West Nile virus and other mosquito-borne diseases surveillance in Canada: Annual edition 2021

On this page

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

Surveillance highlights

West Nile virus

• A total of 46 human cases of West Nile virus (WNV) were reported in 2021. Of these, 43 cases acquired their infection within Canada (37 clinical cases and 6 asymptomatic cases).
• Of the 37 WNV clinical cases: 62% were neurological, 19% were non-neurological and 19% were unclassified. Two WNV-associated deaths were reported.
• Nineteen WNV positive horses, 31 WNV positive dead wild birds, and 1.4% of mosquito pools tested were positive for WNV.

Eastern equine encephalitis virus

• Seven cases of Eastern equine encephalitis in horses were reported.

California serogroup viruses

• Eighteen human California serogroup virus infections were reported: 2 identified by the National Microbiology Laboratory and 16 separately reported by Québec.

Introduction

West Nile virus (WNV), Eastern equine encephalitis (EEE) virus, and the California serogroup (CSG) viruses, specifically Jamestown Canyon virus and snowshoe hare virus, are known to cause human infection in North America. All four of these mosquito-borne diseases are endemic in various regions of Canada^{Footnote 1}. It is anticipated that climate change will impact range expansion and local abundance of mosquito species that carry these pathogens that cause human disease^{Footnote 1}. It has been proposed that the changing climate has already impacted mosquito-borne diseases in Canada which have increased in incidence by approximately 10% over the past 20 years^{Footnote 1}.

Canada's endemic mosquito-borne diseases have complex transmission cycles with viruses circulating between specific avian or mammalian hosts and competent mosquito vectors. However, a broad range of other mammals, including humans and horses (equines), can also be infected. Mosquito-borne disease surveillance requires a One Health approach in recognition of the interdependence of human health, animal health and their shared environment. This report integrates findings from human and animal health surveillance, conducted in collaboration with multi-disciplinary health partners, with the goal of achieving awareness and optimal human health outcomes.

Methods

West Nile virus

West Nile virus has been a nationally notifiable disease since 2003, and human cases in Canada are reported voluntarily to the Public Health Agency of Canada (PHAC) by provincial and territorial health authorities via the National WNV Surveillance System.

During the typical mosquito season (spring, summer, fall) or for at-risk travelers during the winter, blood donations are routinely screened using a nucleic acid amplification test (NAAT) by Canadian Blood Services and Héma-Québec. Blood donations may be screened using a NAAT specific to WNV or a NAAT that tests for viruses in the same serological family as WNV (Japanese encephalitis serocomplex). Positive blood donors, most commonly asymptomatic, then get reported to provincial/territorial health agencies. Following a positive blood donor screening, supplementary tests may be performed by provincial/ territorial labs or the National Microbiology Laboratory (NML).

Human cases include both clinical and asymptomatic infections. In this report, human cases are classified according to the national surveillance case definition. Of note, Saskatchewan reports WNV neurological syndrome cases only.

Epidemiological analyses included in this report capture human cases where the WNV infection was acquired within Canada. These cases either did not report travel or reported travel only within Canada. For the purposes of this report, cases that were acquired outside of Canada, due to international travel, are excluded from overall epidemiological analyses and only included in the "Travel-related cases" section. If travel was reported but the location was not specified, then the case was also excluded from overall epidemiological analysis since it could not be determined if the infection was acquired within Canada. Cases that occur outside the typical mosquito season and did not report travel were validated by the province or territory, and reflect the data in provincial and territorial surveillance systems. The earliest available episode date (e.g., symptom onset date, diagnosis date, laboratory sample date or reporting date) for each WNV case was used to assign the case to an epidemiological week using the Centers for Disease Control and Prevention (CDC) epi week calendar. Cases reported outside of the typical mosquito season were excluded from Figures 2 and 3.

In addition to human cases, other indicators of WNV activity in Canada include seropositive animals and positive mosquito pools. In 2021, 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 National WNV Surveillance System. The Canadian Food Inspection Agency (CFIA) provides nationwide data on WNV veterinary cases, such as horse cases, reported via the Immediately Notifiable Disease Regulations. Mosquito surveillance data are collected and provided by four participating provinces and one territory: Saskatchewan, Manitoba, Ontario, Québec, and the Northwest Territories.

Other mosquito-borne diseases

Unlike WNV, human CSG and EEE virus infections are not nationally notifiable diseases in Canada, and therefore, the true number of cases and burden of disease is unknown. Some provincial/territorial laboratory partners perform their own testing and there may be differences among their respective case definitions, which can lead to discrepancies in reported numbers. In addition, when requested by other laboratories, the NML tests for EEE virus and CSG viruses in patients who present with symptoms consistent with an arboviral infection. It is possible there is overlap in the cases reported by the NML and those by the provinces. In 2021, CSG virus infection counts were provided by both the NML and the province of Québec.

In addition to human cases of EEE, other indicators of local transmission come from animal and mosquito surveillance. In 2021, mosquito pools were tested for EEE virus in the province of Ontario and the CFIA provides nationwide data on cases of EEE in horses. For CSG viruses, mosquito pools were tested in the Northwest Territories in 2021. As there have only been a few cases of CSG virus ever documented in livestock in Canada, CSG virus is not a notifiable disease in animals, and therefore not reported to the CFIA. As there have only been a few cases of CSG virus infections ever documented in livestock in Canada, CSG viruses are not notifiable diseases in animals, and therefore not reported to the CFIA.

Limitations of reported data

The findings in this report are subject to several limitations. First, the National WNV Surveillance System is a passive surveillance system; therefore, it is likely that the actual incidence of WNV infections in humans is higher due to underreporting. Only approximately 20% of WNV infections are symptomatic; therefore, it is suspected that many cases go undetected. Although symptoms of WNV can be severe, they are usually mild, and most individuals may not be aware of their infections. Detection and reporting of WNV neurological syndrome are considered more complete than that of WNV non-neurological syndrome as severe cases are more likely to seek medical care. As EEE and CSG virus infections are not nationally notifiable or reportable in many provinces and territories, it is likely these counts are also underreported. As EEE and CSG virus infections are not nationally notifiable or reportable in many provinces and territories, it is likely these counts are also underreported.

Second, data collection (e.g., mosquito pool testing, dead wild bird collection and testing), public health follow-up (e.g., only neurological cases are investigated by public health in Saskatchewan), and case definitions (e.g., CSG viruses) vary within Canada, which many lead to challenges with interpretation. In addition, provincial and territorial disease reporting systems may receive updated case information, which can result in differences between what is presented in this report and provincial and territorial reports.

This report is based on the latest data provided to PHAC for 2021 (data as of 2022-04-06). Modifications and updates to case information made after this date may not be captured in this report.

West Nile virus

Human case surveillance

A total of 46 human WNV cases were reported to PHAC between January 1 and December 31, 2021. Cases were reported by four provinces in 2021: Ontario (n=25), Québec (n=13), Manitoba (n=6) and British Columbia (n=2). In total, 93% of cases (n=43) acquired infection within Canada, including cases with no travel history (n=36) and cases with travel within Canada (n=7). There were two cases that reported travel outside of Canada and one case that reported travel, but the location was not specified (see section on Travel-Related Cases).

Of the 43 infections acquired within Canada, 37 were clinical and 6 were asymptomatic. The majority of the infections acquired within Canada resided in the southern regions of Ontario, Québec and Manitoba (Figure 1).

Figure 1 – Text description

The map shows the distribution of reported human West Nile virus cases (clinical and asymptomatic), positive dead wild birds reported, and positive horse cases reported in Canada for 2021.

Province/Territory	Human Infections (n)	Positive birds (n)	Positive horses (n)
British Columbia	1	0	0
Manitoba	6	0	6
Ontario	24	11	3
Quebec	12	14	0
Saskatchewan	0	6	9
Total	43	31	18

Note: The map excludes cases acquired outside of Canada due to international travel, as well as cases with travel location unspecified. Of note, there are five human cases included in the map that reported travel outside their province or territory but within Canada, and two cases that reported travel within their province or territory of residence.

Cases of WNV acquired within Canada occur during the mosquito season, the period when mosquitoes are typically active, with the majority of WNV cases reported in the summer and fall. The earliest onset date for a human WNV case acquired within Canada during the typical mosquito season in 2021 was July 15 (epidemiological week 28). Most (77%) of the reported human WNV cases occurred with an onset date between epidemiological weeks 32 and 37 (early August to mid-September), peaking in week 35 (late August) (Figure 2). Typically, the peak week varies from year to year, ranging from week 33 to 37 (early August to mid-September) (Figure 3). The number of human cases observed in 2021 represents one of the lowest number of cases reported in the preceding five seasons (2016–2020) (Figure 3).

Figure 2 – Text description

In 2021, between May and end of October, 40 human infections (clinical cases) of West Nile virus were reported to the Public Health Agency of Canada by 4 provinces: British Columbia, Manitoba, Ontario and Quebec.

Month	Epidemiological weekFootnote d	Province
		British Columbia	Manitoba	Ontario	Quebec
May	week 18	0	0	0	0
	week 19	0	0	0	0
	week 20	0	0	0	0
	week 21	0	0	0	0
	week 22	0	0	0	0
June	week 23	0	0	0	0
	week 24	0	0	0	0
	week 25	0	0	0	0
	week 26	0	0	0	0
July	week 27	0	0	0	0
	week 28	0	1	1	0
	week 29	0	0	1	0
	week 30	0	0	0	0
August	week 31	0	0	0	0
	week 32	0	1	2	0
	week 33	0	2	2	0
	week 34	0	1	1	3
	week 35	1	0	4	4
September	week 36	0	0	1	4
	week 37	0	1	6	0
	week 38	0	0	1	0
	week 39	0	0	1	0
October	week 40	0	0	1	1
	week 41	0	0	0	0
	week 42	0	0	0	0
	week 43	0	0	0	0
	week 44	0	0	0	0
Total		1	6	21	12

Figure 3 – Text description

This graph shows the number of the West Nile virus human infections (clinical and asymptomatic cases) reported in 2021, which are below the average of the five preceding seasons (2016-2020).

Month	Epidemiological weekFootnote d	Mean (2016-2020)	Min-Max (2016-2020)	2021
May	week 18	0	0	0
	week 19	0	0	0
	week 20	0	0	0
	week 21	0	0	0
	week 22	0.2	1	0
June	week 23	0.4	2	0
	week 24	0.6	1	0
	week 25	0.4	2	0
	week 26	0.8	1	0
July	week 27	1.0	3	0
	week 28	1.8	6	2
	week 29	2.8	6	1
	week 30	3.6	8	0
August	week 31	8.2	20	0
	week 32	17.8	41	3
	week 33	24.6	67	4
	week 34	27.6	74	5
	week 35	27.2	60	9
September	week 36	23.8	60	5
	week 37	15.4	38	7
	week 38	8.8	22	1
	week 39	7.4	14	1
October	week 40	4.0	6	2
	week 41	1.4	2	0
	week 42	0.8	3	0
	week 43	1.6	4	0
	week 44	0.6	1	0

Of the 37 clinical cases acquired within Canada, 62% (n=23) were reported as WNV neurological syndrome, 19% (n=7) as WNV non-neurological syndrome and 19% (n=7) as unclassified/unspecified (Table 1). Among the clinical cases, two deaths associated with

WNV infection were reported. In addition, 6 WNV asymptomatic cases were reported. In 2021, the incidence rate for reported WNV clinical cases (n=37) acquired within Canada was 0.10 per 100,000 population.

Table 1. Reported human West Nile virus cases by disease classification (clinical and asymptomatic ^{Footnote f}) and rates^{Footnote g} (per 100,000) of clinical cases by province in Canada, 2021

Province/Territory	Clinical cases					Asymptomatic casesFootnote f
	Neurological	Non- neurological	Unclassified/unspecified	Total	RateFootnote g (per 100,000)
British Columbia	0	1	0	1	0.02	0
Manitoba	3	2	0	5	0.36	1
Ontario	8	4	7	19	0.13	5
Québec	12	0	0	12	0.14	0
Canada	23	7	7	37	0.10	6

The overall incidence rate for reported WNV clinical and asymptomatic cases (n= 43) acquired within Canada in 2021 was 0.11 per 100,000 population. The incidence of reported WNV clinical and asymptomatic cases was the same in females and males (0.11 per 100,000 population). The incidence rate for reported WNV clinical and asymptomatic cases increased with age; the incidence was highest in individuals aged 50-59 years and lowest in the 0-19 and 20-29 age groups (Figure 4). Rates should be interpreted with caution, given the low number of human infections in 2021.

The age of cases ranged from 4 to 90 years (mean = 56 years, median = 57 years). The mean age for neurological cases was 57 years (median = 59 years) and for non-neurological cases it was 57 years (median = 57 years). The mean age for asymptomatic cases was 50 years (median = 50 years). There were two deaths associated with WNV infection, both cases were over 70 years of age.

Figure 4 – Text description

Age-specific incidence rate (per 100,000 population) of reported human West Nile virus cases (clinical and asymptomatic) in Canada, 2021

Age group	Age-specified incidence rate Footnote h (per 100,000 population)
0-19	0.02
20-29	0.02
30-39	0.07
40-49	0.12
50-59	0.23
60-69	0.17
70-79	0.22
80+	0.18

Travel-related cases

There were two cases that reported travel outside of Canada to the United States (US) and one case reported travel but did not specify a location.

There were seven cases that reported travel within Canada, including two cases that traveled within the province or territory of residence (Ontario: n=1, Québec: n=1) and five cases that traveled outside the province or territory of residence but within Canada (British Columbia: n=1, Ontario: n=2, Québec: n=2).

Mosquito, wild bird and horse surveillance

During the 2021 mosquito season, 17, 562 mosquito pools were tested for WNV in four provinces and one territory: Ontario (n=13,585), Québec (n=1,987), Manitoba (n=1446), Saskatchewan (n=360) and Northwest Territories (n=184). Of these, 240 (1.4%) pools tested positive for WNV: 105 in Ontario, 116 in Manitoba, 10 in Québec, and nine in Saskatchewan (Table 2). In 2021, the percent of mosquito pools positive for WNV was highest in Manitoba (8.0%) and Saskatchewan (2.5%). The percent positivity for tested mosquito pools is within the typical range and is similar to the percent positivity reported in 2020.

The CWHC tested 164 dead wild birds for WNV. Of these, 31 (19%) were positive for WNV in three provinces: Ontario (n=11), Québec (n=14) and Saskatchewan (n=6) (Table 3). In 2021, WNV was detected in dead wild birds from late May to early November. The number of dead wild birds positive for WNV in 2021 was similar to the number of positive birds in 2019 and 2020, and less than the average annual number of dead wild birds positive for WNV in the previous five years (mean = 105) (Figure 5).

The CFIA was notified of 19 horse WNV cases in the following four provinces: Alberta (n=1), Saskatchewan (n=9), Manitoba (n=6) and Ontario (n=3) (Table 3). The number of horse cases in 2021 (n=19) was greater than the number of horse cases in 2019 (n=8) and 2020 (n=6), but lower than the average annual number of horse cases in the previous five years (mean = 47) (Figure 5).

Table 2. Number of mosquito pools tested for West Nile virus, number of positive mosquito pools and percent positivity by testing province in Canada^{Footnote i}, 2021

Province/Territory	Number of positive Pools	Total pools tested	Percent positive pools	Month with highest percent positivityFootnote j
Saskatchewan	9	360	2.50%	August
Manitoba	116	1,446	8.0%	August
Ontario	105	13,585	0.80%	August
Québec	10	1,987	0.50%	August
Northwest Territories	0	184	0%	N/A
Total	240	17, 562	1.40%	-

Table 3. West Nile virus-positive birds and horses by province^{Footnote k} in Canada, 2021

Province/Territory	Number of positive birds	Number of positive horses
Alberta	0	1
Saskatchewan	6	9
Manitoba	0	6
Ontario	11	3
Québec	14	0
Total	31	19

Figure 5 – Text Description

Reported number of human West Nile virus cases (clinical and asymptomatic), positive dead wild birds and positive horse cases in Canada, 2016-2021

	Year
Number of cases	2016	2017	2018	2019	2020	2021
Humans	106	194	421	36	162	43
Horses	46	54	123	8	6	19
Birds	131	145	164	44	41	32

California serogroup (CSG) viruses

In 2021, the NML identified a total of two human CSG virus infections. Further testing confirmed one infection was Jamestown Canyon virus and the other was snowshoe hare virus. Additionally, the province of Québec separately reported 16 human CSG virus infections (11 confirmed, 5 probable); of which, 7 were confirmed as Jamestown Canyon virus, 6 were confirmed as snowshoe hare virus, and 3 were unspecified. However, these 16 infections were classified using a different case definition^{Footnote 2} than the two infections reported by the NML.

In 2021, the Northwest Territories tested 184 mosquito pools for CSG viruses; of which, 42% (n=78) tested positive. This is slightly higher than the proportion of pools that tested positive in the Northwest Territories in 2020 (33%). The collection dates for positive mosquito pools in 2021 ranged from May 27 to August 26. The epidemiological week with the highest number of mosquito pools positive for CSG viruses was week 27 (n=23), in early July. The proportion of mosquito pools positive for CSG viruses was highest in epidemiological week 21 (100%) at the end of May and epidemiological week 31 (100%) in early August (Figure 6). Note that in week 21 the number of mosquito pools tested was very low (n=2).

Figure 6 – Text description

Epidemiological WeekFootnote d	Negative mosquito pools	Positive mosquito Pools	Total mosquito pools	Percent positive mosquito pools
week 20	1	0	1	0
week 21	0	2	2	100
week 22	1	3	4	75
week 23	15	0	15	0
week 24	5	0	5	0
week 25	9	0	9	0
week 26	14	0	14	0
week 27	6	23	29	79.3
week 28	29	3	32	9.4
week 29	9	13	22	59.1
week 30	10	4	14	28.6
week 31	0	10	10	100
week 32	1	10	11	90.9
week 33	2	8	10	80
week 34	4	2	6	33.3

The collection date of the mosquito pools is used to assign an epidemiological week using the CDC epi week calendar.

Eastern equine encephalitis (EEE) virus

In 2021, no known human cases of EEE were reported in Canada. The CFIA was notified of seven cases of EEE in horses in Ontario. During the 2021 mosquito season, 87 mosquito pools were tested for EEE virus in Ontario, of which 3% (n=3) tested positive.

Discussion

The 2021 season marks the second lowest incidence of human WNV infections reported in the last six years. The low incidence of WNV in humans was accompanied by low evidence of WNV activity in birds and mosquitoes across multiple jurisdictions in Canada during the 2021 mosquito season.

The low numbers of human WNV cases reported in Canada differs from what was experienced in the US in 2021. The CDC reported an increase in the number of cases of WNV infections in humans in 2021 (n=2,911) compared to 2020 (n=731), and an increase in number of deaths in 2021 (n=227) compared to 2020 (n= 66)^{Footnote 3}. Part of this increase in cases can be attributed to a large outbreak of WNV infections in the southwestern state of Arizona. Three counties in Arizona (Maricopa, Pima, and Pinal) reported more than 50% of all WNV cases across the US in 2021^{Footnote 4}. In addition, the US had its highest national rate of WNV neuroinvasive disease since 2012 (0.61 per 100,000 population), which surpassed the median rate during 2010–2020 (0.39 per 100,000 population)^{Footnote 4}. In contrast, the rate of WNV neurological syndrome cases (0.06 per 100,000 population) was low in Canada in 2021.

Lower numbers of WNV infections were observed in jurisdictions outside of North America. Incidence rates for locally acquired cases in Europe in 2021 decreased when compared to 2020, with the European Centre for Disease Control (ECDC) reporting 159 locally acquired cases of WNV infections in humans, including 10 deaths^{Footnote 5},^{Footnote 6}.

Annual fluctuations in the reported number of human WNV cases, dead wild birds and horse infections, and percent positivity in mosquito pools are expected. The annual incidence of WNV is impacted by a variety of factors including but not limited to climatic conditions, vector abundance and human behaviour. In a study looking at mosquito development in Manitoba performed over two mosquito seasons, Culex tarsalis, the main vector in the Prairies, favoured high degree days, which was noted as consistent with their biology^{Footnote 7}. The authors also described the 2021 summer in Manitoba as dry and found higher trap counts for Culex tarsalis in 2021 compared to 2020^{Footnote 7}. In addition, Canada continued to face public health challenges in 2021 due to the COVID-19 pandemic. There are many factors related to the COVID-19 pandemic that may have impacted surveillance data, although the extent of their impact is unknown. For example, during the COVID-19 pandemic there were changes in the availability of healthcare and in health-seeking behaviours, which may have affected the diagnosis and reporting of vector-borne diseases, particularly for cases with less severe signs, symptoms and outcomes^{Footnote 8}. In addition, travel restrictions during the COVID-19 pandemic could have altered the probability of exposure to infected mosquitoes.

In Canada, two CSG virus infections in humans were reported in 2021 by the NML and 16 infections were reported by Québec. California serogroup viruses, such as Jamestown Canyon virus and snowshoe hare virus, are endemic in Canada.

As CSG virus infections are not nationally notifiable diseases in Canada, there is currently no formal surveillance system in place to monitor, track and report cases. Furthermore, infections caused by CSG viruses are likely under-diagnosed due to factors such as a lack of awareness among healthcare professionals in Canada^{Footnote 1}. In 2021, the proportion of mosquito pools positive for CSG viruses in the Northwest Territories was high (42%) compared to other mosquito-borne diseases such as WNV.

Mosquito surveillance strategies may vary for different diseases and between jurisdictions, which may contribute to differences in the number of mosquito pools tested and the percent positive. There is some evidence that suggests the prevalence of CSG virus infections in Canada may increase with climate change^{Footnote 9}. Climate change is warming the Arctic at an increased rate and could change the diversity and activity of mosquito vectors in the area^{Footnote 10}.

There were no reports of human cases of EEE in Canada in 2021, though seven cases of EEE in horses were reported. To date, only two known human cases of EEE have ever been reported in Canada, a case in 2016 and another case in 2020, both residents of Ontario. Cases of EEE remain relatively rare. As with CSG virus infections, EEE virus infections are not nationally notifiable in humans and there is no formal surveillance system in place to classify and count cases. There were very few human cases of EEE reported in the US in 2021 (n=5) compared to the average annual number of human cases of EEE in the previous 10 years (mean = 11)^{Footnote 11}.

Public health conclusions

West Nile virus is the leading cause of domestically acquired mosquito-borne disease in Canada. WNV illness can occur in people at any age, but groups at higher risk of developing neurological disease include people over the age of 50 and some immunocompromised persons^{Footnote 12}. There continues to be annual fluctuations in the number of WNV infections in people and other indicators of WNV activity such as infections in birds and horse, and positive mosquito pools. Other viruses such as EEE virus and CSG viruses cause sporadic infections in humans. There are no vaccines or specific treatments for human WNV, EEE virus and CSG virus infections. Therefore, prevention strategies including education and promotion of personal protection (i.e., long-sleeved clothing, permethrin-treated clothing, window screens and mosquito repellents) from mosquito bites, as well as mosquito control, are critical for decreasing the risk of mosquito-borne infections in people. Ongoing national surveillance is needed to help target prevention and control efforts, and increase awareness among healthcare professionals, especially of CSG and EEE viruses.

For more information including populations-at-risk, symptoms and treatment, please refer to Canada.ca.

Acknowledgments

The Public Health Agency of Canada would like to acknowledge the provincial and territorial WNV and other mosquito-borne disease programs, Canadian Blood Services, Héma-Québec, the Canadian Wildlife Health Cooperative (CWHC), and the Canadian Food Inspection Agency (CFIA) for their participation in the National WNV Surveillance Program.