West Nile Virus

West Nile virus (WNV) belongs to a family of viruses called Flaviviridae. It is spread by mosquitoes that have fed on the blood of infected birds. WNV can infect a variety of mammals, including horses, humans and domestic and wild birds (particularly crows, blue and grey jays, ravens and magpies).

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Risk to human health

An infected mosquito transmits the virus by biting a human. Most people infected with the virus either have no symptoms or flu-like symptoms. Rarely, people can become very ill resulting in hospitalization and even death.

There is more information about WNV in people on the Public Health Agency of Canada (PHAC) WNV page.

Almost every province of Canada has had human cases of WNV.

Signs

Animals (particularly horses) infected with the virus may show the following clinical signs:

  • ataxia (lack of coordination)
  • depression or lethargy
  • fever
  • head pressing
  • head tilt
  • impaired vision
  • inability to swallow
  • loss of appetite
  • muscle weakness or twitching
  • partial paralysis
  • coma
  • death

Horses and humans show signs of disease more than other mammals.

Most wild birds show no clinical signs. However, birds such as crows, ravens, blue and grey jays are very susceptible to infection with WNV. They usually die once infected.

In the domestic bird population, chickens and turkeys usually show no signs of infection. Geese often show neurological signs.

The clinical signs of WNV can be confused with rabies in mammals. It can look like Newcastle Disease (ND) and Avian Influenza (AI) in domestic birds.

Where it is found

Europe, Africa and Asia have reported cases of WNV. In 1999, the virus was found in the U.S. for the first time. It was found in Canada in 2001. In 2003, some South American countries reported their first WNV cases.

The Canadian Wildlife Health Cooperative and the PHAC monitor the progression of WNV in Canada. You can find reports of surveillance information on these websites.

Transmission and spread

WNV is usually spread by the bite of a mosquito that has fed on an infected bird. Mosquitoes transmit the virus from wild birds to mammals and domestic poultry.

Very rarely, the virus can be spread through contact with infected animals, their blood or other tissues.

Diagnosis

In horses and geese, neurological signs may suggest infection. Only laboratory tests can confirm the diagnosis of WNV.

During the outbreak in 2001, scientists used crows as an indicator of virus spread. Community members reported findings of dead crows. Scientists used this information to track the distribution of the disease in Canada.

Treatment

There is no treatment available to kill the virus. Fluid therapy and anti-inflammatories can reduce the severity of clinical signs.

Horses can be vaccinated to prevent infection. There are currently several WNV vaccines for horses in Canada. The vaccines need to be given every year for continued protection. Contact your veterinarian for more information on WNV vaccines for your horse.

Vaccinated horses may test positive on certain blood tests. This may affect their eligibility for export to countries that require negative blood test results for the virus. Some other countries require that horses be certified as vaccinated against the virus prior to import. For information on specific import/export requirements, contact your Canadian Food Inspection Agency (CFIA) district office.

Canada's role in protecting livestock from WNV

We consider WNV a domestic disease, meaning that it is commonly found in Canada. Your vet will submit samples to a provincial or other lab for diagnosis. You will have to pay for WNV testing if your vet suspects it.

This means that all laboratories must notify us when they suspect or diagnose this disease.

Response

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, we 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.

We have prepared documents on how it will respond to situations where neurological clinical signs are reported in horses and domestic poultry. The main objective of these documents is to provide guidelines on how we will manage situations where the primary task for the agency would be confirmation or exclusion of the reportable diseases which clinically could resemble WNV (for example rabies in horses or ND in avian species).

Response to central nervous system signs (CNS) in domestic geese and ducks in West Nile Virus endemic areas

Epidemiology and pathogenesis of West Nile Virus

Domestic geese

From the limited experience in the literature, it appears that geese are most susceptible to WNV infection at a young age. Morbidity and mortality rates can be as a high as 30% to 40% in some flocks. The incubation period for WNV in geese is established to be 2 to 3 days, with most mortalities occurring 5 to 10 days post infection (PI). Transient viremia develops 1 to 5 days PI with peak virus titers 104-6 2 days PI. This level of viremia is of sufficient magnitude to infect mosquitos, meaning the geese can serve as a reservoir and amplifying host for WNV. WNV was isolated from the brain, heart, kidney, and intestine during 5 to 10 days PI, and from the oropharyngeal swabs of clinically ill birds. The viremic stage in geese coincides with the manifestation of clinical signs of infection. Infected geese will show signs of depression, weight loss, torticollis, opisthotonous and rhythmic side-to-side movement of the head. These clinical signs cannot be distinguished from Newcastle disease (ND) and highly pathogenic avian influenza (HPAI) - both reportable diseases of poultry. The WNV titer of infected geese is declining by 10 days PI, with no WNV isolated from plasma or tissues from goslings 21 days PI, which also corresponds with the disappearance of clinical signs.

Ducks

They may become infected with WNV; however, limited data indicate that ducks have a low to moderate viremia of short duration. The pathogenesis of WNV infection in ducks is unknown at this time. However, there is no scientific evidence that domestic ducks could pose a threat of WNV infection to humans when handled at slaughter.

Flock investigation

Geese flocks that exhibit any neurological signs must be reported to us for investigation. A CFIA veterinarian will examine the flock to rule out ND and HPAI, in accordance with Foreign Animal Disease Strategy protocols established for these 2 diseases. Once the ND and HPAI have been ruled out, the quarantine will be released, and the owner and the attending private or provincial veterinarian may pursue confirmation of WNV diagnosis. The district veterinarian will notify the owner in writing of the necessity that his/her geese be free of neurological signs for 30 days before scheduled slaughter in registered establishments in Canada, or 90 days before being eligible for export. Prior to shipment for slaughter to a federally registered establishment, geese that originate from flocks with confirmed or suspected WNV will require certification by a licensed veterinarian, stating that they have been free of clinical signs for 30 days. An attestation by the producer, must be submitted to the slaughtering establishment prior to shipment of the affected flock.

ND and AI in geese and ducks, and WNV infection in geese are potentially pathogenic to humans, and thus any suspicion of these diseases must be communicated to the provincial health authorities, as well as the Provincial Department of Agriculture.

Note: WNV is an immediately notifiable disease under the Health of Animals Regulations. All persons involved in the handling of these animals should follow occupational health and safety standards for handling slaughter animal species, as recommended in the document West Nile Virus - Protect Yourself! by Public Health Agency of Canada.

Response to central nervous system signs in horses in West Nile Virus endemic areas

Clinical signs of WNV in horses could be indistinguishable from those seen in rabies. Thus, any horses showing neurological signs suggestive of and clinically indistinguishable from rabies have to be reported to us for investigation.

The following provides a brief description of the epidemiology and pathogenesis of WNV in horses and based on this, the course of action we will take to fulfill its mandate of protecting human and animal health.

Epidemiology and pathogenesis of West Nile Virus in horses

Natural WNV infections in horses have been reported in Europe, Africa, the Middle East and recently in North and South America. It appears that WNV affects horses of all ages, breeds and sexes. Its occurrence in North America is seasonal and coincides with the presence of the mosquito vector. Most cases of WNV in horses are reported from mid-August to late October. It is estimated that between 10 to 40% of horses in endemic areas can be infected with WNV, but only 8% of these will manifest clinical signs of the disease. The WNV incubation period is usually from 5 to 15 days with a low level of transient viremia less than 102.5 plaque-forming unit per millilitre (PFU/ml) of serum (range 101.0 to 103.0) developing 1 to 2 days post infection. Four to 8 days following the infection, the WNV is no longer detectable in the blood of infected horses. Neurological signs may become apparent from 5 to 22 days post infection and most horses are usually not viremic at that stage.

The clinical signs of WNV in order of their frequency include:

  • ataxia
  • weakness of limbs
  • recumbency
  • muscle fasciculation
  •  fever
  • paralyzed or drooping lip
  • twitching face or muzzle
  • teeth grinding
  • blindness
  • traumatic lesions of the forelimbs and head due to compulsive movement

WNV in horses does not result in any gross pathological lesions and the virus can only be isolated from the brain and spinal cord of clinically ill horses. Approximately 60% to 70% of horses with clinical signs may fully recover.

Considering the sporadic occurrence of WNV-associated diseases in horses, the development of low-magnitude and short-duration viremia, as well as limited amount of antigen detected in CNS tissue, horses are considered an incidental and dead-end host of WNV. Consequently, horses do not play a significant role in the epidemiology of WNV and do not pose a risk to humans. Slaughter of clinically healthy horses under normal circumstances does not constitute any WNV health hazard to inspection staff or plant employees.

Field investigation

All horses showing neurological signs indistinguishable from rabies must be reported to us. A CFIA veterinarian will investigate those cases to rule out rabies as per internal procedures. After rabies has been ruled out (horses surviving 7 days post the onset of neurological signs), the horse will be released from quarantine and it will be up to the owner and the attending private veterinarian to pursue eastern equine encephalomyelitis/western equine encephalomyelitis (EEE/WEE) and WNV diagnosis if they wish to do so.

Once they have recovered from clinical signs of WNV, horses may be sent to slaughter without any further restrictions.

Registered establishment investigation

All horses showing CNS neurological signs on ante mortem inspection will be placed in the subject pen and will be dealt with as per internal procedures. Also, if a reportable disease is suspected, the veterinarian- in-charge will follow the internal guidelines for dealing with other reportable diseases in registered slaughter establishments.

As in the field investigation situations, a CFIA Veterinarian will investigate these cases to rule out rabies. If for humane reasons the animal must be condemned immediately, blood samples can be drawn for EEE/WEE and WNV testing prior to euthanasia. Brain tissue will be collected for rabies diagnosis. Appropriate tissue can be collected as a courtesy and, along with the blood samples and held by the District Veterinarian should the owner wish to pursue a diagnosis of EEE/WEE or WNV.

Response to West Nile Virus in slaughtered birds of the order of Galliformes (commercial chickens, turkeys, guinea fowl, pheasants, quails), rheas, emus and ostriches

Although no natural infection or clinical signs of WNV have been reported in domestic commercial poultry (Galliformes), and only sporadic cases of WNV infection have been reported in ostriches, we will treat all situations where domestic birds show neurological signs as having the potential for hosting a foreign animal disease (FAD), such as ND or HPAI. We will investigate them accordingly, as per appropriate FAD disease strategies.

Pathogenesis of West Nile Virus in commercial chickens and turkeys

Experimentally, WNV infected chickens and turkeys have not shown observable clinical signs of infection during the 21 days post infection (PI) observation period. Chickens tend to develop high level of viremia with virus titers as high as 105.0/ml at fourth day PI, and WNV was isolated from blood plasma up to 8 days PI. Turkeys, however, develop viremia from 2 to 10 days PI, but the average level of viremia is low and the virus could not be isolated from plasma. WNV was also isolated from myocardium, spleen, kidney, lung and intestine collected from chickens at 3, 5 and 10 days PI; however no virus was isolated from chickens after 10 days PI. No virus was isolated from similar organs of infected turkeys. WNV does not cause gross pathological lesions in experimentally infected chickens and turkeys. After 3 weeks of age chickens appear to be resistant to natural WNV infection, thus commercial chickens reaching slaughterhouse at approximately 37 to 40 days of age should have passed the viremia stage and be free of virus in an unlikely event of being infected with WNV before 3 weeks of age.

Considering the remote possibility of natural WNV infection in chickens and turkeys, combined with experimental evidence that turkeys develop negligible levels of viremia and chickens are refractory to WN infections past 3 weeks of age, we can conclude that neither of those classes of poultry pose a risk of WNV transmission to humans.

Note: in 2002, the Wisconsin Division of Public Health (WDPH) investigated and found a high prevalence of WNV antibody among some farm turkey workers. Although, the mode of transmission to these workers remains unknown, the assumption was made that transmission by less typical routes might have occurred. Despite uncertainty, epidemiological evidence suggested that, in this particular situation, percutaneous injury or fecal-oral, or respiratory exposure to aerosolized-infected turkey feces could have caused human infection.

Other Galliforme slaughter birds (guinea fowl, pheasants and quails)

Limited research and indirect evidence suggest that the pathogenesis of WNV in these classes of poultry is similar to that seen in commercial chickens. There is no evidence that people become infected with WNV by handling these birds in a normal slaughterhouse environment.

Rheas, emus and ostriches

There is limited knowledge of natural WNV infection occurring in these birds, and only few cases of naturally occurring WNV infection in these species have been reported worldwide. To date, there have been no recorded cases of these birds passing WNV infection to other animals or to people.

Surveillance

When reporting on WNV, our Animal Health Epidemiology and Surveillance section will use the following criteria to classify the results.

Confirmed positive case

A positive case must have compatible clinical signs including ataxia (in other words stumbling, staggering, wobbly gait or incoordination) or at least 2 of the following: circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle fasciculation, proprioceptive deficits, blindness, lip droop/paralysis, teeth grinding, fever, acute death.

Plus 1 or more of the following:

  • isolation of West Nile virus from tissuesFootnote 1
  • detection of IgM antibody to WNV by the Enzyme-Linked Immunosorbent Assay (ELISA) testing in serum or cerebrospinal fluid (CSF)Footnote 2
  • an associated 4-fold or greater change in IgG ELISA testingFootnote 3 or sero neutralization (SN) test antibody titre to WNV in appropriately-timed, paired seraFootnote 4
  • a positive polymerase chain reaction (PCR) to WNV genomic sequences in tissues and appropriate histological changes
  • a positive immuno-histochemistry (IHC) for WNV antigen in tissue and appropriate histological changes

Possible case

In a "possible" case, the animal must present compatible clinical signs of the disease (see previous section) plus 1 of the following conditions:

  • in the case where only 1 serum sample is available, the analysis shows elevated titre to WNV antibody by SN test in serum or positive IgG ELISA testFootnote 3
  • In the case where appropriately-timed, paired seraFootnote 4 are submitted, the samples present static titres to WNV (SN test or IgG ELISA)

It should be noted that the animal will normally be retested to further determine its actual status.

Reactor

An animal for which a single positive SN or IgG ELISAFootnote 3 is reported, if there is no information regarding the presence of clinical signs or if it is reported that the animal didn't show any clinical signs, will be classified as a reactor.

Such animal will be deemed to have been tested for other reasons than clinical signs, such as for surveillance, export or for verification of post vaccination protection.

Suspect case

A suspect case will only be reported by a laboratory that did not perform testing for WNV and has ruled out other possible diagnosis. Testing may not have been performed because it is unavailable or if the owner or the practitioner would not have asked for or authorized further testing.

Additional information

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