IN THIS ISSUE

This eighth issue of the Ontario WNv Bulletin provides a summary of results of the various surveillance activities in Ontario. It also includes an update on other findings elsewhere in Canada and in the U.S.

For the weeks ending September 15 and September 22, the 18 sentinel chicken surveillance sites in Brockville (BR), Picton (PI), Guelph (GU), Niagara (NI), Long Point (LP) and Point Pelee (PP) were visited. Blood samples were collected from 177 chickens in these sites for each of the two weeks of sampling. As of September 22, no chickens have tested positive for WNv.

As of September 29, a cumulative total of 137 persons, ranging in age from 2 months to 84- years-old, were reported as suspect cases through the human encephalitis component of the WNv surveillance program. None has tested positive for WNv. Of the 137 individuals, 87 (64%) were reported by 34 of the sentinel hospitals, and 26 of these were first identified by the public health laboratory. Community physicians and non-sentinel hospitals reported 46 (34%) suspect cases through (enhanced) passive surveillance. Thirteen of the suspect cases had meningitis, 19 had encephalitis and 5 had meningoencephalitis as clinical syndromes. One hundred and twenty-four individuals had an acute serum taken, and of these, 32 (25%) also had convalescent serum drawn. Cerebrospinal fluid (CSF) was taken from 76 individuals (9 with CSF only, 50 with an acute serum also taken and 17 with both sera drawn).

For the weeks ending September 15 and 22, mosquitoes were collected from each of the 21 surveillance sites. It should be noted that three additional mosquito trapping and collection sites were set up in Niagara Region due to the proximity to positive crows in Lewiston, New York. Since the week ending September 15, analysis of data for Niagara was done for six surveillance sites. The results of the enumeration and speciation are available to the week ending September 22. As per last issue of the bulletin, only live mosquito data were examined for the week ending August 18, 2000.

For the 17 weeks of observation, mean mosquito counts at each of the 21 sites in the six regions were compared using analysis of variance. Since data for only two weeks were available for the three new Niagara sites, this region was separated into two groups for analysis: one group included NI-1, NI-2 and NI-3; the other group consisted of the new sites (NI-4, NI-5, NI-6). As with the last issue of the bulletin, one site in each of the regions of Brockville (BR-1), Long Point (LP-3) and Picton (PI-3) had a significantly higher mean mosquito count than the other sites in their respective regions (p=0.01, 0.02 and 0.01 respectively) (Figure 1). PP-3 continued to have a significantly lower mean count of total mosquitoes in comparison to the other sites within the region of Point Pelee. At the end of 17 weeks, the mean mosquito counts for the regions of Brockville, Guelph and Picton were once again on the increase. In contrast, Long Point and Point Pelee exhibited an increase at the end of week 16. However, both regions started to decline afterwards. Niagara continued to show a decreasing trend. Overall, the mean total mosquito counts in each of the six regions did not vary statistically significantly within the region over the 17 weeks of observation.

The number of female mosquitoes collected in Brockville, Guelph and Picton were on the increase as of September 22. Conversely, the number of female mosquitoes in the remaining surveillance regions of Long Point, Niagara and Point Pelee increased after week 16 and then decreased the week after. However, none of the trends in female counts were statistically significant. In looking at the proportion of female mosquitoes in these regions, Brockville, Long Point and Picton had statistically significant results (p=0.03, 0.02 and 0.01 respectively). From week 15 to week 16, both Long Point and Picton have increased in the proportion of female mosquitoes collected. However, Picton remained constant at the end of week 17 while Long Point continued to increase. Although Brockville has the highest mean percentage of female mosquitoes (Table 1), it began to show a decline in the proportion of female mosquitoes at the end of 17 weeks of observation. This is in contrast to what was observed in the last issue of the bulletin (after week 15), where Brockville was starting to increase in proportion. Additionally, the decreasing trend observed in the proportion of female Culex pipiens mosquitoes for the region of Point Pelee continued until the end of week 16. By the week ending September 22, this statistic showed an increase for this region. Along with Point Pelee, Niagara and Picton also showed an increase after 17 weeks of observation. Conversely, Brockville, Guelph and Long Point were on the decline. However, only Point Pelee had a statistically significant result (p=0.03). Although the regions of Brockville, Niagara, Picton and Point Pelee started to decrease in the proportion of female Aedes mosquitoes collected, only Brockville and Point Pelee had statistically significant results (p=0.02 and 0.01 respectively). By the end of week 17, the majority of female mosquitoes present remain to be of the species Aedes.

Table 1

Figure 1

(Note :   Only live mosquito data were used for the week ending August 18, 2000)

As of September 22, a cumulative season total of 1421 bird carcasses have been submitted to the Canadian Cooperative Wildlife Health Centre (CCWHC). For week 16 (ending September 15), 48 dead birds were submitted. Another 19 carcasses were submitted to the CCWHC the following week. As seen in Figure 2, the decreasing trend in the submission of dead birds continues. Of the 1,421 dead birds, none have been found to be positive for WNv, with the exception of a single "equivocal" dead crow from Windsor.

A total of 996 (70%) carcasses underwent an initial autopsy. Of these, 474 (48%) did not require further examination and 243 carcasses had a firm alternative diagnosis. The remaining 231 (49%) did not require further diagnosis due to other reasons. Five hundred and twenty-two of the 1,421 carcasses submitted did require further examination after the initial autopsy. From histology results, it was revealed that 56 of the 522 had a firm alternative diagnosis. No apparent WNv lesions were observed in 153 dead birds. By September 22, 65 specimens have been considered as suspect cases for WNv infection after histology. Of these, 39 have results pending; the rest have been diagnosed as negative for the virus. Results for the remaining 248 carcasses that underwent further examination are still pending.

In the last bulletin, we reported a dead crow collected on August 10 in the city of Windsor, which tested positive for WNv PCR by the Health Canada laboratory in Winnipeg and "suspicious" equivocal immunohistochemistry findings by the Canadian Food Inspection Agency. Confirmatory testing by two U.S. reference laboratories has been done. Both laboratories found equivocal evidence of WNv. The only conclusion that can be made is that the infectivity status of the specimen cannot be determined. Due to poor quality of the specimen samples, the findings are ambiguous.

Figure 2

Figure 3

After consultation with the Ontario Ministry of Health and Long-Term Care (MOHLTC), the Canadian Cooperative Wildlife Health Centre (CCWHC) Ontario Region Laboratory in Guelph, has ceased accepting dead birds submitted specifically for WNv surveillance as of October 6. The only exception is submissions from the Niagara Region Health Unit. This exception has been made due to the continued activity in the U.S. counties adjacent to it. With the onset of frost in most parts of southern Ontario adjacent to WNv-endemic counties in New York, it is expected that mosquito numbers will remarkably decrease over the immediate future. Thus, the risk of WNv transmission among birds and mosquitoes and to people is also expected to diminish. The CCWHC will continue to examine wildlife of any type submitted for determination of cause of death.

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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