Different Ecological Niches for Ticks of Public Health Significance in Canada

Abstract

Tick-borne diseases are a growing public health concern as their incidence and range have increased in recent decades. Lyme disease is an emerging infectious disease in Canada due to northward expansion of the geographic range of Ixodes scapularis, the principal tick vector for the Lyme disease agent Borrelia burgdorferi, into central and eastern Canada. In this study the geographical distributions of Ixodid ticks, including I. scapularis, and environmental factors associated with their occurrence were investigated in New Brunswick, Canada, where few I. scapularis populations have been found to date. Density of host-seeking ticks was evaluated by drag sampling of woodland habitats in a total of 159 sites. Ixodes scapularis ticks (n = 5) were found on four sites, Ixodes muris (n = 1) on one site and Haemaphysalis leporispalustris (n = 243) on 41 sites. One of four adult I. scapularis ticks collected was PCR-positive for B. burgdorferi. No environmental variables were significantly associated with the presence of I. scapularis although comparisons with surveillance data in neighbouring provinces (Québec and Nova Scotia) suggested that temperature conditions may be too cold for I. scapularis (< 2800 annual degree days above 0°C [DD > 0°C]) across much of New Brunswick. In contrast, the presence of H. leporispalustris, which is a competent vector of tularaemia, was significantly (P < 0.05) associated with specific ranges of mean DD > 0°C, mean annual precipitation, percentage of clay in site soil, elevation and season in a multivariable logistic regression model. With the exception of some localized areas, temperature conditions and deer density may be too low for the establishment of I. scapularis and Lyme disease risk areas in New Brunswick, while environmental conditions were suitable for H. leporispalustris at many sites. These findings indicate differing ecological niches for two tick species of public health significance.

Fig 1. Distribution of sampling sites and tick-positive sites.

Sites of occurrence of I. scapularis (green), H. leporispalustris (blue), I. scapularis and H. leporispalustris (pink) and I. muris and H. leporispalustris (yellow) are shown. The black circle indicates a significant cluster of sites where H. leporispalustris ticks were found. The two sites where I. scapularis were found in previous field studies are identified by red stars. The map was created in ArcGIS Version 10.2 (ESRI. Redlands, CA).

Fig 2. The seasonal activity of immature H. leporispalustris.

The seasonality is expressed as the proportion of the total number of ticks of each instar collected in each month of the study.

Fig 3. Relation of temperature conditions and results of field surveillance for I. scapularis in eastern Canada.

The map shows the number of years from 2009–2014 in which DD > 0°C was greater than the model-based threshold for I. scapularis population survival obtained for southeastern Quebec and the Maritime provinces (New Brunswick, Prince Edward Island and Nova Scotia) from MODIS satellite data at a spatial resolution of 250 m. Sites where field surveillance for I. scapularis has occurred are indicated by circles (for locations where I. scapularis were found) and crosses (for locations where I. scapularis were not found). The map was created in ArcMap 10.2.2 (ESRI. Redlands, CA).

Fig 4. White-tailed deer densities.

Estimated densities of white-tailed deer/km² are shown for each wildlife management zone in New Brunswick for 2014. The map was created in ArcGIS Version 10.2 (ESRI. Redlands, CA).