Human surveillance for West Nile virus (WNv) consists of collecting information from human cases of WNv. This information is then used in many ways. Human surveillance allows us to know where cases are occurring, enabling health care workers and the general public to be on alert. Furthermore, human surveillance can also help us understand the risk factors associated with contracting the disease, and factors influencing the outcome of the disease.

A human case is identified when a person visits a physician and the symptoms of West Nile virus infection are identified. The health care provider then submits a blood sample to the Ministry of Health and Long-Term Care (MOHLTC) Central Public Health Laboratory (CPHL). The first test performed is the IgM ELISA (Enzyme-Linked Immunosorbent Assay), which, if positive, is run once more to rule out false positive results. These two tests may be additionally followed by the PRNT (Plaque Reduction Neutralization Test) to confirm the diagnosis. If the IgM ELISA tests are positive, the patient is advised that they are a 'Probable' WNv case. The ELISA results are available within 24 hours, while the PRNT confirmation testing takes an additional 7 days.

If the laboratory tests are positive, the laboratory notifies both the local public health unit and the Public Health Division of MOHLTC. Detailed interviews of the patient are conducted by the health unit staff. If the person has given/received blood or organs, public health authorities notify Canadian Blood Services so that appropriate measures can be taken to protect our blood supply.

The chart below shows probable and confirmed positive WNv tests on humans in Ontario in 2004. Surveillance statistics are current as of 5:00 p.m. EST Monday through Friday.

*Probable cases refer to patients with two positive IgM ELISA (enzyme-linked immuno sorbent assay) tests
**Confirmed cases refer to patients with two positive ELISA tests AND either a positive confirmatory PRNT (plaque-reduction neutralization test) OR three previous cases have been confirmed by PRNT in the same health unit this year
‡ May have been exposed during travel
***Travel case

*  Probable cases refer to patients with two positive IgM ELISA (enzyme-linked immuno sorbent assay) tests
**  Confirmed cases refer to patients with two positive ELISA tests AND either a positive confirmatory PRNT (plaque-reduction neutralization test) OR three previous cases have been confirmed by PRNT in the same health unit this year
‡ Travel cases refer to patients with a history of travel out of Ontario in 15 days prior to symptom onset, while endemic refers to cases with no travel history out of Ontario three weeks prior to symptom onset. Also, a case is called endemic if the case has been in both a region of Ontario with WNv activity and an area outside of Ontario with WNv activity in the three weeks prior to symptom onset.
***  One case with Week of Onset August 8, 2004, and one case with Week of Onset September 5, 2004 may have been exposed during travel out of the province.

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