Academic journal article Environmental Health Perspectives

Expansion of the Lyme Disease Vector Ixodes Scapularis in Canada Inferred from CMIP5 Climate Projections

Academic journal article Environmental Health Perspectives

Expansion of the Lyme Disease Vector Ixodes Scapularis in Canada Inferred from CMIP5 Climate Projections

Article excerpt

Introduction

A cause for concern to the Canadian population and public health sector is the emergence and establishment or re-establishment of climate-sensitive infectious diseases, particularly Lyme disease (Bush et al. 2014). Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted by ixodid ticks. The main vectors are Ixodes pacifus and Ixodes scapularis in western and central/eastern North America, respectively (Eisen et al. 2016). Since the first reported human cases of Lyme disease in the early 1990s, Canada has experienced an increase in both the number of endemic regions for the disease and the number of annual diagnosed human cases (Ogden et al. 2008; PHAC 2015). Not only are these trends placing increased health stress on the Canadian population, but they are also likely resulting in added economic consequences for patients, communities, the healthcare system, and governments (Hinckley et al. 2014; Adrion et al. 2015; Zhang et al. 2006).

Tick survival depends on several factors, including climate suitability, host density, soil composition, and microhabitat characteristics (Leighton et al. 2012). Ogden et al. (2005, 2006, 2008) raised the hypothesis that climate suitability, predominantly temperature, is a key determinant of the survival of I. scapularis populations and consequently that a warming climate may be a key driver of the emergence of the tick in Canada. There are multiple ways in which climate may affect the survival of tick populations. First, climate factors such as temperature and humidity can affect host-seeking activity. Second, the development rate of ticks from one life stage to the next is highly dependent on temperature: the development rate is 0 at 0[degrees]C and becomes faster at higher temperatures (Ogden et al. 2014b).

To simulate the effect of temperature on tick population survival and seasonality, Ogden et al. (2005) developed a mechanistic I. scapularis population model comprised of 12 mutually exclusive states, each one representing a specific state in the ticks' life cycle. The model was designed to simulate I. scapularis populations in specific geographic regions. To accomplish this, temperature data were used to vary the duration that ticks remained resident in the egg-laying and engorged-tick state and to alter questing-tick activities. Several parameters, including interstadial development rates, host-finding rates, and mortality rates, were calibrated for each development stage of the population model. Empirical validation revealed that the model was able to closely replicate the seasonal activity patterns for all three tick instars that were observed in the field. Using this model with temperature input obtained from two climate models, the effect of projected future climate on the possible geographic range of I. scapularis populations was explored under two different emissions scenarios. All the research was in agreement that northern expansion of the geographic range suitable for I. scapularis establishment may take place under future climate conditions (Ogden et al. 2006).

Wu et al. (2013) modified the mechanistic I. scapularis model developed by Ogden et al. (2005). This revised population model allowed prediction of the basic reproductive number ([R.sub.0]) of I. scapularis for specific geographic locations in North America. The authors used the same model framework and many data values from Ogden et al. (2005). However, this version of the model used rates of development in contrast with development duration, allowing a periodic system of delay differential equations to be developed rather than a system of delay difference equations. In turn, this allowed Wu et al. (2013) to employ the next-generation matrix mathematical technique to obtain [R.sub.0] values. [R.sub.0] describes the propensity for a parasite or microparasite to survive and be propagated. Conditions that allow [R.sub.0] > 1 are those under which the tick population can persist, while conditions under which [R. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.