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. 2021 May;62(5):469–476.

Surveillance of West Nile virus in horses in Canada: A retrospective study of cases reported to the Canadian Food Inspection Agency from 2003 to 2019

Antoine Levasseur 1, Julie Arsenault 1, Julie Paré 1,✉
PMCID: PMC8048205  PMID: 33967285

Abstract

The objectives of the study were to describe the regional and provincial incidence rates and the weekly distribution of 842 reported West Nile virus (WNV) cases in horses in Canada between 2003 and 2019. This study also investigated characteristics of cases reported to the Canadian Food Inspection Agency (CFIA) between 2015 and 2019. The western region (British Columbia, Alberta, Saskatchewan, and Manitoba) had higher incidence rates than the eastern region (Ontario, Quebec, and Atlantic provinces) and overall, Saskatchewan registered the highest incidence. Over the study period, an earlier weekly preliminary onset of WNV cases was observed in the western region. The vast majority of cases were unvaccinated (96%), most cases were Quarter Horses (68%) and the risk of mortality was 31.9%. The findings of this study may be useful in informing veterinary equine practitioners about measures to prevent WNV disease in horses in Canada.

Introduction

West Nile virus (WNV) is an arthropod-borne flavivirus first reported in humans and horses in Canada in 2002 (1). Passive surveillance in animals began in 2003 when the Canadian Food Inspection Agency (CFIA) added WNV to its list of Immediately Notifiable Diseases (IND). Under the Health of Animals Regulations, any laboratory that diagnoses or suspects an IND in an animal in Canada shall notify the Minister (CFIA) immediately (2). This activity generates data that are collected uniformly nationwide for trade support, international reporting, and public health purposes. In 2006, WNV became a World Organisation for Animal Health (OIE) listed disease (3), requiring cases to also be reported to the OIE.

The global prevalence of WNV in animals has increased in recent years (3), after rapidly becoming endemic in North America following its introduction in 1999 (4). In North America, WNV maintains an enzootic cycle involving mosquitoes and birds, with the latter acting as the amplifying hosts and reservoir species. The main vectors of WNV in Canada are Culex tarsalis in western Canada and Culex pipiens/restuans in eastern Canada (5). This cycle is influenced by ecological factors such as land use and meteorological conditions (4,6), affecting geographical and temporal variations in WNV transmission (5). Reported changes in mosquito feeding patterns from birds to mammals in summer months may be an important determinant for WNV spillover (7), but has yet to be confirmed as an important determinant for human and horse infections (8). Climate change reportedly has a significant effect on WNV ecology, but this is not fully understood (4). In Canada, establishment and expansion of WNV to new regions may be intensified by climate change, increasing the period of activity and the geographical distribution of WNV vectors (4,5,7).

Horses are frequent accidental hosts for WNV. Following an incubation period of 1 to 2 wk (6), 8 to 10% of WNV-infected horses exhibit neurologic signs of the disease (e.g., ataxia, weakness, paralysis) (9,10). Reported risks of mortality in WNV clinically affected horses vary from 22 to 44% (6), with no specific treatment, other than supportive care, leading to uncertain prognoses and often only partial recovery (6). Several vaccines are available for horses in Canada; they are effective at reducing viremia, clinical signs, and mortality (11). Other recommended risk reduction strategies include indoor housing and mosquito control (e.g., reducing standing waters, mosquito nets for windows) (6,12).

Regional outbreaks and potential risk factors associated with WNV in horses in North America have been described and investigated (13–17), but larger scale studies are warranted to evaluate spatiotemporal dynamic and continuous spread of WNV to new areas (4). The primary objectives of this study were to describe at regional and provincial levels: i) the average and annual incidence rates of WNV; and ii) the weekly distribution of equine case notifications. The regional level (eastern versus western regions) was determined according to main WNV vector species distribution. A secondary objective was to describe demographic data (age, sex, breed group), vaccination status, and subsequent clinical outcome of these cases, in order to better characterize horses infected with WNV in Canada.

Materials and methods

Study design, period, and area

A descriptive retrospective epidemiological study of equine WNV cases reported to the CFIA between 2003 and 2019 was undertaken. Canadian provinces were classified either as part of the western region (British Columbia, Alberta, Saskatchewan, and Manitoba) or the eastern region (Ontario, Quebec, and Atlantic provinces). Territories (Yukon, Northwest Territories, and Nunavut) were excluded from the study, as no evidence currently exists of virus circulation in mammals (unrelated to travel) (18,19), birds (20), or mosquitoes (19) within these regions.

Surveillance data and case definition

All laboratory notifications for WNV for the study period were extracted from the CFIA’s IND database. Only confirmed positive cases were included, as defined by a combination of neurological signs (clinical history) and positive laboratory tests (including detection of IgM antibodies, seroconversion on paired sera, virus isolation, or identification of viral antigens by molecular methods or immunolabeling) (21,22). For each case, the laboratory reference number of the case, the name of the submitting laboratory, the date of sampling, the date of submission (i.e., date laboratory received sample), the date of confirmation (i.e., date laboratory result was released) and the patient province of residence were recorded. In addition, for cases between 2015 and 2019, demographic data (age, sex, breed group), vaccination status, and clinical outcome (dead or alive) of the cases were actively sought and collated. Data cleaning was done in SAS 9.4 (SAS Institute, Cary, North Carolina, USA) to remove any duplicate records.

Incidence

Total incidence counts and average annual incidence rate of WNV cases were reported at national, regional, and provincial levels. The average annual incidence rate for each geographic level was calculated as the average annual number of cases over the average annual horse population. The average annual horse population over the study period was based on Agricultural Census data from Statistics Canada (2001, 2006, 2011, and 2016) (23,24) and on intercensal estimates (all other years). For intercensal years, populations were estimated using a linear function between 2 consecutive census years. The linear function between 2011 and 2016 was then used to extrapolate data until 2019. The annual incidence rates were also calculated for regions and provinces. All incidence rates were reported by 100 000 horses.

Weekly distribution

The weekly distribution of cases cumulated over all years was calculated for each region and their constituting provinces using the date of sampling, date of submission, and date of confirmation. The week was defined in accordance with the Centers for Disease Control and Prevention (CDC), the Morbidity and Mortality Weekly Report (MMWR) (25).

Case characteristics

Over the 2015 to 2019 period, the distribution of WNV cases according to demographic data, vaccination status, and clinical outcome was described. Only horses reported to have been vaccinated for WNV within 12 mo before diagnosis were considered vaccinated. Horses euthanized following the onset of neurologic disease were considered dead due to WNV. For each characteristic, only horses with known values were kept for the descriptive analyses.

Results

Over the study period, 842 confirmed positive cases of WNV in horses were reported to the CFIA (Table 1).

Table 1.

Total number and average annual incidence of equine West Nile virus cases reported to the Canadian Food Inspection Agency between 2003 and 2019 by region and province.

Geographic level Number of cases (% of the total) Average horse population (% of the population) Average annual incidence rate/100 000 horses
Eastern region 155 (18.4) 109 786 (29.5) 8.3
 Atlantica 0 (0) 6716 (1.8) 0
 Quebec 56 (6.65) 23 019 (6.2) 14.3
 Ontario 99 (11.76) 80 051 (21.5) 7.3
Western region 687 (81.6) 262 087 (70.5) 15.4
 Manitoba 89 (10.6) 33 880 (9.1) 15.5
 Saskatchewan 297 (35.3) 52 633 (14.2) 33.2
 Alberta 279 (33.1) 132 407 (35.6) 12.4
 British Columbia 22 (2.6) 43 167 (11.6) 3
Total 842 (100) 371 873 (100) 13.3
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a

Nova Scotia, New Brunswick, Prince Edward Island, Newfoundland and Labrador provinces.

Incidence

The western region registered more cases and a higher average annual incidence rate than the eastern region during the same study period (Table 1). Saskatchewan had the highest count and the highest average annual incidence rate of WNV infections. No cases were reported in the Atlantic provinces.

West Nile virus activity varied between each region over the years (Figure 1). Overall, 36% of cases were notified in 2003. The 2 highest peaks in incidence rates for the western region were in 2003 and 2018, whereas for the eastern region, the peak in incidence rates was in 2017. Between 2011 and 2018, there was a resurgence in the incidence of WNV in both regions studied (Figure 1).

Figure 1.

Figure 1

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a — Annual incidence rate of equine West Nile virus cases in the western region and provinces of Canada [British Columbia (BC), Alberta (AB), Saskatchewan (SK), and Manitoba (MB)]. b — Annual incidence rate of equine West Nile virus cases in the eastern region and provinces of Canada [Quebec (QC) and Ontario (ON)]. No cases were reported in Atlantic provinces. Annual incidence rates are reported as number of cases/100 000 horses. Horse population is based on data from the Canadian Census of Agriculture (23,24).

The annual incidence rates of WNV varied within western provinces (Figure 1a), with multiple high peaks occurring in Manitoba, Saskatchewan, and Alberta. Saskatchewan had the highest annual seasonal peaks of infections. Alberta and Saskatchewan recorded similar overall counts of WNV (Table 1), but the annual incidence rates were consistently higher in Saskatchewan (Figure 1a). British Columbia had the lowest incidence rates of the western provinces.

The trends in annual incidence rates were similar between Quebec and Ontario (Figure 1b). Peaks of infection were identified in both provinces in the same years, but their peaks varied in amplitude between 2012 and 2014.

Weekly distribution

Dates of sampling (n = 524), submission (n = 818), and confirmation (all cases) were reported. Among all cases with a date of sampling, the total delay in notification (i.e., time lag between sampling and confirmation) had a median of 5 d (95th percentile = 15 d). The time lag between sampling and submission dates ranged from 0 to 26 d (median = 2 d, 95th percentile = 6 d) in the western region and from 0 to 12 d (median = 1 d, 95th percentile = 5 d) in the eastern region. Longer delays were observed when the sample was collected at euthanasia. The time lag between submission and confirmation date ranged from 0 to 47 d (median = 2 d, 95th percentile = 11 d) in the western region and from 0 to 35 d (median = 7 d, 95th percentile = 20 d) in the eastern region. Longer delays were observed, for example, when a horse died with neurological signs and a post-mortem rabies investigation was undertaken prior to WNV testing.

The distribution of cases per surveillance week reflected a different seasonal pattern for each region, with an earlier occurrence in the western region for all the dates collated (Figure 2). Also, the eastern region registered more cases later in the season: all cases were confirmed between weeks 24 and 43 (median = week 36, 95th percentile = week 39) in the western region, compared to weeks 27 to 45 (median = week 37, 95th percentile = week 43) in the eastern region. The provincial weekly distributions indicated a similar trend between the provinces of the same region (Figure 2).

Figure 2.

Figure 2

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Weekly cumulative count and proportion of equine West Nile virus cases in western region and provinces of Canada [British Columbia (BC), Alberta (AB), Saskatchewan (SK), and Manitoba (MB)] and in eastern region and provinces of Canada [Quebec (QC) and Ontario (ON)] between 2003 and 2019 based on: a — date of sampling (n = 524); b — date of submission (n = 818); and c — date of confirmation (n = 842). The week was defined in accordance with definition used by the Centers for Disease Control and Prevention (CDC) (25).

Case characteristics

Between 2015 and 2019, 250 equine cases were reported to the CFIA. Age was collected for 234 of the 250 cases. A high proportion of young horses characterized the age distribution, but cases in horses up to 25 y of age were reported (median = 6 y) (Figure 3). No foals were reported as clinically affected by WNV. Sex was available for 239 cases, with the proportion of cases being similar between males and females (Table 2). Information on breed was available for 211 horses, with 19 breeds or breed group represented. Most reported cases were Quarter Horses (Table 2), and 90% of these were in the western region (n = 129). Standardbred cases were mostly reported in Ontario (10/14 cases).

Figure 3.

Figure 3

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Age of equine West Nile virus cases from 2015 to 2019 in Canada (n = 234).

Table 2.

Summary of case characteristics of equine West Nile virus cases in Canada between 2015 and 2019 (n = 250 horses). The percentage of cases were calculated for each characteristic and excluded missing values.

Characteristics Number of cases (%)
Sex
 Male 114 (47.7)
 Female 125 (52.3)
Breed group
 Quarter Horse 144 (68.2)
 Standardbred 14 (6.6)
 Drafta 9 (4.3)
 Warmblood 9 (4.3)
 American Paint 8 (3.8)
 Thoroughbred 7 (3.3)
 Otherb 20 (9.5)
Vaccination statusc
 Vaccinated 10 (4)
 Not vaccinated 238 (96)
Clinical outcome
 Alive 109 (68.1)
 Deadd 51 (31.9)
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a

Belgian (n = 1), Canadian (n = 1), Clydesdale (n = 2), Percheron (n = 5).

b

Appaloosa (n = 1), Arabian (n = 2), Fjord (n = 3), Friesian (n = 1), Haflinger (n = 2), Miniature (n = 3), Morgan (n = 1), Pony, unspecified breed (n = 4), Tennessee Walker (n = 2), Welsh Cob (n = 1).

c

Only horses reported to have been vaccinated by the submitting veterinarian in the last 12 mo prior to the infection were considered vaccinated.

d

Dead includes horses that were euthanized.

Vaccination status was documented for 248 of the 250 cases. Most clinically affected horses were unvaccinated (Table 2). Among the 10 vaccinated cases, the age ranged from 1 to 13 y.

Clinical outcome was known for 160 horses. Most horses recovered (overall risk of mortality of 31.9%) (Table 2). Among dead horses, only 1 had been vaccinated.

Discussion

This is the first extensive study of WNV in horses in Canada, describing 17 y of passive surveillance based on notifiable disease regulations. The results present the minimum number of WNV cases that occurred in the country during this period, as the source population was formed by cases investigated by equine practitioners and for which diagnostics for WNV were sought.

The high incidence of WNV in 2003 was likely due, along with local ecological factors, to the introduction of the virus into a naïve horse population. Although some WNV activity occurred as early as 2002 (1), natural or vaccine-induced immunization of the horse population in 2003 was likely low. Although WNV vaccines were first sold in Canada in 2003, the extent of vaccination is unknown.

The results of this study were consistent with the “boom and bust” epidemiological patterns reported for WNV (26), emphasizing the probable increase of vaccination following an outbreak in horses and the natural immunity in recovered horses (27). This pattern was also likely related to yearly weather variations which influence the virus transmission cycle (5,26). However, for most western provinces, the results of this study indicated a lower decline in incidence between annual peaks in the last decade than when WNV was introduced in the early 2000s. Since 2011, an increase in incidence rates was noted in both regions, suggesting a recent increase in WNV activity.

Surveillance of WNV cases in horses could be relevant to a One Health perspective, to inform both the animal and public health sectors regarding the risk of infection. As reported by others (28), the surveillance in horses provides indication on WNV activity in rural areas. Moreover, according to a study undertaken in Quebec, the seroprevalence in horses was reported to be higher than in humans (29), suggesting that horses could be sentinels for predicting human risk of exposure. However, there is limited information on distribution of various mosquito vectors in Canada and on the role of each in effectively infecting humans and horses. Interestingly, in this study, Saskatchewan had consistently high incidence rates of equine infection, yet few human cases were reported in this province over the same time period (18). This contrasts with the high number of both equine and human cases reported in Manitoba and Alberta for the same period (18). Spatial variations in the risk of infection due to local ecological factors and vaccination coverage in horses may affect the sensitivity of equine WNV surveillance in predicting human exposure and would warrant further investigation.

Two main sources of bias need to be considered when comparing regional or annual incidence of reported cases of WNV. First, the likelihood that a suspected clinical case would be submitted for WNV testing is unknown and may have varied in time and space, depending on factors related to horse owners and their veterinarians as well as on regional initiatives. In fact, throughout the study period, some provinces implemented additional regulations for WNV that may have encouraged veterinarians to submit samples and investigate WNV infections in horses. For example, in Alberta, WNV is an immediately notifiable disease and when suspected, the disease must be reported to the chief provincial veterinarian within 24 h (30). Another example is the free testing offered in Quebec until 2015 as part of the provincial surveillance program (31). A second source of bias may have been an inconsistent underestimation of the horse population by province. In fact, the Agricultural Census was used in this study to estimate the horse population, because it was the only available source of data on horse demographics consistently collected over the area and study period. However, only information on horses housed on agricultural operations (i.e., farms that produce agricultural products for sale) is captured in these census data, whereas declared horse cases of WNV could originate from all types of premises. A study from Equestrian Canada (32) conducted in 2010 indicated that 50% of the horses in Canada were housed on agricultural operations overall, but British Columbia and Quebec were the only provinces with lower numbers of horses on agricultural operations than on non-census operations. This might have led to an uneven overestimation of WNV incidence rates in space and time.

The results supported an earlier yearly onset of cases in the western region than the eastern region, suggesting different regional timing for viral transmission of WNV to horses. Regional differences in the seasonal shift in mosquito feeding preferences could have a role, as this factor has been reported to correlate with the timing of WNV infections in mammals (7). Other ecological factors driving the viral transmission of WNV could be involved, such as regional variations in climatic factors influencing mosquito abundance (5) or differences in feeding preferences of mosquito species specific to each region (4,33). The earlier seasonal incidence in the western region and the late extent of the WNV season in the eastern region were consistently observed for all surveillance dates. However, the date of sampling and the date of submission are likely more relevant than the date of confirmation for capturing ecological differences in the timing of infection since they are not influenced by the local availability of WNV diagnostic tests. In fact, serological tests are readily available in only 1 laboratory that almost exclusively receives samples from the western region, whereas other laboratories send samples for serological testing outside of Canada. This likely also contributed to longer delays between date of submission and date of confirmation for the eastern region.

The different surveillance dates can provide useful information for the seasonal planning of WNV testing by laboratories and horse vaccination by equine practitioners in Canada. When considering the incubation period, it is likely that horses were infected 1 to 2 wk before the date of sampling, which itself occurs 1 to 2 wk before case confirmation. Furthermore, as full immune protection after vaccination can take 2 wk to 1 mo (6,11) and given all the delays in case notifications, to ensure maximum protection, horses should be vaccinated at least 2 mo before the first confirmed cases in each region. In practice, this corresponds to mid-April in western provinces and early May in eastern provinces.

The distribution of age among the affected horses indicated that horses of any age, except for yearlings, can be clinically affected by WNV, even many years after the introduction of WNV into Canada. The number of cases among males and females was similar, as suggested by others (27). Compared to any other breed, Quarter Horses were more frequently observed among the cases in this study, similar to other studies (16,17). The high population of Quarter Horses in western provinces may partially explain their increased cases (34). In addition, as many western Quarter Horses work outdoors on feedlots, they could be at higher risk of exposure to vectors. In a study of owners with a WNV-infected horse, a higher percentage of Quarter Horse owners did not report on-farm use of mosquito control measures compared to owners of other breeds (13).

Most affected horses in our study had not been vaccinated in the year before diagnosis. This was consistent with a previous study from Saskatchewan which indicated that vaccination likely provided protection against development of WNV clinical disease (35). Along with the clinical portrait of affected horses in this study, this illustrated the importance of vaccination of horses of any age, sex, or breed. Mortality risk was similar to that reported in other studies, with approximately 1/3 of affected horses dying (6).

The findings of this study may be useful to inform veterinary equine practitioners on the occurrence of West Nile virus infections in Canada, as well as to characterize the clinically infected horses based on a nationwide standard case definition for all surveillance years. There were apparent differences between western and eastern regions in seasonality and incidence of WNV cases. Furthermore, there was high variability from year to year, but a consistent increase in incidence in the last decade. Moreover, even if higher incidence rates were recorded in western Canada, an important increase in case notifications in eastern Canada in recent years was also recorded. These findings can be used as a starting point for studies assessing seasonality, risk factors, and spatiotemporal patterns of the disease in horses in Canada on a more refined geographic scale.

Acknowledgments

The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) with a student grant attributed to Antoine Levasseur. The project was also supported by the Veterinary Student Internship Program of the CFIA. The authors extend their appreciation to CFIA, Provincial and Laboratory staff for their contributions to data collection. CVJ

Footnotes

Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office (hbroughton@cvma-acmv.org) for additional copies or permission to use this material elsewhere.

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