IN THIS ISSUE

The fifth issue of the Ontario WNv Bulletin provides a summary of results of the various surveillance activities in Ontario. It also updates readers on surveillance findings elsewhere in Canada and in the U.S.

Erratum :   In the last issue of the bulletin (Volume 1, Issue 4; August 18, 2000), the summary of mosquito counts and dead bird surveillance were incorrectly reported as <35% of female population as Culex species and 102 specimens collected respectively. The correct information is <42% of female population as Culex species and 163 specimens collected. In addition, the "Mean % of Female Mosquitoes, Culex pipiens" column in the table found in the Mosquito Surveillance section should have the following values listed for each site: Brockville, 17.4; Guelph, 23.0; Long Point, 41.5; Niagara, 33.0; Picton, 29.7; and Point Pelee, 19.3.

For the weeks ending August 4 and August 11, all 18 sentinel chicken surveillance sites in the surveillance areas of Brockville (BR), Picton (PI), Guelph (GU), Niagara (NI), Long Point (LP) and Point Pelee (PP) were visited.

Blood samples were collected from all 18 sites in all six regions for each of the weeks ending August 4 and August 11. As of August 11, none of the sentinel chicken serums have tested positive for WNv.

Selected specialty programs and all hospitals were notified about the possibility of WNv infection in Ontario in a mailing sent July 5, 2000.

Currently, 55 hospitals enrolled in the active human meningo-encephalitis surveillance program are participating. As of August 18, a total of 26 suspect cases were reported. Twenty-four of these were from 16 sentinel hospital sites, while the remaining two were reported through passive surveillance. Of the sentinel hospital cases, three were initially identified by laboratory rather than by the site nurse. Meanwhile, of the two passively reported cases, one was reported by a physician. Acute and convalescent sera and cerebrospinal fluid have been submitted for testing. From these specimens, there has been no confirmation of WNv. There are still only two cases diagnosed with meningo-encephalitis (i.e., a 14-year-old child from the Ottawa-Carleton health unit region and a 29-year-old individual from the Hamilton-Wentworth region). In addition to the four cases aged 34, 35, 54 and 58 years reported in the last bulletin, a 33-year-old from Peel region is considered a suspect case. Furthermore, an additional two cases (aged 70 and 80 years) from Toronto health unit hospitals have been reported. Thus, there are nine cases reported to date from the Toronto region. Finally, all but six of the 26 suspect cases have recovered or are recovering. These six cases have their status listed as unknown.

Results of mosquito enumeration and speciation are available up until August 11, 2000. For the weeks ending August 4 and August 11, mosquitoes were collected from all 18 Ontario mosquito surveillance sites.

To be able to compare the total mosquito counts at each of the 18 sites in the six regions, an analysis of variance for the 11 weeks observed was carried out. The results indicate that one site in Brockville (BR-1) and one site in the Picton region (PI-3) had higher mean mosquito counts than the other sites in their respective regions. (See Figure 1.) For these two sites, the results were statistically significant (i.e., p=0.01 for BR-1 and p=0.02 for PI-3). Consistent with past results, these sites were observed to have higher mean mosquito counts compared to the other sites in their regions. The results also noticeably show that one site in Long Point (LP-3) had higher counts of mosquitoes than the other sites within its region. However, the difference was not statistically significant.

Mean mosquito counts from the three sites of each of the six regions were also examined. The results are summarized in Table 1. For the 11-week period, it was observed that both the regions of Niagara and Point Pelee were showing an increasing trend of mean total mosquitoes. For these two areas, the results were statistically significant (i.e., p=0.02 and p=0.01 for Niagara and Point Pelee respectively). Furthermore, each of these same regions also reported a significant difference in trend for the total number of female mosquitoes (i.e., p=0.01 for each of the 2 areas). For Niagara, the total number of female mosquitoes is again on an increasing trend by the end of 11 weeks. Point Pelee, on the other hand, has been declining after the increase observed in week 10 from week 9. Meanwhile, for the proportion of female mosquitoes seen, a statistically significant result was seen only for the region of Long Point (p=0.03). After decreasing in week 10, the proportion of female mosquitoes in Long Point is again increasing at the end of week 11. Over the 11-week observation period, a statistically significant decreasing trend in the proportion of female Culex pipiens mosquitoes was observed in the regions of Brockville, Niagara and Point Pelee (p=0.001, 0.01, 0.0001 respectively). Up until week 10, these regions were reporting an increasing trend of female mosquitoes that are Culex pipiens. In addition, these same regions are also showing a significant increasing trend in the proportion of female Aedes mosquitoes (i.e., p=0.001 for BI, p=0.004 for NI and p=0.0001for PP).

Alternatively, at the end of 11 weeks, the region of Picton continues to report a statistically significant increasing trend for the proportion of female Culex pipiens mosquitoes (p=0.002) and a statistically significant decreasing trend for the proportion of female Aedes mosquitoes (p=0.001). It should be noted that for the regions of Long Point, Picton and Point Pelee, the majority of female mosquitoes continue to be of the species Culex pipiens, which is the main vector of WNv.

Table 1

Figure 1

In the last two weeks of observation (weeks 10 and 11), weekly submissions of dead bird carcasses reached a total of 330 birds. In week 11 alone, 199 dead birds were submitted, which is higher than weeks 1 to 9 combined. Due to this large increase, data entry is delayed, and for weeks 10 and 11, a table showing the number of submissions by health unit is not available. The tremendous increase in health unit participation in dead bird surveillance following the reports of confirmed WNv in adjacent U.S. counties will assist in early detection of the virus in Ontario.

In Ontario, 525 bird carcasses have been submitted by August 11. Of the total number of carcasses, 242 (46%) did not require further diagnosis after initial autopsy, and 182 of these had a firm alternative diagnosis. For all of the birds submitted, 211 (40%) required further examination after autopsy. From these, histology results have indicated a firm alternative diagnosis in 35 carcasses. Another 27 of the 211 carcasses did not require further testing; results on a further 133 carcasses are still pending. To date 16 specimens have been classified as suspect cases for WNv infection after histology. Twelve of the 16 have tested negative after further testing, while the remaining four suspect cases are still undergoing testing procedures. As of August 11, no dead bird carcasses submitted in Ontario have tested positive for WNv.

Figure 2

Figure 3

A restriction on the species of submission to the Canadian Cooperative Wildlife Health Centre has been made due to the large surge in submissions following the identification of WNv in U.S. counties immediately bordering on Ontario. WNv dead bird submissions have been restricted to crows, blue jays and hawks of any type. Any other species involved in outbreaks (i.e., more than three to four birds of the same species found in a local area within a 2- to 3-day period except mourning doves due to an outbreak of trichomoniasis) and those with a clear history of nervous system signs will also be examined.

Finally, with two dead birds positive for WNv found in Lewiston, New York (Niagara County), which is located immediately along the Canada-United States border and is east of the Niagara health unit, the region of Niagara will carry out mosquito collection using gravid traps in areas along the Niagara River. The gravid traps to be employed at these areas allows increased sensitivity of catching gravid female mosquitoes which are suspected of carrying WNv.

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

© Queen's Printer for Ontario, 2000  |  Privacy Policy  |  Disclaimers  |   Last Modified :