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West Nile Virus

What is West Nile Virus?

West Nile Virus (WNV) is known to cause West Nile Fever, West Nile Virus Disease or West Nile Encephalitis, a disease that affects wild and domestic birds and a variety of mammals including horses and humans. WNV is a member of the Flaviviridae family, which includes the viruses responsible for Japanese encephalitis, St. Louis encephalitis and Yellow fever, all transmissible by mosquitoes. The Corvidae family of birds such as crows, blue and grey jays, ravens and magpies are particularly susceptible to illness and death from WNV. The West Nile virus geographical distribution is deemed to be best monitored through wild birds surveillance. The Canadian Cooperative Wildlife Health Centre is the main contributor for wildlife disease surveillance in Canada and is already a major participant in monitoring the progression of the virus in Canada. To view Canada’s surveillance efforts on the Internet visit the Canadian Cooperative Wildlife Health Center, West Nile Virus Surveillance Program 2003 or the Health Canada West Nile Virus Surveillance web site.

What is the geographical distribution of West Nile Virus?

The WNV was recognized in many parts of the world, and its geographical distribution includes Europe, Africa and Asia. In 1999, WNV was identified in the United States for the first time, and in 2001 its presence was confirmed in Canada. Some South American countries reported their first WNV cases in spring of 2003.

What is the history of West Nile Virus in Canada?

During 2001, WNV was detected for the first time in Canada. Surveillance efforts by Health Canada and its partners confirmed the presence of the virus in dead birds and mosquito pools. Since then, more positive birds have been found in several provinces. In late summer 2002, the first cases of WNV in horses were reported in Quebec, Ontario, Manitoba and Saskatchewan. During the same time, the first case of WNV in North America was identified in domestic geese in Southern Manitoba. The Ontario Ministry of Health announced the first suspect human cases of WNV in its province in August, 2002. In 2003, the CFIA included WNV in its list of Notifiable Diseases, which means that veterinary laboratories are required to report to the CFIA any positive cases in domestic animal species, including horses. This data is compiled and forwarded to Health Canada for inclusion in its WNV data management system. In 2003, combined presumptive and confirmed positive cases in horses amounted to 445 across 6 provinces: Nova Scotia, Quebec, Ontario, Manitoba, Saskatchewan and Alberta.

For the past and current information on the number of WNV confirmed or suspected cases and the species involved in Canada, see the Public Health Agency of Canada web site.

What is the transmission cycle of West Nile virus?

WNV is one of several viruses known as arthropod-borne viruses or arboviruses, because all are transmitted among vertebrate hosts by insect vectors such as mosquitoes and ticks. The wild bird population is the principle host of the WNV. The main vector of WNV is various species of mosquitoes which primarily feed on birds. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood. Infected mosquitoes may then transmit WNV to other animals or humans while biting to take blood. The virus is located in the mosquito's salivary glands. During subsequent feeding, the virus may be injected into the animal or human blood stream, where it may multiply and possibly cause disease.

Does West Nile virus affect animals other than birds?

Although the wild birds are the principle hosts of WNV, the virus has rarely been isolated from mammals such as camels, cattle, horses, dogs, sheep, mountain goats, cats, chipmunks, skunks, squirrels, bats and domestic rabbits. In contrast to birds, infected mammals generally do not carry much virus in their blood and thus do not act as effective sources of infection for mosquitoes.

What domestic animal species can be affected by West Nile Virus?

Research indicates that many domestic poultry species, at young age, can get infected with WNV, but usually the level of viremia in domestic poultry is low and of short duration. Domestic mammals, including horses do not carry much virus in their blood and thus do not act as an effective source of infection for mosquitoes. Similar to humans and other mammals, horses are generally considered a "dead end" host for WNV. For more comprehensive information on the epidemiology and pathogenesis of WNV in horses, domestic birds of the order of Anseriformes (Domestic Geese and Ducks), and of the order of Galliformes (Commercial Chickens, Turkeys, Guinea fowl, Pheasants, Quails) as well as Rheas, Emus and Ostriches, please refer to the Canadian Food Inspection Agency's Role In West Nile Virus Activities. The CFIA posts detailed information about its response to West Nile Virus and links to Health Canada on its Web site.

What are the clinical signs of West Nile infection in domestic species?

Horses infected with WNV show neurological disturbances which may include stumbling or tripping, muscle weakness or twitching, partial paralysis, loss of appetite, depression or lethargy, head pressing or tilt, impaired vision, inability to swallow, fever coma and death.

Most infected domestic birds do not show symptoms of WNV infection, and only domestic geese appear to be particularly susceptible to disease and/or death when infected. WNV infected geese will show signs of depression, loss of appetite, inability to stand, weight loss and death. The clinical signs of rabies in horses can be confused with WNV and in domestic birds WNV can be difficult to be distinguished from Newcastle Disease and Avian Influenza. Thus, if you see these signs in your animals, contact your veterinarian immediately.

How is West Nile Virus diagnosed?

Birds, even though infected, rarely display clinical signs. In North America, the mortality rate in crows and wild birds may be an indicator of WNV activity. In horses and domestic poultry, and especially geese, any neurological signs may be suggestive of WNV infection. Usually a blood test (Vec Test capture ELISA and PCR) or post mortem examination confirms the WNV diagnosis.

Is there any treatment for animals that become infected with West Nile Virus?

The main treatment is supportive therapy to reduce the severity of symptoms. If your animal shows any symptoms that may be associated with WNV infection, contact your veterinarian.

Is there any West Nile Virus vaccine available for animals in Canada?

Currently there are two West Nile Virus vaccines registered for use in horses in Canada.

The Recombitek is a West Nile Virus vaccine produced by Merial and the Fort Dodge (Wyeth) manufactures Killed West Nile Virus vaccine, and the Canadian importer, Wyeth markets it under the trade name of West Nile - Innovator.

Both of those vaccines require two injections, several weeks apart for primary immunization, and a booster injection is required annually for continued protection. For more information on the WNV vaccines for horses, you may wish to review the Fort Dodge (Wyeth) web site, and the Merial web site.

Note:

Vaccinated horses may develop antibodies to WNV, which may affect their eligibility for export to countries that require negative blood test results for this disease. Some other countries require that horses be certified as vaccinated against West Nile Virus prior to import. Consult your veterinarian if you are planning to travel internationally with your horse.

How can we reduce the risk associated with this virus?

Health Canada publishes an information sheet on this subject. The fact sheet " West Nile Virus - It's Your Health fact sheet" also gives advice and describes the type of surveillance that is being done in order to inform the public on the risk of infection. You may want to consult the document.

What is Canadian Food Inspection Agency’s role?

WNV is an immediately notifiable disease under the Health of Animals Act. This means that all laboratories are required to contact CFIA upon suspicion or diagnosis of this disease. Information pertaining to immediately notifiable diseases without delay must be forwarded directly to the CFIA’s Disease Surveillance Unit by e-mail or by facsimile at 613-228-6675.

For more information, on the immediately notifiable diseases please consult the Department of Justice Canada web site.

Detailed information on the CFIA’s role in and the Agency’s response to West Nile virus activities, with all the pertinent links can be viewed.

You can also contact the nearest CFIA district veterinarian using the listing in the blue pages of your local telephone directory.

Note:

The WNV must be considered as a differential diagnosis to other reportable diseases in horses (rabies) and poultry (Newcastle Disease); these diseases must be ruled out whenever the CFIA is contacted with neurological symptoms in horses and domestic poultry to ensure that other diseases are not occurring, that could be foreign to Canada.

In certain cases, after reportable diseases have been ruled out, if clinical signs of WNV are found in domestic species that are raised for food production, the CFIA will still play a role in ensuring that the animals are allowed to be slaughtered only after they have recovered completely and a period of time has passed after the clinical signs have ended.

Does the Canadian Food Inspection Agency test for West Nile Virus?

Now that the first confirmed cases of WNV in horses and domestic geese in Canada have been reported, the WNV is considered an indigenous disease in Canada, therefore the responsibility for further WNV diagnoses resides in provincial or other laboratories that can offer the test on a fee basis. Because of the zoonotic nature of the disease, CFIA District Veterinarians will provide information to veterinary practitioners, horse owners and poultry producers and the public. CFIA's response to the incursion of WNV in the domestic animal species population is centred around education and prevention.

Is it safe to handle or consume animals or animal products possibly infected with West Nile virus?

For information on the safe handling of various animal species in the wild or slaughter houses and issue related to food safety, please consult Health Canada’s web sites at the following addresses :

• Health Canada West Nile Virus & Animals, Can people get West Nile Virus from eating birds or animals that are infected with West Nile Virus?
• Occupational Health Advisory: Biosafety Information, West Nile virus, Workplace Health and Public Safety Program, Public Health Agency of Canada.

Media Inquiries

Canadian Food Inspection Agency

Dr. Andre Valliere: 514-349-3815

Health Canada

Media Relations: 613-228-6682
Media inquiries:
Aggie Adamczyk: 613-941-8189
Public inquiries: 613-957-2991