ARCHIVED - Descriptive epidemiology of Lyme disease in Ontario: 1999-2004
1 November 2006 Volume 32 Number 21
L Vrbova, Hon BSc, MSc (1), D Middleton, BSc, DVM, MSc (1)
Ministry of Health and LOng-Term Care, Toronto, Ontario
Lyme disease (LD), also called Lyme borreliosis, is a tick-borne zoonosis caused by the spirochete bacteria Borrelia burgdorferi. The disease is the most common tick-borne disease in North America1, and affects both humans and animals.
LD most often presents with a characteristic erythema migrans (EM) or ‘bull's eye rash' around the site of the tick bite, accompanied by non-specific symptoms such as fever, malaise, fatigue, headache, myalgia, and arthralgia2-4. The incubation period from exposure to the onset of EM rash ranges from 3 to 32 days (mean 7 to 10 days)5. LD is rarely, if ever, fatal. However, in some cases LD morbidity may be severe, chronic, and disabling, especially if the disease is treated late in its clinical progression2-4.
LD has been a Reportable Disease in Ontario since November 1988. The purpose of this article is to provide a summary of LD information as it relates to Ontario as well as to provide descriptive epidemiological information on LD cases reported in Ontario for the period 1999 to 2004.
Enzootic Cycle and the Vectors
The principal vectors of LD in Canada are the western blacklegged tick, Ixodes pacificus (in British Columbia) and the blacklegged tick, I. scapularis (in central and eastern Canada including Ontario). In localities where these ticks are established, B. burgdorferi circulates between ticks and reservoir hosts. Because these species of ticks are not host-specific and feed on a wide range of animal species (including birds, mammals and reptiles), many different animal species have the potential to act as reservoirs for the LD bacteria. However, in eastern North America, the white-footed mouse (Peromyscus leucopus) is considered to be the principal natural reservoir for B. burgdorferi6.
Vector ticks typically acquire the bacteria when they feed as larvae or nymphs and once infected they are capable of transmitting the pathogen to the next animal they feed upon (i.e., as nymphs or adults). Ticks have a 2-year three-instar life cycle. The three instars, larva, nymph and adult, each feed only once. Nymphs are active in the spring and early summer, and adult females feed in April-June and October-November. These therefore represent times of higher risk for people and pets.
It should be noted that the two species of ticks most commonly found on people and pets in Ontario are the American dog tick (Dermacentor variabilis) and the groundhog tick (I. cookei); however, based upon evaluations in the field7 and laboratory8 neither of these species are considered effective vectors of Lyme borreliosis.
In North America, human cases of LD are mostly found in the north-eastern United States, although they are also present in the mid-Atlantic, and north-central regions, and in several counties in north-western California9.
In Canada, the incidence of LD in humans is low. The distribution of competent tick vectors is used to identify areas where there is a higher risk of acquiring Lyme disease. Lyme disease endemic areas can be defined as those with
established populations of vector ticks and
evidence of enzootic transmission of B. burgdorferi between the tick vectors and resident animal populations (i.e., detection of bacterial infections in ticks and host animals).
The climate in Ontario is suitable for I. scapularis development in the southern Ontario region (south of North Bay) and the Rainy River-Kenora area of the north-western region. The regions of Thunder Bay and other areas in the north-central or northeastern Ontario region are not suitable for the tick's life cycle10. Reproducing populations of I. scapularis are established along the north shore of Lake Erie: Long Point, Point Pelee National Park, Rondeau Provincial Park, and Turkey Point10-13.
Enzootic transmission was demonstrated at Long Point, Rondeau Provincial Park, and Turkey Point, but not Point Pelee National Park. Populations of I. scapularis infected with B. burgdorferi were found in all four areas, however antibodies to B. burgdorferi in resident animal populations were only found at Long Point, Rondeau Provinical and Turkey Point13-15. Thus, as per the definition above, these three are the only areas in Ontario endemic for LD.
LD cases in humans and dogs without histories of travel to endemic areas, as well as the collection of I. scapularis ticks infected with B. burgdorferi, have occasionally been reported from non-endemic areas of Ontario16-17 and elsewhere in Canada18-20. Given the tendency of I. scapularis to feed upon birds21, it seems likely that the blacklegged ticks detected outside of endemic areas were introduced into these localities on migratory birds infested in endemic areas in the United States or Canada. These “adventitious” ticks, if infected with B. burgdorferi, could transmit the bacteria to humans or animals residing in areas outside of LD endemic areas22. Thus, it is possible for people or animals to acquire LD in non-endemic areas across Canada.
It also seems likely that establishment of vector ticks into new localities is most often facilitated by infested migratory birds, though translocation of deer or other animals may also assist in movement of ticks. The shipment of white-tailed deer from Minnesota (an LD endemic area) to Long Point in 1886 may have been the primary mechanism by which blacklegged ticks became established on Long Point14
Risk Factors and Prevention
The highest risk of acquiring LD is associated with frequenting an area with an endemic population of competent tick vectors. Therefore, individuals at higher risk of acquiring LD are those who live or work in residential areas surrounded by woods or overgrown brush infested by vector ticks, participate in outdoor recreational activities such as hiking, camping, fishing, and hunting in tick habitat, and engage in outdoor occupations such as landscaping, brush clearing, forestry, and wildlife and parks management in endemic areas23-24.
Preventive measures are designed to minimize exposure to ticks. Light-coloured clothing increases the likelihood of seeing ticks clinging to clothing. Hats and pantlegs tucked into socks provide physical barriers to the skin. Insect repellent containing DEET (maximum deterrence in concentrations around 30%) applied to clothing and skin is highly effective in repelling I. scapularis25. ‘Tick checks' conducted at the end of the day to detect and remove attached ticks are very important, especially the groin, axillary region, and head above the hairline. Tick removal within 24 hours of attachment reduces the probability of infection26. Detection of ticks can be difficult because unfed nymphs are very small (approximately 1 mm) and unfed adults, although larger, are also not very big (approximately 5 mm).
In Ontario, LD is a Reportable Disease under the Health Protection and Promotion Act. Public health units investigate cases of LD and transmit their reports of cases meeting the Ontario LD case definition to the Ontario Ministry of Health and Long-Term Care (MOHLTC) via the Reportable Disease Information System (RDIS).
Ontario Lyme Disease Surveillance Case Definition
Endemic*: isolation of B. burgdorferi from a clinical specimen OR erythema migrans observed by physician OR at least one clinically compatible late mani- festation (neurologic, cardiac or musculoskeletal) and laboratory evidence of B. burgdorferi infection).
Non-endemic*: erythema migrans observed by a physician and laboratory evidence of B. burgdorferi infection.
* Definition of an endemic area: one in which the presence of an established vector population (e.g. identification of the three stages of I. scapularis - larva, nymph, and adult) is known to be infected with B. burgdorferi (currently Long Point on the north shore of Lake Erie is the only endemic area in Ontario).
A provisional definition of an endemic area can be made where there is the occurrence of at least three well defined confirmed cases, for whom there are no histories of exposures in a previously identified endemic area. However, the three confirmed cases must be based on the revised case definition and the geographic limits of the endemic area will be defined by the Ministry of Health and Long-Term Care.
A case was categorized as “travel-associated” if the ‘risk setting' category in RDIS indicated that the case travelled to or lived in an endemic area outside of Ontario. All other cases were classified as either “endemic” or “unknown” depending on the additional information provided. For those cases categorized as endemic, maps of cases were created by health unit of residence as well as location of “most likely exposure”. Location of most likely exposure was defined as the health unit in which the case resided unless travel to another region in Ontario was specified in the record.
Incidence rate calculations were calculated using the 2001 Canada Census population counts. SPSS v.12.0 was used for statistical analysis and graphs were created using MS Excel XP. Mapping was done using Environmental Systems Research Institute Inc (ESRI) ArcGIS 9.0.
There were a total of 172 cases of LD reported on RDIS with dates of onset between 1999 and 2004 inclusive. Of these cases, 31 (18%) were acquired in Ontario, 110 (64%) were travel associated, and travel history was unknown for the remaining 31 (18%) cases. Figure 1 shows the number of cases over the 5-year period by travel status. Most of the reported cases had a date of onset between June and September (Figure 2). There were approximately the same number of females (n = 87) as males (n = 84), and the highest number of cases occurred in the 50 to 59 age category for females, and the 40 to 49 age category for males (Figure 3).
For those that reported symptoms, the two most commonly reported symptoms were fever and rash (both reported for 12% of cases), followed by erythema migrans (10% of cases) and stiff neck (7% of cases) (Figure 4).
Endemic cases accounted for 18% (31/172) of the cases reported between 1999 and 2004. There were almost twice as many female cases as male cases (n = 20 and 11, respectively). The largest proportion of endemic female cases occurred in the 30 to 39 and the 50 to 59 age-groups, while the largest proportion of male cases occurred in the 50 to 59 age-group (Figure 3). The endemic cases were reported from various regions in Ontario (Figure 5).
Travel-associated cases accounted for 64% (110/172) of the cases reported between 1999 and 2004. Most travel cases of LD reported in Ontario resulted from travel to the United States, accounting for at least 48% of all travel cases. Europe (25% of travel cases) is the other major source of infection. Within Europe, the countries most often specified were the Czech Republic and Germany accounting for 15% and 12% of European travel cases, respectively.
In Ontario, 64% (110/172) of LD cases were travel-associated between 1999 and 2004. Most cases of travel-associated LD occurred in travellers to endemic regions of the United States and Europe.
Endemic cases accounted for 18% (31/172) of the cases. This percentage would be as high as 36% (62 cases) if all the cases with unknown travel status were classified as endemic. Therefore, approximately 5 to 10 endemic cases of LD were reported annually between 1999 and 2004. Between 1984 and 1994, approximately nine to 10 endemic cases were reported annually27. These figures suggest that the annual number of LD Lyme cases has remained constant over the past two decades. This contrasts with the increasing incidence of LD in the USA over the same time period28.
Endemic cases identified from 1999 to 2004 were distributed geographically throughout Ontario with no apparent clustering (Figure 5). This was similar to that observed for the cases identified between 1984 and 199427. The detection of new endemic areas in Ontario is limited by the small number of cases identified each year and by the lack of information accurately identifying the location of most likely exposure. While the broad geographic distribution of cases may also be explained by adventitious ticks, further research into new endemic areas should be undertaken.
The dates of onset of symptoms for most cases were consistent with the infection being acquired in the summer or fall months. There were three endemic cases that reported their date of onset in December and one in February (Figure 2). The dates of onset occurring in the winter months are probably best explained by a long incubation period. One of the cases with a date of onset in December reported that they went camping in early September.
There were a total of 172 LD reported between 1999 and 2004. Of these cases, 31 (18%) were acquired in Ontario, 110 (64%) were travel-associated, and travel history was unknown for the remaining 31 (18%) cases. The average annual number of endemic cases of LD for the 5-year period ranges between five and 10 if cases with unknown travel history are included with the endemic cases. Cases were reported from various regions of Ontario, with no apparent clustering.
The authors would like to thank the staff at Ontario public health units as well as the staff at Ontario laboratories for their work in investigating and reporting cases of Lyme disease. The authors would also like to thank Dr. R. Lindsay, National Microbiology Laboratory, Public Health Agency of Canada, for his assistance with the manuscript, and S. Johnson, Infectious Diseases Branch, Ontario Ministry of Health and Long-Term Care, for creating the GIS maps.
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