Mosquito-borne diseases surveillance report: Annual edition (2019 - Preliminary)

Organization: Public Health Agency of Canada

Date published: February 2022

ISBN: 978-0-660-40499-8
Cat: HP40-298/1-2019E-PDF
Pub.: 210344

On this page

• Surveillance highlights
• Introduction
• Methods
• West Nile virus
• Travel-related cases
• Mosquito, wild bird and equine surveillance
• California serogroup viruses
• Eastern equine encephalitis virus
• Discussion
• Public health conclusions
• Acknowledgements
• Erratum
• References

Introduction

West Nile virus (WNV), Eastern equine encephalitis virus (EEEV), and the California serogroup viruses (CSGV), specifically the Jamestown Canyon and snowshoe hare virus, are known to cause human infection in North America. All four mosquito-borne diseases are endemic in parts of Canada with 10% increase in incidence observed over last 20 years, largely attributed to climate change^{Footnote 1}. Canada's endemic mosquito-borne diseases have complex transmission cycles with viruses circulating between specific avian or mammal hosts and competent mosquito vectors. Sometimes, however, a broad range of other animals, including humans and horses, can be infected. Recognizing that the health of humans is interconnected with that of animals and the environment, mosquito-borne disease surveillance requires a One Health approach. This report therefore integrates findings from human and animal health surveillance, conducted in collaboration with multi-disciplinary health partners, with the goal of achieving optimal human health outcomes.

Methods

WNV is a nationally notifiable disease and human cases of WNV in Canada are reported voluntarily to the Public Health Agency of Canada (PHAC) by provincial and territorial health ministries via the Canadian WNV Surveillance Program. Other agencies such as Canadian Blood Services and Héma-Québec also report human infections of WNV in Canada via provincial and territorial health ministries. In this report, human cases are classified using the national surveillance case definition. Human infections include both clinical cases and asymptomatic infections, and travel cases, either domestically or internationally acquired, are excluded from overall epidemiological analyses, except for Figure 5.

Other indicators of WNV activity in Canada include information from animals and mosquitoes. In 2019, information on WNV-positive dead wild birds was largely provided by the Canadian Wildlife Health Cooperative (CWHC); however, some provinces also provide this data directly to the Canadian WNV Surveillance Program. The Canadian Food Inspection Agency (CFIA) provides nationwide data on WNV veterinary cases, while mosquito surveillance data are collected and provided by four participating provinces: Saskatchewan, Manitoba, Ontario, and Québec.

Unlike WNV, neither EEEV and CSGV are nationally notifiable diseases in Canada in humans. The National Microbiology Laboratory (NML), however, tests for EEEV and CSGV in patients who present with the symptoms consistent of arboviral infection^{Footnote 1} when requested by laboratories or for confirmation purposes. Human CSGV cases are those where an acute infection is detected. In addition to the NML's testing for CSGV, some provincial partners conduct their own testing and use provincial case definitions that do not match the national case definition, given that CSGV is not a nationally notifiable disease.

This report is based on the latest data provided to PHAC for the 2019 transmission season (data as of: 2020/11/13). Data collection methods and case definition interpretation vary within Canada, and data are subject to change as new information is received.

West Nile virus

Human case surveillance

A total of 45 WNV human infections were reported to PHAC during the 2019 season. Of these, 3 were locally acquired clinical cases, eight travel associated cases and four asymptomatic cases. The locally acquired clinical cases were reported across four provinces: Ontario (n=17), Québec (n=12), Alberta (n=2) and Manitoba (n=2). The majority of cases were geographically distributed in the southern regions of these provinces (Figure 1). The four asymptomatic infections were reported by Ontario (n=2) and Québec (n=2).

The earliest onset date for a WNV human case in 2019 was July 3 in Ontario (surveillance week 27). Most (99%) of reported cases occurred with an onset date between surveillance weeks 27 and 41 (early July to mid October), with a peak in week 36 (early September) (Figure 2). Typically, the peak weeks vary from year to year, ranging from week 32 to 36 (early August to early September) (Figure 3). The number of human cases observed in 2019 represents the lowest number of cases reported since 2015 (Figures 3 and Figure 5).

Of the 33 human infections (confirmed and probable), 61% (n=20) were reported as WNV neurological syndrome, 30% (n=10) as WNV non-neurological syndrome and 9% (n=3) as unclassified/unspecified (Table 1). Among the clinical cases classified as neurological, two deaths were reported. In addition, four WNV asymptomatic infections were reported.

In 2019, the incidence rate of reported infections (clinical and asymptomatic) increased with age; incidence was higher for females than males in the 50-59 age group, and lower in those 0-19, 20-29, 40-49 and over 60 years old (Figure 4). It is important to interpret these differences with caution, given the low number of human infections in 2019.

Travel-related cases

Between January 2, 2019 and November 22, 2019, a total of eight travel-related clinical cases were reported from Ontario (n=5), Alberta (n=2) and British Columbia (n=1). Of these, 12.5% (n=1) were reported as WNV neurological syndrome, 75% (n=6) as WNV non-neurological syndrome and 12.5% (n=1) as unclassified/unspecified. The median age of cases was 56 (range: 35-71). International travel locations included United States, Mexico, Cuba, Barbados and Ireland. These cases were excluded from analyses in this report.

Mosquito, wild bird and equine surveillance

During the 2019 transmission season, 11,586 mosquito pools were tested for WNV in four provinces: Ontario (n=8,400), Québec (n=1,820), Manitoba (n=1,166) and Saskatchewan (n=200). Of these, 96 (<1%) tested positive for WNV: 71 in Ontario, 19 in Québec, and 6 in Manitoba (Table 2). In 2019, the percent of mosquito pools positive for WNV infection was highest in Québec (1.0%). Similar to human cases, percent positivity for tested mosquito pools was low (<1%), compared to the average of the previous five years (1.9%) (Figure 5).

The CWHC tested 220 dead wild birds for WNV. Of these, 46 (21%) were positive for WNV across three provinces (Table 3, Figure 1 and Figure 5). Overall, WNV activity was detected in birds from June to late October. Despite maintaining rigorous surveillance activities, fewer WNV-positive birds were identified than previous years (Figure 5).

The CFIA was notified of eight cases of WNV in domestic horses in the following five provinces: Alberta (n=3), Saskatchewan (n=2), British Columbia (n=1), Ontario (n=1) and Québec (n=1) (Table 3). This is the lowest number observed in Canada in a six-year span (Figure 5).