Canadian Wildlife Federation: West Nile

All images, unless otherwise stated, are from Hemera.

West Nile Virus is an exotic and intimidating sounding disease that stirs fear in many people. Unfortunately, it is a disease that is all too likely here to stay. However, despite its more serious implications, an understanding of what it is and how it works can make it seem less daunting. A bit of insight into the world of mosquitoes will go a long way in contributing to a better comfort level with this new fact of life.

West Nile Virus

West Nile Virus is what’s termed an arbovirus. This stems from the phrase “arthropod-borne” which refers to viruses that are carried from host to host by blood-feeding arthropods (such as mosquitoes, ticks, and no-see-ums). In the case of West Nile Virus the arthropod involved is the mosquito. West Nile Virus is also a bird virus – birds are the main host for the virus.

It has been found in more than 100 bird species. A bird’s susceptibility to the virus is largely species dependent. Birds of the Corvid family – crows, ravens, magpies, and jays – experience the greatest mortality from West Nile Virus, but some other species, including some hawks and owls, have also suffered. Over the long term, it is believed that most birds will evolve resistance to the disease; however, in the shorter term there is worry for any bird population under threat.

Contracting West Nile Virus

West Nile Virus is spread by the bite of an infected mosquito. It is not spread through person-to-person contact. There is some evidence that infection can occur through handling a dead bird, but this risk is very low. As it is never a good idea to handle dead animals with your bare hands, be sure to wear gloves or use a thick plastic bag when picking up a dead bird.

In areas where West Nile Virus occurs, it is only a minority of mosquitoes that are infected. If bitten by an infected mosquito, approximately 80 per cent of people will not suffer any kind of illness, but will instead develop immunity to the disease. Twenty per cent of infected people will experience a mild illness, and less than one per cent will develop the more serious and potentially fatal form of the disease.

For health related information on West Nile Virus contact your local public health office. You can also check out Health Canada’s Web site.

The horse is one mammal that is vulnerable to West Nile Virus, suffering a relatively high mortality rate. A vaccine has been developed for use on horses. Horse owners should talk to their vet about having their horses vaccinated well before mosquito season.

How It’s Transmitted

A mosquito bites an infected bird and picks up the virus particles that are in the bird’s blood. If this same mosquito then bites another bird, the virus particles are injected into the second bird. New virus particles are produced and these begin circulating in the bird’s blood where they can be picked up by another mosquito that bites the now infected second bird.

If a mosquito bites an infected bird, acquires the virus, and then bites a human or other mammal, then the virus can be passed on to this new host. However, the virus does not build up in the blood of humans or other mammals in sufficient quantities to allow the virus to be passed on to another mosquito. Therefore, humans and other mammals are considered dead-end hosts because they can’t pass the virus on.

Even among birds, the quantity of virus that builds up in their blood varies among species. This can be confusing because how sick a creature becomes doesn’t necessarily reflect how likely it is to pass it on. While house sparrows are not very vulnerable to West Nile Virus, it builds a large density of the virus in its blood enabling it to pass it on easily via mosquito. Most crows, on the other hand, become very sick once infected but carry lower densities of the virus in their blood.

Understanding Mosquitoes

Most people think a mosquito is a mosquito. They are annoying creatures – swat them away and good riddance! However, there is a lot more to mosquitoes than meets the eye.

First of all, there are actually more than 2,500 species of mosquito worldwide. Seventy-five of these can be found in Canada. All of these species differ in significant ways.

Female mosquitoes are very particular about where they will lay their eggs. Practices vary depending on the species. Mosquito eggs fall into two basic categories, desiccation resistant and non-desiccation resistant. Desiccation resistant basically means they can survive being laid in an area where there is currently no standing water but will later flood. Non-desiccation resistant mosquito eggs would die if the water source dries up before they hatch. Mosquitoes of the first group lay their eggs in a spot where it will later flood (such as at the edge of tidal pools), while those in the second group must lay their eggs directly on the water surface.

The type of water a mosquito will choose for egg laying varies among mosquito species. Some species prefer clean water while others like dirty polluted water. Certain mosquito species are found in urban areas while other species will only be found in woodlands. In order to effectively control any one species, it is important to understand where it likes to breed and live.

Another characteristic that differentiates various mosquito species is the time that it likes to “hunt” for its blood meal. Some are day biters, while others bite at night.

Each species also has a definite preference for what it will feed on. One species, for example, likes to feed on frogs and snakes, while another prefers rodents. Some species will feed on a range of hosts. In order to act as a human-related vector for West Nile Virus, a mosquito must feed on both birds and humans.

All mosquitoes have one thing in common – they need water to complete their life cycle. Although some mosquito species can lay their eggs in soil, this soil must be flooded before the eggs will hatch. The larval and pupal stages will only survive in water. This means that mosquitoes cannot breed in tall grass or shrubs – only standing water.

Understanding how a particular mosquito species functions – where it breeds, when it feeds, and what it likes to feed on – helps to identify whether it is a significant risk factor for West Nile Virus transmission. It also allows its populations to be better controlled. A targeted effort aimed at certain high-risk species will be much more effective than an all out war against mosquitoes as a whole. Mosquitoes are a fact of life in Canada and no amount of effort will ever eradicate them.

Target Mosquito Species

There are specific characteristics that make certain mosquito species more likely than other species to transmit West Nile Virus to humans.

It must readily feed on both birds and humans.
It must be found in places where it will encounter human hosts.
The mosquitoes must live long enough to be able to acquire the virus from a bird and then go on to take a second blood meal from a human.

Although West Nile Virus has been isolated in a number of different species of mosquito, not all these species will be successful transmitters of the disease. Some, for one or more of the reasons listed above, may not be able to pass the virus on to humans. The mosquitoes most predominantly found to transmit the disease to humans and horses are the Culex mosquitoes. Culex pipiens, in particular, is the most effective vector.

Culex Pipiens

Culex pipiens is the scientific name for the northern house mosquito. This mosquito is the most common mosquito of urban and suburban areas of eastern Canada and British Columbia. It loves to feed on birds but will also take advantage of human hosts, particularly later in the season when Culex populations reach their peak of activity. This mosquito is called the house mosquito because it tends to live and breed around houses. It will also readily enter dwellings in search of a blood meal.

Culex pipiens likes to lay its eggs in stagnant, or even polluted, water. It is able to find plenty of suitable breeding sites in and around urban areas. The abundance of plastic containers has been a boon to mosquito populations. Discarded containers provide the perfect breeding conditions. Even a small bottle cap can support the growth of these mosquitoes – eggs to adults. Discarded tires, unused wading pools, buckets, and clogged rain gutters can all serve as egg-laying sites. Culex pipiens will lay its eggs in almost any container that holds water with decomposing organic material. It does not like to lay its eggs in water that is too clear and clean.

Culex pipiens, like all mosquitoes, must pass through four life stages – egg, larva, pupa, and adult. Three of these stages are aquatic – they only occur in water. Any water source must last long enough for the mosquitoes to go through the transformation from egg to adult in order for them to survive. The time it takes for this process depends on the temperature – the hotter it is the faster it occurs. Generally this takes eight – 12 days in the summer.

Culex pipiens is replaced by Culex restuans in some areas of Eastern Canada and the Prairie Provinces. It also likes to breed in stagnant or polluted water in urban areas, readily using containers.

Culex tarsalis is considered to be the most likely carrier of West Nile Virus in Alberta. Primarily a rural species, it will migrate to urban areas. Culex tarsalis breeds in warm, shallow pools of standing water.

Controlling Mosquito Populations

The most effective way to control any mosquito population is to control its breeding sites. Here’s how:

Remove empty flowerpots, drill holes in garbage cans or flowerpot bases so that water can drain. Be aware of any children’s toys left outside that could hold water.
Store kiddie-pools upside down, cover garbage and compost bins so water can’t accumulate, and maintain a clean and chlorinated swimming pool. Remove pool covers to get rid of pools of water on the cover.
Store unused tires inside a garage or shed or cover them to prevent water from accumulating inside. If you have a tire swing, drill a hole in the bottom sufficient to allow all water to drain.
Don’t litter or dump garden debris at roadsides, streams, or in storm drains. Recycle or properly dispose of tin cans, plastic cups, drinking boxes, bottle caps, and other containers.
Check flat roofs for standing water.
Clean up trash around your neighbourhood – in parks, along streams and roadsides, etc. Be sure to check under bushes.
Clean and refresh the water in birdbaths and pet bowls at least once a week. Sweep out puddles that last more than a few days.
Screen water barrels and other water tanks so that mosquitoes can’t get in.
Keep your gutters clear – water that accumulates among the leaf litter is prime breeding habitat.
Encourage natural predators such as birds, frogs, and dragonflies to frequent your yard.

Some mosquitoes are inevitable so minimize your likelihood of getting bitten. Here’s how:

Limit your outdoor activities between dusk and dawn (especially during early morning or evening when the main carrier species are most active).
Wear long sleeves and loose clothing when outdoors.
Use insect repellent (be sure to read precautions and follow directions). Seek advice from your doctor regarding the use of repellents on young children.
Cover infant carriers with mosquito netting when outdoors.
Ensure that screens are in good repair.

Wetlands

A healthy functioning wetland supports a wide diversity of natural mosquito predators. These help to keep mosquito populations under control. Studies have shown that the restoration of a healthy wetland can actually reduce the number of mosquitoes (by 90 per cent in one case). Wetlands provide habitat for predators such as birds, frogs, fish, and other insects, which in turn feed on mosquito larvae, pupae, and adults. Some of these creatures feed not only on the mosquitoes produced in the wetland but also those produced in other water sources, allowing overall mosquito numbers to drop.

Some mosquito predators are:

Dragonflies and damselflies – Dragonfly adults and nymphs are both predators of mosquitoes. Adult dragonflies are nicknamed “mosquito hawks” because of their voracious appetite for these creatures. Though smaller than dragonflies, damselflies have similar appetites. Both dragonflies and damselflies like to live in permanent waters that have some standing and aquatic vegetation.

Water striders – Water striders live on the surface of ponds. They use their front legs to grasp prey such as mosquito larvae and midges.

Water boatmen – these aquatic bugs are both predators and scavengers. They help break down decaying vegetation but also feed on the larvae of mosquitoes and midges. Water boatmen prefer the shallow, vegetated areas of a pond or wetland.

Backswimmers – Backswimmers like to hang out in the shallow waters of still waters and ponds where they feed on the larvae of mosquitoes and midges.

Giant water bugs – these large bugs like to stalk their prey among the vegetation in the shallows of ponds and lakes. They are predators that will feed on small fish and tadpoles, but also insects.

Birds – Insectivorous birds, such as swallows, red-winged blackbirds, and nighthawks all make mosquitoes part of their diet.

Bats – Canadian bat species are all insectivorous. Little brown bats, in particular, prey on small flying insects that include mosquitoes.

Though no one predator will completely eliminate mosquitoes from your area, by welcoming a diversity of natural predators you will ensure that mosquito populations are kept under control.

Backyard Ponds

Ponds can function similarly to a wetland if they are created in such a way that they form a healthy functioning ecosystem. This means that they are able to support a good population of aquatic insects, such as water beetles and dragonflies, toads, and frogs. With their natural predators present in healthy numbers, mosquitoes should not be a problem. The pond may even help control your local mosquito population by acting as a trap for mosquitoes to lay their eggs where the emerging larvae are quickly eaten up. A pond can also provide a healthy crop of dragonflies to scour the evening skies for adult mosquitoes.

If the pond is constructed in such a way that it is not able to support a diversity of mosquito predators you can add your own by introducing fish to your pond. Fish will happily feed on your local mosquito population. However, only add fish to closed bodies of water. Never introduce new species into natural ecosystems. Avoid the use of non-native mosquito fish, Gambusia spp., as some species are very aggressive and have caused serious problems for native fish in some areas of the world. Native fish are just as effective at controlling mosquito populations.

If your pond is too small for even fish then add a recirculating pump – mosquitoes like to breed in still water. If your pump is left off for extended periods of time, be sure to change the water weekly to ensure that mosquito larvae don’t have a chance to develop into adults.

Be sure to maintain your pond. Check it regularly and remove any dead vegetation or leaves that may have fallen in the water. The water in your pond should remain relatively clear. If it becomes murky this is an indication that something is out of balance. It may be that you have been over-feeding your fish or that fertilizer has made its way into the pond. (Beware of using fertilizer in the areas around your pond.)

Many of the insects and other creatures associated with ponds and wetlands prey on mosquito larvae and adults. In order to survive, mosquitoes have adapted a rapid lifecycle that allow them to develop in temporary water sources where their predators are unable to survive – such as plastic containers, used tires, and long-lasting puddles. It is these standing water sources that create burgeoning mosquito populations around urban areas.

Mosquito Traps

Be wary of claims made by some companies selling mosquito traps. There is not much scientific evidence to support many of them. In particular, avoid the use of bug zappers. These machines work by light which makes them ineffective at mosquito control, but, unfortunately, very effective at killing many beneficial insects. Don’t be fooled by the presence of mosquito look-a-likes, such as some non-biting midges that can be attracted to bug zappers.

Bacillus thuringiensis var. israelensis (Bti)

Bti is a naturally occurring bacterium found in soils. This bacterium produces a protein crystal that is only toxic to the larvae of mosquitoes and their close relatives, such as black flies. When the crystals are ingested by these larvae, and they reach their alkaline intestines, the crystals dissolve and are converted into a toxin that destroys the insect’s stomach, subsequently causing it to die. The stomachs of humans and other animals are acidic and so don’t activate the toxins. This means that Bti is very targeted and has a minimal effect on the environment, making it a very popular tool for many municipalities in controlling mosquito populations.

Bti is now available for purchase at some garden centres. It is considered safe to use on private land where there is no outflow to other water sources. It may not be used in any water meant for human consumption. Be sure to follow product directions carefully to ensure effective mosquito control.

As the long term, widespread use of such an organism is unknown, it should only be used where necessary. Preference should be given to modifying human-created breeding sources.

Bacillus thruingiensis var. israelensis or Bti should not be confused with Bacillus thuringiensis var kurstaki or Btk. Btk controls the larvae of lepidopterans (butterflies and moths). It is used to target gypsy moth and spruce budworm, but also kills the caterpillars of many other butterfly and moth species.

Spraying

The spraying of chemicals targeted at adult mosquitoes is the least effective means of control. It is also the most harmful. This is for several reasons:

Previous attempts to eradicate mosquitoes using chemical pesticides such as DDT have lead to the development of massive resistance. Essentially the weaker mosquitoes are killed; leaving resistant mosquitoes to breed and quickly give rise to a new generation of mosquitoes unaffected by the chemicals.
No chemical pesticide is 100 per cent safe. Pesticides usually have effects on non-target organisms – often the natural predators of the problem species.
To be effective these pesticides must be sprayed when the mosquitoes are most active – early morning and evening – which is when people are most active. The chemicals must come into contact with the mosquitoes when they are flying. If a mosquito is resting in the shelter of a house the spray will be ineffective in killing it.
Health Canada supports the use of malathion for adult mosquito control. According to a Health Canada fact sheet “Malathion is highly toxic to insects, including beneficial insects such as honeybees.” Also the “PMRA (the Pest Management Regulatory Agency) has determined that malathion is highly toxic to fish and aquatic invertebrates.” In addition “higher application rates (than recommended) may damage automobile paint.”

Mosquitoes are an inevitable part of life in Canada. As much as they are a bother to us, they are also an important part of the ecosystem, providing food for many birds, bats, amphibians, and other insects. Through a better understanding of the world of mosquitoes, we can more effectively reduce our exposure to West Nile Virus without harming those creatures that can help us in our goal.

If you are looking for more information on West Nile Virus in Canada, check out some of the following websites: