IN THIS ISSUE

This is the second issue of the Ontario WNv Bulletin. This bulletin is intended to provide a summary of results of the various surveillance activities in Ontario, as well as update readers on surveillance findings elsewhere in Canada and the U.S.

Another successful week of Sentinel Chicken Surveillance was carried out. All 18 sites located in the six potentially high-risk areas of Brockville (BR), Picton (PI), Guelph (GU), Niagara (NI), Long Point (LP) and Point Pelee (PP) were visited.

For the week ending July 7, blood samples were collected from all 18 sites in all six regions. No further difficulties in transport of samples were reported. Laboratory analysis has been completed for all samples collected the week ending June 30. No sentinel chickens in Ontario have tested positive for WNv.

Efforts to establish passive enhanced and active surveillance in Ontario have been proceeding. On July 5, a letter was sent to Ontario physicians in selected specialties (emergency medicine, critical care, infectious disease, medical microbiology, neurology) and public hospitals (chief executive officers and infection control practitioners) notifying them of the possibility of WNv in the province this season.

Enrolment of sentinel hospitals for the active human meningo-encephalitis surveillance system has been progressing well. As of July 13, 2000, 35 hospitals have agreed to participate. These hospitals have been sent binders providing detailed instructions on how to report cases of viral meningo-encephalitis cases, what samples are needed for testing and where they are to be sent along with background information on WNv. A total of 60 hospitals are targeted for enrolment into the active human surveillance system.

Mosquito surveillance includes the monitoring of the composition and abundance of local mosquito populations in 18 sites spread across six regions in Ontario. It is important to remember that, the mosquito species Culex pipiens is the primary vector for WNv and only female mosquitoes bite and therefore have the potential to transmit the virus. For the week ending July 7, mosquitoes were collected from all 18 sites.

Analysis of variance over the six-week period of total mosquito counts between sites within each region indicated no significant differences in mosquito counts. However, one site in each of the Brockville (BR-1), Long Point (LP-3) and Picton (PI-3) regions showed markedly higher counts of mosquitoes than other sites within the same region (see Figure 1). The lack of statistical significance is likely due to the small number of samples available for analysis. As the mosquito season progresses these particular sites may warrant particular attention if the WNv is identified in Ontario. There was an increase in total mosquito counts in 10 (55.6%) of the sites when compared to the previous week. The total mosquito counts from the three sites of each of the six regions were pooled for further analysis.

The table below provides a summary of findings during the first six weeks of collection. Regression analysis indicated that the Brockville and Picton regions had a significant increasing trend over the six-week period (p=0.03, 0.01 respectively). However, the proportion of females did not show a significant increase over the six-week collection period in any of the six regions. The data indicated that Brockville was the only region that indicated a significant increasing trend of the proportion of female mosquitoes that are Culex pipiens (p=0.02). There were no other significant increases over the six-week period in either proportion of female mosquitoes or proportion of Culex pipiens species in any of the six sites.

Table 1

During the months of May, June, and the first week of July, 20, 40 and 11 dead bird carcasses were collected, respectively. Figure 2 shows the weekly submissions of dead bird carcasses to the week ending July 7. The figure indicates a slight drop in number of submissions in the last two weeks of June, but the first week of July does show an increase. Health unit participation in dead bird surveillance to the week ending July 7, has resulted in 28 submissions (each submission could involve multiple carcasses) by 15 (40.5%) health units.

As of July 7, 71 bird carcasses have been submitted for examination. Of these 71, 29 (41%) carcasses were classified as not requiring further examination after initial post mortem, with 19 (90%) having a firm alternative diagnosis. For 42 (59%) of the carcasses, autopsy results indicated the need for further examination. To date histology results are available for 25 (48%) of the specimens requiring further examination. Histology results indicated that for 13 (52%) specimens, no further WNv testing was required because a conclusive alternative diagnosis was found. For 7 (28%) specimens, histology testing revealed no characteristic WNv lesions. As of July 7, there have been no confirmations of WNv in dead bird specimens collected in Ontario.

Comments, questions and suggestions about surveillance results in this bulletin
should be directed to :

Public Health Branch
Sheila Artes
WNv Surveillance Coordinator
Disease Control Service

Tel : 416-327-3508
E-mail : sheila.artes@moh.gov.on.ca

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