Surveillance for Ixodes scapularis and Ixodes pacificus ticks and their associated pathogens in Canada, 2021

Abstract

Background: Ixodes scapularis and Ixodes pacificus ticks pose risk of infection with tick-borne diseases in Eastern and Pacific Western Canada, respectively.

Objective: In 2021, passive and active tick surveillance programs collected ticks and associated data elements, including location, infection and other characteristics, to monitor their populations and inform public health prevention and mitigation activities.

Methods: Surveillance data for ticks were compiled from the National Microbiology Laboratory (Public Health Agency of Canada), provincial public health, Canadian Lyme Disease Research Network and eTick (an image-based online platform). A descriptive analysis of tick records and infection prevalence of tick-borne pathogens is presented. Seasonal trends are described.

Results: During 2021, 6,892 I. scapularis ticks were identified across all ten provinces via passive surveillance with 777 I. pacificus ticks collected from British Columbia. Most were adult female ticks, collected from human hosts in the spring (March–May) or fall (October–November) seasons. The most common pathogen, Borrelia burgdorferi, was detected in 18.6% (95% CI: 17.2%–20.1%) of samples. Active surveillance resulted in 1,929 I. scapularis and 18 I. pacificus ticks collected in six provinces. Among I. scapularis, 22.3% were infected with B. burgdorferi, 11.8% with Babesia odocoilei and 4.3% with Anaplasma phagocytophilum. Fewer than 1% were infected with each of Borrelia miyamotoi (0.7%), Babesia microti (0.1%) and Powassan virus (0.1%).

Conclusion: As the risk of infection with tick-borne diseases continues to grow in many parts of Canada, monitoring trends in infection prevalence and the geographical range expansion of ticks provides essential data to inform public health actions and messaging.

Introduction

Ixodes scapularis and Ixodes pacificus ticks are known to transmit several bacterial, viral and protozoan pathogens to humans in Eastern/Central and Western Canada, respectively. They are doing so in increasing population numbers and across a broader geographical range due to climate and environmental changes ^{Footnote 1Footnote 2Footnote 3Footnote 4Footnote 5}. The resulting increase in potential for tick-borne diseases in the country, especially in Southern Central and Eastern Canada, has been emphasized in previous reports and in research literature and requires ongoing surveillance so prevention efforts can be successful ^{Footnote 1Footnote 6Footnote 7Footnote 8}. Case numbers of Lyme disease reported in Canada in 2022 have increased more than sevenfold since 2012 ^{Footnote 9}. Additional tick-borne diseases transmitted by I. scapularis or I. pacificus, namely, anaplasmosis, babesiosis and Powassan virus disease, are nationally notifiable diseases in Canada as of early 2024 ^{Footnote 10Footnote 11Footnote 12Footnote 13}.

Although tick surveillance has been conducted in Canada since the 1990s, data started to be summarized annually at the national-level by the Centre for Food-borne, Environmental and Zoonotic Infectious Diseases, Public Health Agency of Canada (PHAC) in 2019, and provide a baseline for tick-borne disease risk that, over time, will help to identify trends ^{Footnote 14}.

The objective of this annual surveillance report is to update the summary of characteristics of the main Lyme disease vectors in Canada, I. scapularis and I. pacificus, collected through passive and active surveillance during 2021. This article also summarizes the prevalence and spatial distribution of their associated pathogens.

Methods

Data sources

This report uses two types of surveillance data from more than 20 different providers. Passive surveillance datasets were provided by the National Microbiology Laboratory (NML) branch of PHAC, British Columbia Centre for Disease Control, Saskatchewan Ministry of Health and eTick. Active surveillance datasets were provided by the Canadian Lyme Disease Research Network, 12 Ontario health units, Laboratoire de santé publique du Québec, University of Manitoba, Manitoba Health, Seniors and Long-Term Care Department, New Brunswick Department of Health, University of New Brunswick and University of Ottawa.

Passive tick surveillance: As in 2020, this analysis was limited to I. scapularis and I. pacificus collected in Canada in the pertinent year ^{Footnote 6}. Provinces with five or fewer ticks of a given species submitted for species identification and laboratory testing were excluded to avoid misinterpretation of results. Ticks with a location of acquisition outside of the province of submission were not geocoded.

Additional regional passive tick surveillance programs have been discontinued since the publication of the previous report due to laboratory capacity constraints and as I. scapularis populations have become established. As before, ticks (or their images) acquired in these jurisdictions could be submitted by the public directly to NML or eTick.

eTick is a web-based, community-science project inviting the public to help with population tick monitoring and is used as a passive surveillance system for ticks in Canada ^{Footnote 15}. Individuals submit images of ticks they encounter online or via the mobile application, which are then examined by trained personnel to identify the species. Only one tick can be submitted in a single image-based submission.

Ticks collected and submitted from Alberta, Manitoba, Ontario, Québec, New Brunswick and Nova Scotia and tested for Anaplasma phagocytophilum, Borrelia burgdorferi, Borrelia miyamotoi and Babesia microti at the NML using methods previously described were included in this report ^{Footnote 16Footnote 17}. Among ticks tested by the British Columbia Centre for Disease Control, only results for B. burgdorferi were included in this report. Additional details regarding methodology are available in the previously published annual report ^{Footnote 6}.

Active tick surveillance: In active surveillance, ticks are collected from the environment using drag sampling or capturing host mammals that are then examined for ticks. This analysis used data from efforts to collect ticks from 10 sites in British Columbia, six in Alberta, at least eight in Saskatchewan, nine sites in Manitoba, more than 60 in Ontario, 36 in Québec, 14 sites in New Brunswick and 10 in Nova Scotia. Drag sampling took place in late spring/summer (May–July) and fall (September–November). Ticks were tested for some or all of the following pathogens: A. phagocytophilum; B. microti; B. odocoilei; B. burgdorferi; B. miyamotoi and Powassan virus.

Analysis

Tick characteristics: For passive surveillance, descriptive statistics were calculated for submission type (sample-based or image-based), tick species, province of acquisition, stage (larva, nymph, adult female or adult male), level of engorgement (unfed or engorged), host (human, dog, cat or other) and month of collection. For active surveillance, descriptive statistics were calculated for province of collection and stage (larva, nymph, adult female or adult male). All data were cleaned and analysed in R (version 4.0.2).

Ticks submitted through passive surveillance that were acquired in Canada and not associated with a travel history to other provinces or countries were mapped using QGIS software (version 3.34.7) based on their location of acquisition. Ticks submitted with a record of history of travel in the previous 14 days within the same province as the locality of acquisition were geocoded to the submitter-provided location of exposure during travel. In active surveillance, the site location of tick dragging was geocoded from data obtained from the NML and mapped for all data.

Infection prevalence: To account for pooled testing of ticks collected by passive surveillance from some jurisdictions, maximum likelihood estimates of prevalence were calculated with 95% confidence intervals (CI) using the PooledInfRate R package (version 1.6) ^{Footnote 18Footnote 19}. This estimates the probability of infection for an individual tick in the population using the results of testing of the pooled samples (i.e., a group of one or more ticks submitted and tested together). The package was developed by the United States’ Centers for Disease Control and Prevention ^{Footnote 19}. Co-infection prevalence was calculated among single submissions only to ascertain true co-infections, that is, two or more pathogens in a single tick. Where ticks were not tested in pools, prevalence was the number of positive ticks divided by the number of ticks tested.

Results

Passive surveillance tick characteristics

In 2021, 7,669 I. scapularis (n=6,892) and I. pacificus (n=777) ticks were submitted by provinces across Canada, with at least 14 submissions per province (Table 1, Figure 1). Image-based submissions comprised 54.9% of ticks submitted (n=4,210) and the remainder were sample-based submissions (n=3,459). Ticks from Ontario, Québec and Nova Scotia comprised 83.5% of all ticks submitted. The majority (98.3%) of ticks were from single submissions.

Figure 1: Descriptive text

This map shows the probable location of acquisition of Ixodes scapularis and Ixodes pacificus ticks submitted through passive surveillance. Ixodes pacificus ticks were present in British Columbia. Ixodes scapularis ticks were present across all provinces to varying extents.

 

Tick stage, level of engorgement and host were available for 98.2%, 89.3% and 100% of I. pacificus records and for 80.3%, 40.0% and 99.9% of I. scapularis records, respectively (data not shown in table). Ticks submitted only via eTick did not include information about engorgement. The majority of ticks submitted in sample-based submissions were adult female ticks (I. pacificus: 96.2%; I. scapularis: 86.8%) (Table 2).

A larger proportion of I. scapularis were engorged upon submission than I. pacificus (41.8% vs 11.1%, respectively) (Table 2). Most I. pacificus submissions were obtained from human hosts (91.4%) while a majority of I. scapularis were obtained from human and dog hosts (69.8% and 24.2%, respectively).

Records including both month of acquisition and tick stage made up 98.2% of I. pacificus and 80.2% of I. scapularis submissions (Figure 2). All submissions missing tick stage information were image-based submissions, comprising 32.6% of those submissions (n=1,374/4,210).

Figure 2: Descriptive text

2A)

Month of collection	Adult	Larva	Nymph
January	4	0	0
February	1	0	0
March	201	0	2
April	1,281	1	7
May	1,004	1	75
June	417	0	280
July	122	19	198
August	19	3	47
September	77	0	15
October	1,205	0	9
November	520	0	0
December	23	0	0
Unknown	1	0	0
Total	4,875	24	633

2B)

Month of collection	Adult	Nymph	Total
January	40	0	40
February	39	0	39
March	126	0	126
April	131	0	131
May	212	5	217
June	110	10	120
July	14	1	15
August	5	0	5
September	3	0	3
October	26	0	26
November	33	0	33
December	9	0	9
Total	764	16	764

 

Adult I. scapularis submissions, comprising 80.2% of total I. scapularis submitted through passive surveillance, peaked in April and again in October and nymph submissions, comprising 11.4%, peaked in June. For adult I. pacificus, submissions peaked in May with a subsequent marginal peak in November.

Infection prevalence in passive surveillance

Data on laboratory testing was available for 98.6% of I. pacificus and 99.4%–99.8% of I. scapularis from sample-based submissions, depending on pathogen. Nearly one in five I. scapularis ticks in Canada is estimated to be infected with at least one tick-borne pathogen (A. phagocytophilum, B. burgdorferi, B. miyamotoi or B. microti) (95% CI: 18.3%–21.3%). The most prevalent pathogen was B. burgdorferi, detected in 18.6% of I. scapularis (95% CI: 17.2%–20.1%). Anaplasma phagocytophilum was detected in 1.1% of I. scapularis (95% CI: 0.8%–1.6%). Other tick-borne pathogens were estimated to have a prevalence of fewer than 1% each (0.04% positive for B. microti (95% CI: 0.00%–0.18%) and 0.4% positive for B. miyamotoi (95% CI: 0.2%–0.7%) (Table 3).

Among 684 I. pacificus ticks, 0.9% indicated presence of B. burgdorferi (95% CI: 0.4%–1.8%) (Table 4). Co-infections in I. scapularis ticks were also estimated to have a prevalence of fewer than 1.0% each (Table 3).

Ticks infected with tick-borne pathogens were primarily found in Southern and Eastern Ontario, Southern Québec, New Brunswick and in Nova Scotia (Figure 3, Figure 4). Of the seven provinces where sample-based submissions were included in our analysis, B. burgdorferi-infected tick specimens were found in five: British Columbia, Ontario, Québec, New Brunswick and Nova Scotia (Table 4).

Figure 3: Descriptive text

This map shows the probable location of acquisition of Ixodes scapularis ticks submitted through passive surveillance that were infected with Borrelia burgdorferi. The map also shows Lyme disease risk areas. Infected ticks are found in Ontario, Québec, New Brunswick, and Nova Scotia to varying extents. Most infected ticks are found in southern Ontario and Québec.

 

Figure 4: Descriptive text

This map shows the probable location of acquisition of Ixodes scapularis ticks submitted through passive surveillance that were infected with Anaplasma phagocytophilum, Borrelia miyamotoi or a co-infection with two or three of the following: A. phagocytophilum, Borrelia burgdorferi, B. miyamotoi and B. microti. Ticks infected with A. phagocytophilum were found in Saskatchewan, Ontario, and Québec. A tick infected with B. microti was found in Québec. Ticks infected with B. miyamotoi were found in Ontario, New Brunswick, and Nova Scotia. Ticks with co-infections were found in Ontario, Québec, New Brunswick, and Nova Scotia.

 

Anaplasma phagocytophilum was found in I. scapularis in all provinces where ticks were tested except Alberta; B. burgdorferi in all except Alberta and Saskatchewan (infection prevalence of 1.1% and 18.6%, respectively) (Figure 3, Figure 4, Table 4). Borrelia miyamotoi was found in Ontario, Québec, New Brunswick and Nova Scotia, while a single Babesia microti-infected tick was found in Québec.

Active surveillance tick characteristics

In 2021, I. scapularis (n=1,935) were collected and tested from five provinces: New Brunswick (n=475), Ontario (n=850), Québec (n=393), Manitoba (n=119) and Nova Scotia (n=98). Of these, the majority of specimens were adults and nymphs followed by larvae (14; 0.7%). In addition, 18 I. pacificus were collected in British Columbia.

Infection prevalence in active surveillance

Laboratory testing results for at least one pathogen were available for 99.5% of I. scapularis. The most prevalent pathogen was B. burgdorferi, present in all five provinces where I. scapularis were collected through active surveillance: Manitoba, Ontario, Québec, New Brunswick and Nova Scotia (Table 5). Borrelia burgdorferi was detected in 22.3% of ticks tested, compared to 29.3% in 2020 ^{Footnote 6}.