This page is part of the Guidance Document Repository (GDR).

Looking for related documents?
Search for related documents in the Guidance Document Repository

All veterinary laboratories in Canada are required to report to CFIA upon suspicion or diagnosis of WNV in all domestic animals species including horses by forwarding pertaining information directly to the Animal Disease Surveillance Unit. E-mail: or by facsimile at 450-768-0064.

Under the Health of Animals Act, the CFIA does not have any obligation or commitment to take action when told of diseases listed as notifiable. The CFIA will gather that information primarily to meet Canada's international obligations for surveillance of diseases in livestock and for public health purposes.

Interpretation - West Nile Virus results reported to the Canadian Food Inspection Agency

The clinical manifestation of WNV virus infection may resemble some CFIA's other reportable diseases. In horses (rabies) and poultry (Newcastle Disease and Notifiable Avian Influenza) these must be ruled out in situations where these species show neurological signs of disease. CFIA must be contacted to ensure that no other disease that could be foreign to Canada is occurring.

When reporting on West Nile Virus (WNV), the CFIA's Animal Disease Surveillance Unit will use the following criteria to classify the various results into categories:

Positive Case

Compatible clinical signsFootnote 1 plus one or more of the following:

  • isolation of West Nile virus from tissuesFootnote 2;
  • an associated 4-fold or greater change in IgG ELISA testing or sero neutralization (SN) test antibody titre to WNV in appropriately-timedFootnote 3, paired sera;
  • detection of IgM antibody to WNV by ELISA testing in serum or cerebrospinal fluid (CSF) - see first assumption below;
  • 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

Compatible clinical signs plus one of the following:

  • elevated titre to WNV antibody by SN test in serum or positive IgG ELISA test, but only one sample is available;
  • static IgG titres to WNV (SN test or ELISA) in appropriately-timedFootnote 3, paired sera.

A possible case will normally be retested to further determine its actual status.


An animal for which a single positive SN or IgG test 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.

Other Possible Categories

Generally speaking and in practice, the most frequent categories will be Positive Case,Possible Case and Reactor.

Suspicious: Clinical signs only

A suspicious case will only be reported by a laboratory that does not perform any testing for WNV and would have eliminated other possibilities (rare). In this case, the owner or the practitioner would not have asked for or authorized further testing.

Non-case: A case that was classified as suspicious or was recorded in another category, but for which another cause (which is not an immediately notifiable disease (IND)) was found to explain the presence of the clinical signs.

The non-case category is only meant to take into account the possibility that a case that was classified as a suspicious was later found to have had been affected by another disease that is not another IND (see above). In cases where it is later found that the disease is another IND, the WNV suspect case will be reclassified in the IND database under this other IND and dropped from the total count of WNV records.


Assumptions on which case definitions are based:

  • ELISA testing may be slightly nonspecific; cross reactions to closely related flaviviruses (e.g. SLE virus) may occur, but in Canada, the possibility of exposure to a "closely related flavivirus" is small enough to not consider it unless in very specific circumstances.

  • IgM antibody in equine serum is relatively short-lived; a positive ELISA means exposure to WNV or a closely related flavivirus has occurred (see first assumption), very likely within the last three months.

  • IgG antibodies may not be present in equine serum until two weeks or more after exposure to WNV; it is possible that clinical signs may be present in an equine before a serum is positive.

  • IgG antibodies detected in serum indicate past exposure to WNV; equine exposed to WNV in previous years may test positive.

Date modified: