IN THIS ISSUE

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

Sentinel Chicken Surveillance was carried out successfully to the week ending July 28, 2000. 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 21, blood samples were collected from all 18 sites in all six regions. However, at one site in each of the Picton and Long Point regions, only 9 of 10 samples were taken. At both sites, the reason for incomplete sampling was due to the death of sentinel birds. For the week ending July 28, all 18 sites were visited, but one site in each of the Long Point and Guelph regions had only 9 of 10 samples collected. In the Long Point region, the dead bird from the previous week was replaced and in the Guelph region, one more dead bird was found. No other difficulties in sampling of chickens have been noted. As of the week ending July 28, all sentinel chicken serums have tested negative for WNv. To date, no sentinel chickens in Ontario have tested positive for WNv.

As a notification for the possibility of WNv in Ontario, a mailing was sent July 5, 2000, to selected specialty programs and all hospitals.

As of August 11, 2000, there are 55 hospitals participating in active human meningo-encephalitis surveillance. From 16 sentinel hospital sites, reports of 20 suspect cases have been received. Four of these 20 cases were initially identified by laboratory rather than by the site nurse. In addition, two more suspect cases were reported through passive surveillance. Of these two, one was reported by a physician. For disease diagnosis, various specimen types, such as acute and convalescent sera and cerebrospinal fluid, were used. From these, WNv has not been confirmed. Out of the 22 suspect cases, two were diagnosed with meningo-encephalitis : a 14-year-old child from the Ottawa-Carlton health unit region and a 29-year-old individual from the Hamilton-Wentworth region. Furthermore, four cases, aged 34, 35, 54 and 58 years, were reported from hospitals in Peel region. A 43-year-old case was also seen at a hospital in the Halton health unit region. From Toronto health unit hospitals, seven cases (aged 2 months, 4 months, 2, 11, 32, 21 and 45) were reported as suspect cases.

Mosquito surveillance progressed in Ontario and results of mosquito enumeration and speciation are available to the week ending July 28, 2000. Mosquitoes were collected from all 18 sites for the weeks ending July 21 and 28.

Mean mosquito counts at each of the 18 sites in the six regions were compared using analysis of variance over the nine-week period. One site in Brockville (BR-1) and one site in the Picton region (PI-3) indicated significantly higher mean mosquito counts than the other sites in the region (p=0.02, 0.04 respectively). Previously in Volume 1, Issues 2 and 3 of the WNv Bulletin, these sites were noted as having higher mean mosquito counts than the other sites in their regions. Additionally, one site in Long Point (LP-3) maintained markedly higher counts of mosquitoes than the other sites within the same region, but the difference was not statistically significant (see Figure 1).

Total mosquito counts from the three sites of each of the six regions were pooled for further analysis. A nine-week summary is provided in the table below. The regions of Niagara and Long Point reported a significant increasing trend of mean total mosquitoes over the 9-week period (p=0.02, 0.04 respectively). Of the six regions, none indicated a significant increasing trend in the proportion of female mosquitoes. Over the nine-week period Brockville, Niagara, Picton and Point Pelee regions had a significant increasing trend in the proportion of female mosquitoes that are Culex pipiens (p=0.001, 0.021, 0.015, 0.003 respectively). The regions of Brockville, Niagara, Picton and Point Pelee indicated a significant decreasing trend in the proportion of female mosquitoes that are Aedes. In these four regions, the female population of mosquitoes is increasingly becoming predominantly Culex pipiens, which is the primary vector of WNv.

Table 1

Figure 1

Weekly submissions of dead bird carcasses to the week ending July 28, 2000, are illustrated in Figure 2. The four weeks in July have had higher weekly submissions of dead birds than in all other weeks. The last three weeks in July had 86 submissions of dead bird, which is higher than all previous weeks combined. As of the week ending August 4, 30 (81%) health units have made a total of 140 submissions (each submission could involve multiple carcasses). An increase in health unit participation was seen in the last three weeks, as 13 health units made their first submissions. Thus far, only seven health units have yet to make dead bird submissions. This increase in health unit participation is important, since dead bird surveillance is likely the most sensitive component of Ontario's efforts to detect any virus activity as early as possible.

As of July 28, 163 bird carcasses have been submitted for examination in Ontario. Initial autopsy indicated that 55 (87.3%) carcasses did not require further diagnosis, and 49 of these had a firm alternative diagnosis. Further examination was required after autopsy for 101 of the 163 dead birds submitted. Histology results have indicated a firm alternative diagnosis in 23 of the 101 carcasses, with another 18 not requiring further testing since no indications of WNv lesions were present. Results on a further 45 carcasses are pending. To date, 14 specimens have been classified as suspect cases for WNv infection after histology, of these, 10 have tested negative after further testing with four suspect cases still awaiting testing. Thus far, no dead bird carcasses submitted in Ontario have been confirmed positive for WNv.

Figure 2

Figure 3

The risk of WNv infection for Ontario has increased with the discovery of positive dead birds in various areas in the State of New York that are in close proximity to the province (i.e., Syracuse, Town of Tonawanda and Franklin County). With Niagara Region located adjacent to the Town of Tonawanda, it has consequently entered into stage two of the Ministry of Health's and Long-Term Care (MOHLTC) WNv Protocol. Niagara Region residents are being warned to protect themselves against mosquitoes and to report dead birds to the local health department for testing.

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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