IN THIS ISSUE

This issue is the ninth and final issue of the Ontario WNv Bulletin for this year. Its purpose is to provide a summation of the results of the various surveillance activities in Ontario and an update on other findings elsewhere in Canada and in the U.S.

On October 13, 2000, the sentinel chicken component of the WNv Surveillance Program closed for the season.

The sentinel chicken surveillance sites located in the regions of Brockville (BR), Picton (PI), Guelph (GU), Niagara (NI), Long Point (LP) and Point Pelee (PP) were visited. In all 18 sites, blood samples were collected from 178, 179 and 178 chickens for the weeks ending September 29, October 6 and October 13 respectively. By the end of the surveillance season, none of the chickens had become infected with WNv.

Up to October 20, a total of 174 individuals, ranging from 2 months to 92 years of age, were deemed suspect cases. Of these, 117 were reported by 37 sentinel hospitals through active surveillance (78 were reported by sentinel hospitals while 39 were initially identified by the public health laboratory). Community physicians and non-sentinel hospitals reported another 54 suspect cases through enhanced passive surveillance. Clinical syndromes include 14 suspect cases with meningitis, 24 with encephalitis and another six with meningoencephalitis. Acute serum was taken from 155 people. Of these, 47 also had a convalescent serum drawn. In addition, 94 of the suspect cases also provided cerebrospinal fluid (CSF) for testing (15 with CSF only, 54 also with an acute serum and 25 with both acute and convalescent sera and CSF). At the end of the surveillance season, no human cases of WNv infection has been confirmed.

The mosquito component of the surveillance for this season was discontinued on October 13, 2000.

For the weeks ending September 29 (week 18), October 6 (week 19) and October 13 (week 20), mosquitoes from the mosquito surveillance sites were collected, enumerated and speciated. The three additional Niagara Region mosquito trapping and collection sites mentioned in the last bulletin were not operating as of the week ending September 29. These three sites were only included for analysis for weeks 16 and 17 (weeks ending September 15, and 22) of observation. In addition, data for the week ending September 1 (week 14) has been updated. For this issue of the bulletin, the results of the enumeration and speciation up until October 13 were included. Finally, as with the two previous issues of the bulletin, only live mosquito data were examined for the week ending August 18, 2000.

For each week of observation, analysis of variance was used to compare mean mosquito counts at each of the sites in the surveillance regions. At the end of the surveillance season, one site in each of Brockville (BR-1), Long Point (LP-3) and Picton (PI-3) had a significantly higher mean mosquito count compared to other sites within their respective regions (p=0.01, 0.02 and 0.01 respectively). In addition, lower mean mosquito counts prevailed in site 3 in Point Pelee compared to the other sites in this region (Figure 1). The result for PP-3 was statistically significant (p=0.01). Although the mean mosquito counts in all six regions increased from week 18 to week 19, with the exception of Guelph, the regions of Brockville, Long Point, Niagara, Picton and Point Pelee were showing a decline in the mean mosquito counts after 20 weeks of observation. However, the result within the region over the surveillance season was not statistically significant for each of the regions.

After week 18, the number of female mosquitoes collected in the six regions increased. However, by the end of 20 weeks, these numbers have started to show a decline. The only exception is the region of Guelph, where the number continued to increase. None of these trends were statistically significant. In addition, for the entire surveillance season, it was observed that Brockville had the greatest mean number of female mosquitoes relative to the other regions (Table 1). However, in considering the proportion of female mosquitoes among the total number of mosquitoes collected, Picton had the largest mean proportion at 93.4% (Table 1). With respect to the proportion of female mosquitoes, the only regions that had statistically significant results were Long Point and Picton (p=0.02 and p=0.001 respectively). At the end of 20 weeks, Long Point was exhibiting a declining trend while Picton had remained constant in the last five weeks the season. In terms of species, the majority of mosquitoes remain Aedes (Table 1). However, the mean proportion of Culex pipiens mosquitoes in Guelph has increased since the last bulletin (from 16.8% to 24.3%). This is because the female mosquitoes from this region have been exclusively Culex pipiens in the last two weeks of observation. Finally, only Brockville showed a statistically significant result within the region upon observation of female mosquitoes by species over the observation period (p=0.001). After showing an increase in the proportion of female Aedes mosquitoes after 18 weeks, this region was showing a decline at the end of 20 weeks.

Table 1

Figure 1

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, 2000.

The CCWHC received 41 carcasses for the week ending September 29, week 18, 23 dead birds in week 19 and 19 bird carcasses in week 20. A total of 1,531 dead birds were submitted to the CCWHC this surveillance season. During the 20 weeks of observation, the mean number of dead birds submitted was 63.79 per week (range of 2 to 462). None of the 1,531 dead birds were found to be positive for the WNv.

Of the total number of carcasses received by the CCWHC, 1,255 (82%) underwent an initial autopsy, of which 643 (51%) did not require further examination and 612 required further examination. A total of 262 (41%) out of the 643 had a firm alternative diagnosis, while the remaining 381 did not require further diagnosis due to other reasons. However, of those that required further examination after initial autopsy, 69 had a firm alternative diagnosis after histology testing. Another 30% had no apparent West Nile lesions and 112 were considered as suspect cases of WNv infection. Of the 112, all but 61, with results still pending, have been diagnosed as negative for the virus.

There have been no positive reports of WNv infected dead birds this surveillance season. After further testing by two U.S. reference laboratories, the possible positive dead crow collected on August 10 from the city of Windsor was found to have equivocal evidence of WNv.

Figure 2

Figure 3

The provinces of Manitoba, New Brunswick, Newfoundland and Saskatchewan have stopped dead bird surveillance. Quebec and Nova Scotia plan to continue until the end of October. If suspicious dead birds are identified after surveillance cessation, the CCWHC and the Canadian Food Inspection Agency (CFIA) will do testing. Furthermore, sentinel chicken surveillance has also been discontinued in Manitoba, New Brunswick, Nova Scotia, Quebec and Saskatchewan. With the exception of Quebec, these provinces have also ceased mosquito surveillance. Quebec has decided to continue mosquito surveillance at selected sites until no mosquitoes are detected.

Conducted by the New York Health Department and the U.S. Centers for Disease Control and Prevention (CDC), the door-to-door WNv survey of households on Staten Island was completed on October 8, 2000. Samples from a total of 877 voluntary participants in 634 households in randomly selected neighborhoods on Staten Island were collected for this survey. The survey participants are scheduled to receive their individual WNv test results within the next three months. The Health Department and CDC will conduct further analysis of survey data, with the results being made available to the public once the analysis is complete.

The Camarague region in the south of France is suffering from an outbreak of West Nile fever. As of October 11, the disease has struck only horses. However, so many horses have died and have been infected that the public health authorities have been forced to introduce emergency measures. The herds of free-ranging horses are being rounded up in order to contain the spread of the disease and also to assist in the rehabilitation of recovered animals.

The contribution of the following agencies and individuals in the publication of this year's issues of the WNv Bulletin is greatly appreciated :

Ontario Ministry of Agriculture and Food (Dr. David Alves, Paul Innes, Melisa Kakuda)

Canadian Cooperative Wildlife Health Centre (Dr. Ian Barker, Ron Templeman, Nicole Rolfe)

Hospital for Sick Children, Toronto (Dr. Lee Ford-Jones, Pam Baker Greene, Bev Cole, Moira Myszak)

University of Guelph (Dr. James Heal, Dr. Doug Campbell, Marjorie Ferguson-Gratton)

Health Canada (Dr. Peter Buck, Brian Winchester)

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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