Lyme disease, the most common tick-borne disease in North America, is a multisystemic illness of humans and domestic animals caused by the spirochete Borrelia burgdorferi sensu lato ("in the broad sense").1 Small mammals are the main reservoir hosts for this bacterium, and Ixodes scapularis (blacklegged tick; Fig. 1) and Ixodes pacificus (western blacklegged tick) found in British Columbia are the principal vectors in Canada.2
Fig. 1: Left: Blacklegged tick, Ixodes scapularis, unfed adult female. This tick is a competent vector for the spirochete causing Lyme disease, Borrelia burgdorferi. Total body length ranges from 2.0 to 3.5 mm; this specimen is 3.2 mm in length. Right: American dog tick, Dermacentor variabilis, unfed adult female. Total body length ranges from 3.0 to 5.0 mm; this specimen is 4.5 mm in length. This is the tick species most frequently submitted for identification in Ontario; it is not a competent vector for B. burgdorferi. Both ticks are found in southern Ontario.
Figure 1. Continued.
As part of collaborative research between the Vector-borne Diseases Laboratory, BC Centre for Disease Control, and the Lyme Disease Association of Ontario, Ontario veterinarians submitted 139 I. scapularis ticks (collected from dogs in 1997 and 1998 in southern Ontario locations) to the laboratory to be tested for B. burgdorferi. For removal, a tick was held as close to the host's skin as possible with fine-pointed tweezers, and was removed from the skin with gentle but steady pressure. Each tick was stored in a small plastic vial with a moist paper towel for shipment to the laboratory for analysis.
The hosts of 121 (87%) of the ticks had no reported history of out-of-province travel. Two (1.6%) of these 121 ticks (1 from Mississauga and 1 from Etobicoke [part of Toronto]) produced motile spirochetes, which were subsequently identified as B. burgdorferi by monoclonal antibody and polymerase chain reaction tests (Table 1). In an additional 7 (5.8%) of the 121 ticks, B. burgdorferi was detected directly by polymerase chain reaction (Table 1). The first blood sample from host dogs was normally taken 4-6 weeks after tick removal. Subsequently, serum samples from all 9 dogs were tested by indirect immunofluorescence antibody assay and Western blot to identify specific antibodies against B. burgdorferi; such antibodies were observed in all samples. In a few cases the indirect immunofluorescence antibody titres were low (Table 1); however, all of the Western blots were reactive, with 5 or more bands needed for identification. Because most dogs with Lyme disease are asymptomatic, Western blot provides valuable diagnostic information about the presence of B. burgdorferi infection.
Table 1.
The wide distribution of I. scapularis ticks and the fact that some of them carry B. burgdorferi shows that the bacterium is present in southern Ontario. The fully engorged female ticks collected in Mississauga and Etobicoke not only produced live cultures of B. burgdorferi, but also laid eggs that hatched into larvae. The fact that these females had mated recently indicates that I. scapularis may be established in these locations, as well as at Long Point, Point Pelee National Park and Rondeau Provincial Park (on the north shore of Lake Erie), where the species is endemic. The Mississauga occurrence is the first report of a gravid I. scapularis female producing larvae in Ontario at a location other than Long Point. The other locations at which I. scapularis were positive for B. burgdorferi include Bramalea, Chatham, Hamilton, Ottawa, Point Pelee, Scarborough (part of Toronto) and Westport (Fig. 2).
Fig. 2: Locations in southern Ontario (asterisks) where Ixodes scapularis ticks carrying Borrelia burgdorferi were collected, in 1997-1999, from dogs with no history of out-of-province travel. Ixodes scapularis ticks carrying B. burgdorferi are endemic at Long Point. At Rondeau Provincial Park I. scapularis ticks are established and B. burgdorferi is present.3 Ixodes scapularis ticks are also established at Point Pelee National Park.
On Apr. 28, 1997, a domestic cat from Aylmer, Que., was taken to an emergency veterinary clinic in nearby Ottawa where a fully engorged I. scapularis tick was removed; the tick later tested positive for B. burgdorferi (by polymerase chain reaction). Eight days before removal of the tick, the cat had been symptomatic (displaying anorexia and lethargy) and had been treated with antibiotics. This incident demonstrates that people and pets can bring I. scapularis ticks infected with B. burgdorferi into Ontario from bordering areas. Current evidence suggests that I. scapularis does not have an established population across southern Ontario. However, I. scapularis ticks are dropped haphazardly by birds during spring migration,4 and in a recent study B. burgdorferi was isolated from an I. scapularis nymph retrieved from a common yellowthroat, Geothlypis trichas, in Nova Scotia during spring banding.5
A total of 280 cases of Lyme disease in humans were reported from 1981 to 1998 in Ontario. In 127 of these cases there was no history of out-of-province travel. In only 14 cases was there exposure at Long Point (Charles A. LeBer, Ontario Ministry of Health and Long-Term Care: personal communication, 1999). A recent study also reported co-infection by B. burgdorferi and Babesia spp. in a patient who had travelled to Nantucket, Mass.6
Across Canada, B. burgdorferi has been isolated in Prince Edward Island,7 Nova Scotia,5 New Brunswick, Quebec, Ontario,8 Manitoba (unpublished data; isolate provided by Harvey Artsob, Laboratory Centre for Disease Control, Health Canada, Winnipeg, 1997), Alberta9 and British Columbia.10 Even though the risk of acquiring Lyme disease in Ontario is low, the possibility of B. burgdorferi infection should not be ignored.
The present study indicates that I. scapularis is widespread in southern Ontario and may act as a source of B. burgdorferi infection for humans, domestic animals and wildlife. Consequently, the medical community should be aware that people who frequent the outdoors may be exposed to I. scapularis ticks and are at risk of contracting Lyme disease.
Addendum
In 1999 isolates of B. burgdorferi were cultured in our laboratory from I. scapularis ticks collected in southern Ontario at Kingston, Cobourg and LaSalle. The ticks were removed on Oct. 25, Nov. 11, and Nov. 18 respectively, from dogs with no history of out-of-province travel.
Footnotes
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This article has been peer reviewed.
We thank the veterinarians who submitted ticks and drew canine blood for serological testing. For designing the polymerase chain reaction (PCR) primers for this study, we thank Dr. Sean Byrne, British Columbia Centre for Disease Control. We also thank the Lyme Disease Association of Ontario for financial support and for providing information on the findings of I. scapularis in Ontario.
Competing interests: None declared.
Reprint requests to: Dr. Muhammad G. Morshed, Department of Pathology and Laboratory Medicine, University of British Columbia, 655 W 12th Ave., Vancouver BC V5Z 4R4; fax 604 660-6073; mmorshed@interchange.ubc.ca
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