For the first time this season, the Moab Mosquito Abatement District found West Nile virus in mosquitoes it collected at the Matheson Wetlands preserve northwest of Moab.

West Nile virus is transmitted by Culex mosquitoes that bite at night anywhere in the valley or during mild periods of the day in the bulrush marshes of the preserve.

The peak flight time for the vector Culex mosquitoes is in the two hours after the first stars become visible at sunset.

If insecticide spraying, or fogging, is needed, district officials say it will be done at that time and in those areas where Culex numbers pose significant risk. Currently, Culex numbers outside the marsh are very low and below the district’s spray threshold, but trapping and testing will continue to see if there are any changes.

People should avoid mosquito bites by wearing clothes with long sleeves, and by using insect repellent. It is especially important to prevent night mosquito bites by having good window screens and by using a screened tent if sleeping outside.

West Nile virus activity will continue in birds and mosquitoes throughout the valley for about two weeks and then be restricted to the preserve and decline as temperatures cool down, district officials say. The area’s primary vector mosquito, Culex tarsalis, stops biting around the fall equinox, which lands on Thursday, Sept. 22, this year as it prepares to go into its winter diapause. The valley’s secondary vector mosquito, Culex erythrothorax, continues biting until frosts kill it off, but stays in and near the bulrush marshes northwest of town.

Because of declining mosquito numbers and the imminent arrival of fall, district officials say that spraying of adult mosquitoes is not likely. However, if spraying is needed, they say it will be based on local Culex mosquito trap counts and infection rates.

The insecticide spray that the mosquito district uses for adult mosquitoes is a pyrethrum and piperonyl butoxide mixture. It is relatively odorless, does not harm paint, is legal for use on crops and pastures, and breaks down within a few hours after application. However, it can cause allergic responses, including severe asthma attacks, in susceptible individuals.

If spraying needs to be done, announcements will be given to KZMU and KCYN, and an attempt will be made to notify those in the spray area who are on the district’s “chemically sensitive” list so that individuals may take personal precautions. District officials say the information will be kept confidential and be used only for spray notification. Because it has been several years since the last list was compiled, those who called before should call again to be placed on a new list.

Even if a person is infected by West Nile virus, the risk of serious disease is low.

Four-fifths of those infected will not get sick. Most of the remainder will have a mild to severe flu-like illness with muscle aches, fever, rash, and headache that usually lasts a few days but can last months. Less than one in 100 people will get meningitis or encephalitis. Those at greatest risk of serious disease are those with weakened immune systems, diabetes, high blood pressure or kidney disease. The elderly are at greatest risk for severe complications.

Proper diagnosis of West Nile virus disease is important. The incubation period is usually one to two weeks, and many other viruses – from herpes to influenza – can cause similar symptoms, including encephalitis.

Culex mosquitoes breed in water that stands for more than a week. Unmaintained swimming pools, hot tubs, wading pools, water-filled buckets, livestock water troughs and flood-irrigated fields breed these mosquitoes. If people remove stagnant water from their property and irrigators properly manage their water, district officials say that fewer Culex will be produced.

Residents advised to take simple precautions to prevent bites

To report stagnant water, or to add your name to the district’s “chemically sensitive” list, call Mosquito Abatement at 435-259-7161.