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West Nile Virus

What is the West Nile virus?

West Nile virus is mainly spread to people through the bite of an infected mosquito. Mosquitoes transmit the virus after becoming infected by feeding on the blood of birds which carry the virus. The disease has been and continues to be a risk in Canada every year.

How is it spread?

Mosquitoes become infected with West Nile virus when they feed on the blood of infected birds. Birds such as crows, blue jays, and sparrows are particularly susceptible to the virus. The virus is carried in the mosquito’s salivary glands and it is transmitted when mosquitoes bite humans and animals. The virus is not contagious and is not spread from person-to–person contact except in extreme cases such as blood transfusions.

In general, temperature is the major influence on the level of WNV activity. If the weather remains warm, with enough rain, Ontario could experience more WNV activity. Mild winter conditions allow more mosquitoes that are carrying the virus to survive into the next year. If the season turns out to be hot, with little rain, then Ontario would likely see lower numbers as there would be fewer pools of stagnant water for mosquitoes to breed in. If the weather is cool, but there is enough rain, Ontario could see some cases but not as high as with hot weather. There is no way to predict how many people will be infected by the West Nile virus in any given year.

Who is at risk?

People of all ages can become infected with West Nile virus however most will have a mild illness. Of those who are infected, 80% have no symptoms. About 20% of those infected will experience West Nile fever, which may consist of fever, headache, muscle ache and rash. The most serious illnesses caused by West Nile virus are encephalitis and meningitis, which only occur in approximately 1% of infections. The risk of severe illness increases with age. Those with chronic diseases, weakened or developing immune systems are also at greater risk for severe illness.

West Nile Virus Transmission Cycle

In nature, West Nile virus cycles between mosquitoes (especially Culex species) and birds. Some infected birds, can develop high levels of the virus in their bloodstream and mosquitoes can become infected by biting these infected birds. After about a week, infected mosquitoes can pass the virus to more birds when they bite. Mosquitoes with West Nile virus also bite and infect people, horses and other mammals. However, humans, horses and other mammals are ‘dead end’ hosts. This means that they do not develop high levels of virus in their bloodstream, and cannot pass the virus on to other biting mosquitoes.

Click here for a visual of the West Nile Virus Transmission Cycle

What are the symptoms of West Nile virus?

West Nile virus is usually a mild disease with signs and symptoms that may include fever, headache, muscle ache, swollen lymph nodes, and skin rash. In some cases the disease may progress to a more severe stage with signs and symptoms that can include disorientation, coma, and even death. In its most serious state, West Nile virus infection can cause an inflammation in the brain, otherwise known as encephalitis.

How is West Nile virus treated?

There are no antibiotics that are effective against the infection and there is no vaccine which humans can use to prevent themselves from becoming infected with the virus. Suspected cases are often hospitalized where supportive treatment can be provided.

What is the Mosquito Life Cycle?

Mosquitoes like to lay their eggs in standing water and they go through 4 stages of development: egg, larva, pupa and adult. In ideal conditions, a mosquito can complete its life cycle in little as seven days.

What is the egg phase of a mosquito?

Female mosquitoes lay their eggs from late spring until early fall. Hundreds of eggs are laid at a time in groups that float on the surface of the water. Most mosquito eggs hatch into larva within 48 hours.

What is the larva stage of a mosquito?

Mosquito larvae, commonly called wigglers because of the way they move, hatch from the eggs and feed on algae and other nutrients found in the water. They come to the surface frequently to obtain air. The larva becomes a pupa in 7-14 days.

What is the pupa stage of a mosquito?

The pupa stage of development is where the mosquito turns into an adult. This process takes 2-3 days where the pupa rests and doesn’t feed before turning into an adult.

What is the adult stage of a mosquito?

The adult mosquito will rest on the surface of the water to dry its wings. The female adult can live for several weeks, the male only about one week.

Do all Mosquitoes Bite Humans?

Only female mosquitoes bite humans as they need blood to develop their eggs. Male mosquitoes feed on nectar and plant juices.

Do all Mosquitoes transmit the virus?

There are specific species of mosquitoes that are most likely to spread West Nile virus to humans. Culex pipiens mosquitoes are the main transmitters of the virus. This species is most commonly found in an urban environment. Aedes vexans is another mosquito species that is an important transmitter of West Nile virus.

Mosquitoes need water to breed. Mosquito eggs left in stagnant water, even small amounts allowed to stand for a week or more, can develop into adult mosquitoes capable of flight. Reducing or eliminating standing water is an effective and economical way to control mosquitoes. Mosquito-breeding season runs from Mid-May until the end of September, so this is the ideal time to eliminate potential breeding sites. The following are methods for preventing stagnant water and thus mosquito breeding:

  • containers, buckets, pots, barrels, cans, wheel barrows, tires, children’s toys, etc. should be emptied, inverted, punctured, disposed of, or covered;
  • ensuring screens on windows and doors are tight fitting with no holes;
  • eaves troughs should be kept clean to allow proper drainage and to ensure that water does not stand;
  • rain barrels should have a tight fitting screen over the top to keep mosquitoes from breeding;
  • shaded flat roof tops should be properly drained to ensure that standing water is eliminated;
  • plastic covers on lawn chairs and other outdoor equipment should be drained weekly;
  • children’s tire swings should have a hole in the bottom so that water drains out;
  • tires should be stored inside a garage or shed, or disposed of through recycling programs;
  • change water in bird baths at least twice a week;
  • small boats and canoes should be stored upside down, and large boats should be covered to prevent accumulation of water with drains kept open;
  • swimming pools, spas, and hot tubs should be kept chlorinated. If not in use, they should be kept tightly covered, and rain water should be kept from accumulating on the cover;
  • ornamental ponds should have a pump that will keep water circulating, or fish that will eat mosquito larvae; and
  • lawns and shrubs should be trimmed and maintained as they are a common spot for mosquitoes to rest.

In 2003, the City of Peterborough created bylaw 03-107 to decrease the prevalence of stagnant water that can serve as breeding sites for mosquitoes that have the potential to carry West Nile Virus. The bylaw states that standing water may not be present on your property for more than four days. For more information on this bylaw please visit the City of Peterborough website.

Personal Protective Measures for Yourself and Your Family:

Personal protective measures that people can take to reduce mosquito bites include:

  • Staying indoors at dusk and dawn because mosquitoes are most active during these times;
  • Using a bug repellent containing DEET, following the manufacturer’s instructions;
  • Covering up with light coloured, long clothing. This includes long-sleeved shirts, long pants, and socks. Also, wear footwear that covers exposed skin.

Insect Repellents:

  • Apply a bug repellent to clothes and exposed skin when it is necessary to be outdoors;
  • Avoid using personal insect repellents on children less than six months of age;
  • When using a repellent containing DEET use the following guideline
  • Use a product with a low concentration if you are only going to be outside for a short time;
  • Re-apply, or use a higher concentration product for longer periods of time. Studies have shown that 30% DEET will provide protection against mosquito bites for 6.5 hours, and 10% will protect for 3 hours;
  • Use a product with no more than 10% DEET, not more than once per day on children between the ages of 6 months and 2 years;
  • Use a product with no more than 10% DEET, applied 3 times per day maximum on children ages 2 to 12 years, avoiding application of repellent to children’s hands and faces;
  • When applying repellent to your face, spray the repellent on your hands and then wipe it on to your face avoiding the eyes, nose, and mouth. Wash your hands following application.
  • Avoid spraying repellent in enclosed areas and breathing in the spray

Alternative products to DEET:

  • Repellents containing 2% soybean oil are an alternative that provide protection against mosquitoes for approximately 3.5 hours. There are no age or application restrictions for use of these products;
  • Products containing P-methane 3,8-diol will provide up to 2 hours of protection against mosquitoes and should not be used on children younger than 3 years of age or applied more than twice daily for those older than 3 years of age;
  • Avoid the use of citronella-based or lavender-based personal insect repellents due to the proposed phasing out of their use in Canada. For further information please check out Pest Management Regulatory Agency.

Each summer, Peterborough Public Health traps mosquitoes which are submitted for species identification and West Nile virus testing.

Why does Public Health trap mosquitoes?

The Public Health sets up mosquito traps as a part of mosquito surveillance in the City and County area. Mosquitoes that are caught in the traps are sent away for testing. During testing, the total number of mosquitoes collected can be counted, the species of mosquitoes can be identified, and most importantly, it can be determined if the mosquitoes are carrying the West Nile virus.

If I see a trap what should I know?

The mosquito traps encourage mosquitoes to fly to the area of the trap and away from people.

The traps are equipped with a light and dry ice that attracts the mosquitoes. The mosquitoes will then fly into the trap and are held in a net. Peterborough Public Health staff monitor and maintain the traps daily.

Does Peterborough Public Health collect dead birds?

Peterborough Public Health does not collect dead birds. In previous years dead birds were used to determine whether or not West Nile virus was in Peterborough and surrounding areas. However, now that it is known that West Nile is well established here, Peterborough Public Health traps mosquitoes for testing because they give a better indication of which specific areas have mosquito populations that carry the virus. For more information on the safe disposal of dead birds, please visit the Canadian Cooperative Wildlife Healthcare Centre website.

For current status of West Nile in Canada, please visit the Public Health Agency of Canada.

For current status of West Nile in the United States, please visit the Centers for Disease Control and Prevention