September 26, 2008 Report by Manager Jenny Mullins:

Our larvaciding for this season began on April 24th and will continue into October as long as larva continue to be found.  Snow pack levels early this year were the highest in our brief history.   The river rose to 20,500 cfs at Peshastin in mid-May just as it did in 2006, but there was not as much rainy weather as in 2006 so larvaciding was successful throughout the district. 

The similarity to the high snowpacks of 2002 and 2006  caused us to predict a resurgence in cases of the West Nile virus and this has proved to be true.   So far this year 29 horses, 9 birds and 41 mosquito samples have tested positive in Washington for the disease.  At first the disease centered in Yakima, Benton and Grant counties but recently two crows tested positive on the west side in Thurston and King Counties.  Three people who probably were exposed to the virus in eastern Washington have been found to be infected with the virus: a King County blood donor with no symptoms, a Yakima County woman in her 50's who developed a fever with headache and muscle aches, and a King County man in his 30's who developed meningitis, a more serious symptom of the disease.

West Nile virus was consistently found late in the season in animals Yakima County from 2005 through 2007. That county was also the site of a mosquito-borne encephalitis outbreak in the 1940's which afflicted over 100 people resulting in 18 deaths.  This outbreak led to the creation of mosquito districts beginning in the 50's.  This year the virus first emerged in early July in mosquito samples in Yakima County.  The earliness of its arrival this year indicated that this would be a bad year and we have been glad that so far very few people have been infected.

Since the Culex mosquito is the main carrier of this disease, this year I acquired a stereomicroscope and learned to identify Culex pipiens and Culex tarsalis, the two Culex species present in our area.  Culex numbers remained low until mid-August. On August 18th they increased to 10 or more at two trap sites so we quickly obtained a VecTest kit and tested these mosquitoes on August 19th.  The test results were negative for West Nile virus.

Since then the weather has been cooler so we have not done any more trapping for adult mosquitoes which are typically less active (so fewer are caught in traps) as the weather cools.  However the majority of mosquitoes that are present now are likely to be Culex, a mosquito which will take advantage of any stagnant water to lay its eggs.  Some common sites for these mosquito larva are puddles in fields and along the river, watering troughs, tires and anything which holds water.  I have continued to find larva throughout this week and the weather is expected to warm so residents are encouraged to check their property and drain any potential sites or call me at 548-3316 if the site is not drainable.

Historically the floodwater mosquito Aedes has been the predominant mosquito in our area.  This mosquito lays its eggs on land in anticipation of flooding due to snowmelt or a rising water table.  Due to its preference for mammals it is only an incidental carrier of West Nile virus.  It is an important vector of malaria, dengue fever and other diseases which do not require birds for their amplification but these diseases are not expected in our area in the near future.

Assistants help us to deal with periods of high water flow when the water table rises to produce more larval habitat.

 Last year's new assistant, Bruce Hill of Wenatchee, returned this season.  Bruce has a M.S. in entomology and retired a few years ago.  He was responsible for most of the East Leavenworth Road area. 

Our new assistant this year was Christopher Ward.  He worked in the Ski Hill area and Waterfront Park.

David Wood has stepped down to follow other pursuits.  He had covered the Ski Hill area along with all of the City Parks for much of 2004 through 2006.  His efforts helped to demonstrate that ground larvaciding can be successful even during high water years like 2006.

Any time is a good time to make sure that screens are in good repair.  As summer arrives be sure to use an effective repellent during those times when contact with mosquitoes is likely. Do not allow mosquitoes to bite you!

The newest pesticide in our arsenal is methoprene, a hormone which prevents the completion of metamorphosis in mosquitoes. The formulation we use provides control for 30 days. We have used it in storm drains and hard to reach areas (such as islands which will become inaccessible) where the presence of pupa will not bother us.

We continue to use Bti, Bs, and Agnique (a monomolecular film) as appropriate. Bti (Bacillus thuringiensis israelensis) is our workhorse: it works well on both Culex and Aedes and usually kills larva overnight by dissolving their midgut after they ingest it. Other strains of Bt are used to control a variety of forest and agricultural pests and are commonly used by organic farmers. In its granular form it is our cheapest pesticide at about $16 per acre. It also comes in a longer lasting doughnut-shaped briquets which are used in sites which are under 1000 square feet.

Our cheapest longer lasting material is Bacillus sphaericus (Bs). It costs about $100 per acre and works in a similar manner to Bti but is not effective on Aedes the floodwater mosquito which lays its eggs on dry land and then hatches in great numbers when flooded. Agnique, a monomolecular film drowns pupa by reducing the surface tension of the water. The rotation of various larvacides helps to reduce the likelihood of pesticide resistance.

Storm drain treatment is a very important part of our disease prevention program and that of many other cities. The storm drains are treated once a month by bicycle along with other small sites in Leavenworth.

Most of our district has not been sprayed with adulticide (malathion) since 2003. About an eighth of the district was sprayed in 2006.

Complaints help us to track down mosquito breeding areas. Call us at 548-5904 if you see two or more mosquitoes in an evening.

Residents need to be sure to check their yards for any standing water: standing water collected by any container will attract the Culex mosquito which lays its eggs on water. Drain the water if possible, otherwise call us at 548-5904 for treatment.  In 2005 at least three complainants were inadvertantly breeding mosquitoes on their own property: in an old bathtub, a container for cuttings, and in a tarp. In other cases, the neighbors were breeding mosquitoes: in an untreated swimming pool, a horse trough and a large cooking pot. If you have a swimming pool, then make sure it is empty or chemically treated.  If you have a horse trough, make sure the water is fresh and if unable to do this or if you will be away on vacation make sure to get Bti briquets from us for treatment every 3 to 4 weeks.

Horse owners are reminded to vaccinate their horses: a two shot series is needed the first year, followed by a booster shot each year thereafter. For the best protection these shots should be completed in the spring. West Nile virus is fatal in about 30% of those cases diagnosed in horses.

Although West Nile Virus is not usually deadly in humans, its effects can be quite debilitating.  About 1/3 of confirmed cases require hospitalization and 2% die.  All ages can be affected but the average age (median and mean) of all cases is in the late 40's with  an average in the late 50's for the worst symptoms.  In order to avoid the disease, avoid contact with mosquitoes. This can be done by maintaining home screens and by avoiding mosquito laden areas and wearing long sleeves, particularly in the evening. There are several effective repellents on the market now: products containing DEET have endured the test of time, the military uses a 33% DEET formulation: higher concentrations may irritate more sensitive skin, formulations between 7 and 20% are quite effective as well. For those who are apprehensive about the use of DEET, products containing Picaridin or oil of lemon eucalytus also offer long-lasting protection.

Why does our district put so much effort into education (to reduce larval habitat) and larvaciding as opposed to adulticiding? There are several reasons: enough to persuade those of almost any political persuasion. Because larvaciding is applied when mosquitoes are most concentrated, it is cheaper and more likely to be effective. (Where possible habitat reduction is by far the cheapest and most effective route.) The materials used for larvaciding are much better targetted to the mosquitoes and thus have much less impact on other species and much less risk to humans. At present there are no documented cases of mosquitoes becoming resistant to Bti, whereas resistance to malathion and other organophosphates is not uncommon. Finally larvacide kills the mosquito before it has a chance to transmit disease.