IN THIS ISSUE

This is the first 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 about results of surveillance elsewhere in Canada and the U.S. It will be produced every two weeks (or more frequently following the occurrence of positive findings in or near Ontario) until the end of the surveillance season in late October 2000.

Sentinel chicken surveillance has been successfully implemented in Ontario and is fully operational. Eighteen chicken coops with 10 chickens each are located in six areas, selected based on the migratory patterns of birds. These areas are : Brockville, Picton, Guelph, Niagara, Long Point and Point Pelee. Weekly blood collection began the week of June 5 and has been completed to the week ending June 30. Sampling has been complete in all but the Picton site, which submitted 47% of target samples due to problems with transport. Laboratory analysis has been completed to the week of collection ending June 23, and is negative by haemagglutination inhibition test.

Both passive enhanced and active surveillance is being established in Ontario. Enhanced passive surveillance : viral encephalitis and viral meningitis are reportable under Ontario Regulation 59/91 under the Health Protection and Promotion Act.

Ontario physicians in selected specialties (emergency medicine, critical care, infectious disease, medical microbiology, neurology) and public hospitals (chief executive officers and infection control practitioners) will soon be receiving a letter alerting them to the possibility of WNv in the province this season. They have been asked to consider this etiology in the differential diagnosis of meningo-encephalitis, especially in adults, and particularly if associated with profound muscle weakness. The mailing included recommendations for diagnostic specimen collection and submission to the Central Public Health Laboratory.

An active human meningo-encephalitis surveillance system has also been established the week of July 4 at 59 selected hospitals in Toronto-Peel, the university teaching hospitals and hospitals from six potentially high-risk areas in Ontario. Participating hospitals have been asked to report all possible cases of WNv Encephalitis weekly based on specific diagnostic guidelines. Results of surveillance will be reported in subsequent issues of this bulletin.

Since WNv is transmitted by mosquitoes, mosquito surveillance is also being undertaken in Ontario. This will permit the monitoring of the composition and abundance of local mosquito populations and allow a better understanding of the risk of infections to humans. Light traps positioned in proximity to the sentinel chicken coops have been set up, and mosquitoes are collected weekly and transported to the University of Guelph for enumeration and speciation. The mosquito species Culex pipiens is the primary vector for WNv. Only female mosquitoes bite and therefore have the potential to transmit the virus, if present.

Collection of mosquitoes was initiated on the week ending June 2, 2000. During the first week of collection, only 14 (77%) mosquito traps were visited. In the subsequent four weeks, mosquitoes were collected from all 18 sites except during the week ending June 23, 2000 when mosquitoes were collected from 15 (83%) sites. An analysis of variance of total mosquito counts between sites within each of the six areas indicated no significant differences in mosquito counts except in one instance. One site in the Brockville area had a significantly higher number of total mosquitoes as compared to the other two sites in the Brockville area (p<0.05). This difference was caused by a very high count of total mosquitoes in the final week of collection. It is possible that this was an outlier since counts from previous weeks do not exhibit a significant difference. Future collections at this site will provide more evidence to confirm if this was an atypical count. The total mosquito counts from the three sites of each of the six regions were pooled, since no other significant differences were present.

Figure 1 indicating the number of mosquitoes by week shows a general increasing trend in numbers of mosquitoes, which is to be expected as the season progresses. Although mosquitoes can breed year round, they are more common during the warmer seasons of summer and early fall. The figure below provides a summary of findings during the five-week collection period. Regression analysis indicated that only the Picton area had a significant increase in number of female mosquitoes over the five-week period (p=0.03). There were no other significant increases over the five-week period in either proportion of female mosquitoes or proportion of Culex pipiens species in any of the six sites.

Table 1

Dead bird carcasses in Ontario have been collected and tested for WNv since May 1, 2000. During the months of May and June, 20 and 37 dead bird carcasses were collected respectively. Figure 2 shows the weekly submission of dead bird carcasses for the first two months of enhanced passive dead bird surveillance, and suggests increasing submissions. An analysis of health unit participation to date indicates that for the months of May and June, 16 submissions (one submission could involve multiple birds) were made from eight (22%) health units.

Of the 57 bird carcasses tested in May and June 2000, 35 (61%) carcasses were classified as requiring further examination after initial post mortem. For 16 (28%) of the carcasses, a firm alternative diagnosis was apparent and no further examination was required. Of the 35 carcasses requiring further examination, histology revealed a firm alternative diagnosis in 13 (37%) and 7 (20%) had no characteristic WNv lesions. To date, there have been no confirmations of WNv in bird specimens from Ontario.

Figure 2

Currently, the Ontario WNv Working Group is in the final stages of completing a comprehensive evidence based response plan. The theme of this plan is to provide an escalating response based on increasing risk. The plan will outline the prevention, response and control systems that will be implemented in Ontario, and describe the surveillance activities that are being undertaken.

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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