West Nile Virus: You Can Prevent West Nile Virus Infection
27 May 2003
Q. I am a mother of two young children and I am pregnant now with our
third child. I have heard a lot of scary stories about this West Nile
Virus epidemic that is going to hit us this summer. I am really worried
about our young children. I don't think I can keep them indoors for
the whole summer. Is there any way that I can let them enjoy playing
outside without getting sick?
A.
Relax! The chance of your children getting seriously ill with West
Nile Virus (WNV) is exceedingly small. You can reduce this chance even
further by following a few simple steps that I am going to explain here.
However, before I go into these preventative measures, let me give
everyone an overview on this much dreaded illness.
WNV got its name because it was first isolated in 1937 in the West Nile
District of Uganda, Africa. Since then, outbreaks of WNV infection
have occurred in many parts of the world. The first recorded epidemic
in North America, however, didn't happen until 1999 in New York City.
It is amazing so many years had passed before it finally landed here
(compared with the jet-spread SARS Coronavirus). How the virus arrived
in New York is still a mystery.
WNV belongs to a family of viruses called Flaviviridae. There are
several other viruses within this family that produce similar illnesses,
and most of them are spread by mosquitoes.
Believe it or not, humans are not the preferred target of West Nile Virus.
The natural hosts in North America are birds like crows, jays, ravens,
and magpies. A female mosquito becomes infected when it feeds on the
blood of a bird that is already infected with the virus. About two
weeks later, the infected mosquito is able to pass the virus onto the
next victim, which is most often another bird.
Humans and other animals are infected by accident when bitten by infected
mosquitoes. Horses are very susceptible to WNV, thousands of them have
contracted this infection in the last few years. In United States,
a small number of cats, dogs, rabbits, squirrels, chipmunks, skunks,
and bats have also been infected. Humans do not get infected when they
contact infected animals.
In the summer of 2002, a new mode of transmission of WNV was recognized.
This happened after a number of patients became sick following blood
transfusion and organ transplantation. It appeared that some people
may not develop recognizable infection and the virus can be present in
small quantities in the blood and tissue, but enough to cause infection
in transfusion and organ recipients.
In countries where WNV infection has occurred for many years, new
infections mostly happen in young children who would develop a mild
febrile illness. Adults in these countries have already been infected
during their childhood years and have developed life-long immunity to
the virus.
In United States and Canada where the virus was introduced only recently,
most serious infections occur in adults, especially those with underlying
chronic illnesses or weakened immune system. Within 3-14 days after the
mosquito bite, symptoms of meningitis (inflammation of the covering of the
brain), encephalitis (inflammation of the brain itself), or combination
of both (this is called meningoencephalitis) can develop. The symptoms
include high fever, severe headache, stiff neck, nausea and vomiting,
drowsiness, confusion, loss of consciousness, poor coordination, muscle
weakness and paralysis.
Fortunately, only a small percentage of people infected with WNV will end
up with these serious illnesses. The great majority, especially children,
develop a mild febrile illness that is similar to other viral infections.
Since its discovery in New York City in 1999, outbreaks of WNV infection
have occurred in most states in United States. The first report of
WNV in Canada occurred in Southern Ontario in the summer of 2001, when
it was discovered in infected birds. Last summer, outbreaks of human
disease were reported in Ontario and Quebec. Furthermore, infected birds
were found in Nova Scotia, Quebec, Ontario, Manitoba, and Saskatchewan.
More extensive outbreaks of human disease is expected all across Canada
within these few years.
Since birds are the natural hosts and many of them succumb to WNV
infection, surveillance of the virus in dead birds is the best way of
gauging the presence of infected mosquitoes in the area. It is important
to recognize that of the 74 known species of mosquito in Canada, WNV
has been found in 10 of these species. The most common ones that cause
human infection belong to the Culex mosquitoes.
Since infected mosquito is the main cause of human infection, the best
way to prevent WNV is by avoiding mosquito bites. This can be done
in two ways: reduce the number of mosquitoes in the environment, and
prevent them from biting us.
If we want to reduce the number of mosquitoes, we have to understand
how they breed. After mating with male mosquitoes, the females require
feeding with blood meals to produce fertilized eggs. These eggs are
then laid in still water, and hatched within 48 hours. The larvae feed
on micro-organisms in the water and transform into pupae and then adult
mosquitoes. The cycle from eggs to adult mosquitoes is about 4 days.
The most effective way to reduce mosquito population in a community
is by removal of still water. Even a small quantity of water left in
a toy bucket can allow hundreds of thousands of mosquitoes to develop.
Therefore, it is everyone's responsibility to check around the backyard
for items that can collect water and become breeding grounds for
mosquitoes.
If there are wading pools, bird baths, and pet bowls outside, the water
should be changed at least twice a week. If you are conscious of water
conservation and have rain barrels, put a fine screen on top so that
mosquitoes cannot get inside to lay eggs. If you have an ornamental pond,
you should put fish in there: mosquito eggs and larvae are nutritious
food for your fish, you won't have to feed them. Regularly inspect
gutters for leaves and other debris that can clog up the gutters and
provide good breeding spots for mosquitoes.
It is important to remember that mosquitoes do not just breed in your
backyard and stay there. They fly significant distance to seek food.
Therefore, we all have responsibility to prevent mosquitoes from breeding
in our backyard, which can become infected with West Nile Virus and
transmit infection to the neighbours. If everyone does his or her part,
we can reduce the number of mosquitoes that were merely nuisance before,
but now a carrier of potentially serious and fatal illness.
Although your children are unlikely to develop serious WNV infection,
it is still very important to prevent them from getting infected.
They should avoid going out around dawn and dusk, the two periods when
mosquitoes are most likely to bite. Wearing light colour clothing with
long sleeves and long pants as well as a hat can reduce the chance of
being bitten. If your child is in a stroller, put a screen around it.
Mosquito traps and ultrasound devices designed to distract mosquitoes
have not been proven effective as yet.
Most homes already have screens over windows and doors. It is important
to check and make sure all the screens are intact. The screen doors
should be able to close tightly so that mosquitoes cannot get through
the crack. More importantly, close the door as soon as someone gets
in or out so that mosquitoes cannot sneak into the house and wait until
everyone sits down for a meal or lays in bed, you are totally defenceless
inside your own home!
In spite of our best effort, there are times that we have to go out
when mosquitoes are around. We have to depend on mosquito repellents
to defend ourselves from those hungry creatures. The most effective
mosquito repellent available is DEET. There have been reports of side
effects, mostly from excessive use of this chemical. In Canada, products
with more than 30% DEET are not permitted any more, although occasional
bottles are still available in some stores. The effectiveness of DEET
depends on the concentration: 5% DEET can provide protection for 1 ½
hours, while 25% products are effective for 5 hours.
Parents should help young children when applying DEET. It should not be
put on their hands in case if they put them in the mouth. Don't spray
it on the face, always put it on the hands and then apply on the face.
I like to spray the repellent on the hat and clothing so that I don't
have to use as much directly on the skin. In this way one can reduce
direct exposure to the chemical and minimize the chance of any potential
harmful effect.
Since you are pregnant, it is even more important for you to prevent
being bitten by mosquitoes. There was one documented case of West Nile
Virus being transmitted from a mother to her foetus prior to delivery.
It resulted in a serious infection that was recognized after the baby
was born. We don't know how common is this form of transmission from
mother to foetus, however, you should use reasonable precaution to avoid
WNV infection during pregnancy.
Finally, I want to emphasize that we still have to enjoy the Canadian
outdoors by going to our national, provincial, and municipal parks in the
summer. With our long and cold winters, we should take every opportunity
to enjoy the outdoor environment as much as possible. We have to take
all the precautions and reduce breeding grounds for mosquitoes, but don't
allow the fear of West Nile Virus to control our lives. Nature is too
important to be missed. It can heal our souls, rekindle our spirits,
and prepare our bodies to face the next winter which for sure will come,
whether we like it or not.
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