West Nile Risk Reduction Initiative

Under agreement with the Ministry of Health, the Union of British Columbia Municipalities in April of 2006 announced a program to assist BC communities in the planning and implementation of programs designed to control mosquito populations. $4 million dollars was provided by the Ministry to assist communities wishing to take a proactive approach to mosquito control with the intent of reducing the risk of occurrence of West Nile Virus.

The Village of Pemberton applied for funding under this initiative and in July 2006 received approval for $5,058.20. Throughout the summer and fall of 2006, the VoP Works Department implemented proactive mosquito control initiatives, including:

Cleaning all municipal catch basins of stagnant water and debris;
Pulling and cleaning all catch basin lids;
Cleaning and repairing four major drainage culverts for the Village, and keeping surrounding vegetation maintained.

There are further plans to have the catch basins pumped in the spring of 2007.

MOSQUITOES & YOUR HEALTH

(The following are excerpts from "Municipal Mosquito Control Guidelines ", Prepared by Roy Ellis, Prairie Pest Management, for Heath Canada, Centre for Infectious Disease Prevention and Control.)

As a nuisance insect, mosquitoes have few rivals in Canada and they can be a burden on certain jurisdictions without posing a threat to public health. Although Canada is fortunate in having relatively few mosquito-borne diseases, several mosquito-borne viruses and various mechanically-transmitted pathogens have periodically represented a risk to the health of Canadians. With the incursion of West Nile Virus (WNV) into Canada in 2001 and its rapid spread throughout much of the country, there has never been a greater need to develop locally-based mosquito control programs.

LIFE CYCLE OF MOSQUITOES

Mosquitoes pass through the egg, larval, pupal and adult stages during their life cycle and all stages except the egg are mobile. Female mosquitoes lay their eggs either on moist soil or on the surface of standing water.

During the larval and pupal stage, mosquitoes remain in the water. Both these stages are adapted to swimming, and can be identified by the 'wriggling' way in which they swim. The larvae of most species can be seen resting at the surface, hanging from the surface film by their respiratory siphon.

The pupal stage looks like an inverted comma. Some people think the pupae look like miniature tadpoles. They use their flattened, paddle-like 'tails' to to propel themselves through the water. They do not feed during this transitional stage of development, as they transform from larvae to adults. When mature, the skin of the pupa splits open along its back and the fresh, adult mosquito slowly emerges into the air. After resting on the surface of the water for a few minutes, the adult mosquito usually moves to a sheltered spot close by to allow its outer skeleton and wings to harden.

TYPICAL LARVAL MOSQUITO HABITATS

The typical habitat for mosquito larvae is a snow-melt pool or a summer rain pool. Small pools of water that are created by melting snow in the spring or by heavy rains during the summer produce most of our nuisance species of mosquitoes. A summer rainfall of three or more centimetres can produce a major hatch of mosquitoes from eggs that were lying dormant in grassy depressions in fields or drainage ditches.

Almost anything, whether natural or artificial, that will hold water for about a week or more may breed mosquitoes. These insects have adapted to a wide variety of larval habitats and every one should be checked for larvae.

A few examples of some larval mosquito breeding sites are given below:

Irrigation ditches, ditches and vehicle ruts along roads, power lines, and railway tracks.
Low-lying pasture pools and woodlot pools.
Tree-holes and rock-pools.
Gutters, barrels, buckets, cans, tires, birdbaths, and boats.
Vegetated edges of sewage lagoons, storm-water retention ponds and reservoirs.
Weedy margins of farm dugouts and sloughs.
Weedy margins of some lakes, ponds, and streams.
Culverts, open wells, catch basins, ornamental pools, and wading pools
Pieces of Polyethylene caught up in short vegetation.

SEASONAL OCCURENCE OF LARVAL & ADULT MOSQUITOES

Larvae of spring floodwater (also know as snow-melt) mosquitoes hatch from the eggs in pools of water formed by melted snow in the woods. The eggs can be found in the leaf litter lining the pools. These larvae develop slowly because of low water temperatures. They emerge as adults in May, before the pools dry up. The female spring floodwater mosquitoes can be very long-lived and may bite several times. They lay their eggs in the shallow depressions in the woods where they will be flooded by melting snow the following year. Spring snow-melt mosquitoes have only one generation per year. Even if summer rains flood these eggs, they will not hatch until the following spring.

Summer floodwater mosquitoes include several of our common nuisance mosquitoes (e.g., Aedes vexans). The larvae hatch from eggs after rainfall in the summer (usually 2.5 cm rains or greater) in shallow flooded areas such as grassy fields, roadside ditches, highway right-of-ways, tire tracks, cow hoof prints, and other habitats. The larvae develop very quickly (7-10 days). Several generations may occur each summer, depending upon the frequency and intensity of rainfall. We cannot predict in advance how bad the summer floodwater mosquitoes will be because we cannot predict rainfall amounts or patterns.

Although there are distinct species differences, generally mosquitoes are most abundant during the summer months. Some species may be more abundant, as adults, during the late spring; others may be most abundant during the late summer. Still others are abundant during the summer months but for only a few weeks after a heavy rainfall.

Heavy snowfall during the winter months, followed by a quick thaw in the spring, tends to favour the development of large populations of the spring floodwater mosquitoes. Heavy rainfalls during the summer months tend to favour massive populations of summer floodwater species.

Temperature is another key factor affecting mosquito populations. During the larval period, warm temperatures speed up larval and pupal development. Warm temperatures, at times when adult mosquitoes are active, facilitate mating and provide favourable conditions for host-seeking by female mosquitoes. Temperature and relative humidity have a major influence on adult mosquito populations. High temperatures, associated with low relative humidity, tend to shorten the adult period and cause the early death of those mosquitoes unable to find cool, moist areas for shelter.

Winds also have their effect on populations. Low winds facilitate swarming by the males and facilitate the female mosquito's search for blood. High winds generally have the opposite effects. However, high winds may be a positive factor for mosquitoes in some regards. For example, steady high winds, following a mass emergence of adult mosquitoes, tend to disperse the population widely, sometimes many kilometres.

ACTIVITY & BITING BEHAVIOUR OF MOSQUITOES

Adult mosquitoes are usually found close to their original larval breeding sites. However, the females may disperse from several hundred metres to several hundred kilometres from their source, often aided by wind. Most mosquitoes rest during the day, seeking shelter in dense vegetation that is close to their swarm sites, egg-laying sites, or blood-meal sources.

The adults of most species are active during periods of twilight, which is when winds are relatively low, temperatures are moderate, and the relative humidity is high. In shady areas, mosquitoes may be active throughout the day. Like male mosquitoes, the female mosquitoes feed on nectar from flowers, aphid excretions on leaves, and sap leaking from broken twigs to obtain the carbohydrates that are required for activities like flight.

Unfortunately for humans and other hosts, the females of most mosquito species require a blood-meal for egg development. Each species seems to have its own range of host preferences but most female mosquitoes will feed on a broad range of hosts. Typical hosts include warm-blooded vertebrates, like birds and mammals. However, the females of a few mosquito species feed on reptiles and amphibians.

RESTING SITES OF FEMALE MOSQUITOES

Most mosquitoes like to rest during the day in cool, moist locations. They are very sensitive to hot, dry weather. A few examples of some adult mosquito resting sites are given below:

Heavy vegetation near their larval breeding sites or their hosts.
Areas of high humidity along the shores of streams, rivers and lakes.
Cool, moist places along roadsides (e.g., in culverts, under bridges, in catch-basins).
Inside buildings (e.g., houses, livestock barns, poultry houses, outhouses).
In piles of firewood, lumber, or other stored materials.

WEST NILE VIRUS TRANSMISSION

West Nile Virus has emerged in recent years in temperate regions of Europe and North America, presenting a significant threat to human and animal (wild and domestic animals) health. This virus was first identified in North America in 1999, when it caused an outbreak of viral encephalitis in humans centered in New York City. This virus is a member of the Flaviviridae family of viruses, which includes St. Louis Encephalitis (SLE). Outbreaks of SLE have been reported in Ontario and Manitoba in the past. In addition, outbreaks of another mosquito-borne virus, Western Equine Encephalitis (WEE, family Togaviridae) have been observed in the Prairie Provinces. However, mosquito-borne diseases have been relatively rare in Canada, prior to the incursion of WNV.

The most serious manifestation of WNV infection is fatal encephalitis (inflammation of the brain or spinal cord) in humans and horses, as well as mortality in certain species of domestic and wild birds. However, the symptoms vary.

Approximately 80% of people who are infected with WNV will not show any symptoms at all. Up to 20% of the people who become infected will display mild symptoms, including fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash on the chest, stomach and back.

This flu-like condition is called West Nile fever and symptoms typically last for a few days. In contrast, about one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. The more severe cases of WNV infection, because they involve the central nervous system, are collectively called West Nile Neuroinvasive Disease (WNND).

West Nile virus is amplified during periods of adult mosquito blood-feeding by continuous transmission between mosquito vectors and bird reservoir hosts. Many different species of birds may act as a reservoir for WNV (up to 138 different species of birds have been reported infected with WNV in North America) and bird surveillance has focused mainly on the Corvidae family that includes crows, ravens, blue jays and magpies. Some bird species (e.g., crows) are very susceptible to infection and often become very ill and/or die as result of an infection.

Infected mosquitoes carry virus in their salivary glands and after a period of incubation (typically 2 to 3 weeks), can infect susceptible bird species during blood-meal feeding. Competent bird reservoirs will sustain an infectious viremia (i.e., have virus circulating in the bloodstream) for 1 to 4 days after exposure, after which these hosts likely develop life-long immunity.

A sufficient number of vectors must feed on an infected reservoir host to ensure the likelihood that some mosquitoes will survive long enough to feed again on a susceptible reservoir host. As a result of this transmission, WNV is "amplified" within this enzootic cycle and under the right conditions, it may "spillover" into human or other animal populations when infected mosquitoes bite them.

People, horses, and most other mammals are not known to develop levels of viremia that would be high enough to infect feeding mosquitoes and thus are probably "dead-end" or "incidental" hosts. WNV is not transmitted from one person to another and provided appropriate precautions are taken when handling infected birds, is unlikely to be transmitted from birds to people. However, birds (and possibly mosquitoes) likely serve to spread the virus from one geographical area to another.

REDUCING YOUR RISK

Here are some things that you can do to reduce the risk of WNV Transmission:

Personal Protection

*Wear proper clothing, ie. a hat or head net, long trousers and a long sleeved shirt
*Use insect repellents. NOTE: products with lower concentrations of DEET are as effective at repelling mosquitoes as products containing higher concentrations of DEET, however products with less DEET remain effective for shorter amounts of time. For guidelines on using products containing DEET, read the article "Safety Tips on Using Personal Insect Repellents"
*Avoid places where mosquito densities are high and avoid being outdoors at times of the day when mosquito activity is highest.

Source Reduction

Installation of a catchment (ie., a small area into which water drains)
Installation of tile leading into a catchment or drain
Modification of grade to facilitate drainage
Filling and leveling water-filled depressions
Removal/maintenance of emergent vegetation from around potential breeding sites

Specific Backyard Source Reduction

Water Gardens: Stock with fish or use Bti; remove excess vegetation.
Swimming Pools: Keep water off the pool cover; maintain water quality at all times.
Tree Holes: Fill hole with sand or mortar.
Plastic Pools: Drain water when not in use, or cover so mosquitoes can not lay eggs.
Containers: Empty Water; store in inverted position; dispose of; or cover
Bird Baths: Change water at least once a week.
Standing Water: Eliminate by draining; fill in low areas.
Street Gutters or catch basins: Keep litter and garden debris out of gutters; do not over water yard.
Septic Tank Fields: Ensure proper drainage so mosquitoes cannot lay eggs.
Roof Gutters/Eaves Troughs: Clean once a year to remove debris.
Irrigated lawns or fields: Avoid over-irrigation. Drain standing water.

Exclusion

The use of screening on all doors and windows of homes and other buildings is an obvious means of excluding mosquitoes. Likewise, doors and windows should always be snug fitting along all four edges.

The above information was taken from "Municipal Mosquito Control Guidelines ", prepared by Roy Ellis, Prarie Pest Management, For Health Canada, Centre for Infectious Disease Prevention and Control.

Last Updated on Wednesday, August 25, 2010