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West Nile virus: surveillance

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West Nile virus (WNV) is an immediately notifiable disease under the Health of Animals Regulations (HAR). Therefore, all veterinary laboratories in Canada are required to report to the Canadian Food Inspection Agency (CFIA) upon suspicion or diagnosis of WNV in all animals, including horses by forwarding pertinent information directly to the Animal Health Epidemiology and Surveillance section via e-mail:

Case definitions

When reporting on WNV, the CFIA's 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 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 above) 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.


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.
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