoes have played a role in human history from the fall of Rome to the outcome of the Civil War and now are becoming more than a nuisance in the Valley. Fortunately for Tulare County, we have Delta Vector Control (DVC) to keep the pests at bay, but only with the public’s help.

Michael Auburn, Delta Vector Control District General Manager, gave a presentation at the September 3rd City Council Work Session, describing the types and numbers of mosquitoes we are dealing with and the diseases they carry.

Auburn said there are two types of mosquitoes endemic in California, the Southern House Mosquito and Aedes aegypti

The Southern House Mosquito feeds on dead birds then passes the disease West Nile Virus on to humans if the bird was infected with the disease. The disease can only be transmitted from mosquito, bird to human and not between humans and has been endemic in the Valley since 2003.

Mir Bear-Johnson, Delta Vector Control District Scientific Program Administrator, said, “This year we have detected unusually high levels of West Nile Virus activity in our mosquito populations, so the focus has been to keep the abundance of these mosquitoes low.”

Auburn said that DVC is using two strategies to combat mosquitoes. One, insecticides, is only used sparingly as the mosquitoes can develop a resistance. The other strategy is to eliminate their breeding ground, standing water in pools or non functioning fountains or bird baths.

Mir said that the infections rate of West Nile peaked at week 31, which is the last week of July and the first week of August. The rate of infection has been declining since and the transmission cycle will break once the temperatures consistently go below 72 degrees.

For most people West Nile Virus is asymptomatic and doctors are loath to even conduct the blood test for the disease. Only people over 50 with compromised immune systems show symptoms such as headache, fever and rash.

In California there have been 57 cases this year of West Nile Virus, said Auburn, and two of those were in Visalia. Of the 57 cases there have been two deaths.

Auburn added that in June Fresno went from two West Nile cases to 26 almost overnight.

The Southern House Mosquito is most active at dawn and dusk and wearing light colored long sleeves and pants and insect repellant is recommended.

The second mosquito prevalent in Tulare County is the Aedes aegypti which does not feed on birds but prefers humans. This mosquito is a carrier of Yellow Fever, Zika, and Dengue.

In the past Visalia has had confirmed cases of Zika and there have been five diagnosed cases of Dengue in Florida in 2019. Even though there are no other current cases of these diseases in the United States besides Dengue, the California Department of Public Health has established protocols in place that the DVC will follow in the event any cases materialize.

Aedes aegypti was first found in Exeter in 2014 and then eradicated. The mosquito then reappeared in Visalia in 2017. The Aedes aegypti has been successful because it can breed in as little as a teaspoon of water and prefers manmade containers. Mir said that the mosquito prefers to lay its eggs on the side of containers instead of on the surface of water  and that the container only needs to be hit by a sprinkler for it to host mosquito eggs.  The eggs can live up to a year and the mosquito can breed inside of houses.

In response to warnings by the DVC the Visalia Public Cemetery and the Tulare Cemetery District have banned the use of flower vases during breeding season. Both cemeteries have received mixed reactions ranging from anger to compliance.

DVC said that it needs significant public participation if it is going to reduce the populations of Aedes aegypti. It is asking the public to eliminate all sources of water such as pet water bowls, plant trays, and standing water in potted plants.

The Aedes aegypti is an aggressive day-biting mosquito and will bite its host multiple times.

Starting this month, Auburn said, DVC will be conducting larviciding trials using truck-mounted and backpack equipment.

Visalia City Council Member Brian Poochigian said he was getting many complaints from the southwest region of Visalia and that it didn’t seem like DVC was fumigating. “I got stung seven times in five minutes when I went outside,” he said.

“Is this something we just need to get used to?” He asked.

“The Aedes aegypti came here with the Hurricane Katrina’s migration and it is here to stay,” said Auburn.


One thought on “Large Number of Mosquitoes Test Positive for West Nile

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  1. Considering the mosquito is capable of killing large numbers of humans ( they do every year and have forever) it’s the smart thing to do.

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