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Column originally published May 27, 2003

A Few Simple Tips To Prevent West Nile Virus Infection

Question: 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?


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 to 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.