IN THIS ISSUE

The seventh issue of the Ontario WNv Bulletin is intended to provide a summary of results of the various surveillance activities in Ontario and an update on other findings elsewhere in Canada and in the U.S.

All 18 sentinel chicken surveillance sites in Brockville (BR), Picton (PI), Guelph (GU), Niagara (NI), Long Point (LP) and Point Pelee (PP) were visited for each of the two weeks ending the September 1 and 8. For the week ending September 1, blood samples were collected from each of the chickens. For these 2 weeks of collection, no chickens tested positive for WNv.

A mailing sent out on July 5, 2000, notified select specialty physicians and all hospitals about the possibility of WNv infection in Ontario.

A total of 125 individuals were reported as suspect cases of WNv infection. Of these, 79 (63%) were reported by 35 of the sentinel sites participating in the surveillance program. Another 42 (34%) were reported by other non-sentinel hospitals and physicians in the community. To diagnose the virus, acute and convalescent sera were submitted. Out of 113 people, at least an acute serum specimen has been submitted for testing. From the 113, 24 (21%) had convalescent sera taken. Cerebrospinal fluid (CSF) was also submitted for diagnostic purposes on 70 cases, of which 60 also had an acute serum drawn. Thus far, all specimens examined were negative. Clinical syndromes included 11 individuals with meningitis, 19 with encephalitis and 6 with meningoencephalitis. All were due to causes other than WNv infection.

The suspect 125 cases ranged from 2 months old to 84-years-old. Since the last bulletin, an additional five suspect cases were reported from Toronto. This brings the total reported by sentinel hospitals in Toronto to 24 suspect cases; suspect cases ranged from 2 months old to 80 years of age. Other health departments also had sentinel hospitals that reported suspect cases. These include: Durham Region (2 suspect cases; aged 8 and 13 years old), Region of Peel (14 suspect cases, ranging from 22 to 75 years of age), Halton Region (5 suspect cases, aged 27, 43, 46, 62 and 78), Niagara Region (11 suspect cases, aged 7 months old to 56 years old), Hamilton-Wentworth Region (5, aged 20 to 64 years old), Ottawa-Carleton Region (7 from 9 to 77 years old) and Windsor-Essex (one 62-year-old suspect case).

For the weeks ending August 18, August 25, September 1 and September 8, mosquitoes were collected from each of the 18 Ontario surveillance sites. Results of mosquito enumeration and speciation are available to the week ending the September 8, 2000. However, due to the number of mosquitoes collected for the week ending August 18, only live mosquitoes have been counted and speciated. Thus, only live mosquito data were examined for this issue of the bulletin.

Analysis of variance of the total mosquito counts at each of the 18 sites in the six regions over the 15-week period indicate that the region of Point Pelee has a significantly lower count of total mosquitoes compared to the other sites within the same region (p=0.03 for PP-3). Conversely, one site in each of Brockville, Long Point and Picton are exhibiting higher counts of total mosquitoes compared to other sites in their respective regions. For these three sites, the results were statistically significant (p=0.01 for BR-1, p=0.03 for LP-3 and p=0.01 for PI-3). (See Figure 1.) As with past results, these sites are observed to have higher mean mosquito counts compared to the other sites in their regions. The mean mosquito counts were also calculated for each of the six regions. Although all regions are showing a declining trend at the end of 15 weeks, none of changes were statistically significant.

When examining only female mosquitoes, the regions of Brockville, Guelph and Picton are reporting high total numbers of female mosquitoes relative to Long Point, Niagara and Point Pelee (Table 1). Furthermore, all regions are exhibiting a declining trend after 15 weeks of observation. However, similar to mean mosquito counts, none of these results were significant statistically. On the other hand, upon examination of the proportion of female mosquitoes, statistically significant results were observed for Brockville, Long Point and Picton (p=0.025, 0.042 and 0.025 respectively). While the proportion of female mosquitoes is increasing in Brockville, both Long Point and Picton are starting to show a decrease in the proportion of female mosquitoes after 15 weeks. Since the last mosquito data analysis (Volume 1, Issue 5), a statistically significant decreasing trend in the proportion of female Culex pipiens mosquitoes continues to be seen in Point Pelee (p=0.009). In addition, this same region is also showing a decrease in the proportion of female Aedes mosquitoes after the 15-week observation period (p=0.004). At the end of 15 weeks, the majority of female mosquitoes have shifted from being of the species Culex pipiens, which is the main vector of WNv, to the Aedes species.

Table 1

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

Figure 1

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

In weeks 14 and 15 of observation (weeks ending September 1 and September 8 respectively), dead bird submissions reached a cumulative total of 1,296 carcasses. In the week ending September 1, the Canadian Cooperative Wildlife Health Centre (CCWHC) received 78 birds for examination. The following week, a total of 41 birds were submitted to the CCWHC. From Figure 2, there is a downward trend in submission of dead birds for WNv surveillance. Dead bird surveillance is considered the most sensitive means for early virus detection and the ability of the surveillance system to detect viral activity is dependent upon the number of submissions. Of the 1,296 dead birds, 85 were deemed unsuitable for autopsy. However, a total of 765 birds underwent an initial autopsy. After initial autopsy, results show that 323 (25%) of the total submissions did not require further diagnosis. Of the 323 birds, 217 had a firm alternative diagnosis. Alternatively, further examination after autopsy was required for 442 of the 1,296 carcasses. From these, histology results indicated a firm alternative diagnosis in 42 birds. Meanwhile, 96 had no apparent West Nile lesions and thus did not require further testing. The results for the remaining 278 carcasses further tested after initial autopsy are still pending. As of September 8, 26 specimens have been classified as suspect cases for WNv infection after histology.

A dead crow collected in the city of Windsor on August 10 is diagnosed as possible positive for WNv. Histopathological findings were in keeping with WNv infection, and a liver examination was deemed to be suspicious for WNv by immunohistochemistry testing carried out by the Canadian Food Inspection Agency laboratories. The Health Canada laboratory in Winnipeg completed nested PCR testing on the liver, brain and heart, and reported a positive result for WNv. Further confirmatory testing by a U.S. reference laboratory is being conducted. An announcement of the finding was provided in a press release by Dr. Heimann, Medical Officer of Health for the Windsor-Essex County Health Unit. There has been no other evidence of WNv activity suggestive of an outbreak among birds or other species in the Windsor area, and bird mortality has consistently preceded the occurrence of human cases in the U.S. This may have been an isolated incident or may turn out to be unconfirmed by other laboratories.

Figure 2

Figure 3

Since August of this year, 157 individuals have survived WNv infection in Israel. However, 12 middle-aged and elderly Israelis have died due to complications associated with this viral infection. The youngest Israeli who died of WNv encephalitis was a 54-year-old woman who initially had myasthenia gravis. Most of the others were aged over 80 and had chronic diseases. With the number of infected people increasing, intensive pesticide spraying has been carried out at night, especially in the hardest hit area of Sharon, north east of Tel Aviv.

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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