Academic journal article Environmental Health Perspectives

Estimated Effects of Projected Climate Change on the Basic Reproductive Number of the Lyme Disease Vector Ixodes Scapularis

Academic journal article Environmental Health Perspectives

Estimated Effects of Projected Climate Change on the Basic Reproductive Number of the Lyme Disease Vector Ixodes Scapularis

Article excerpt

Introduction

Considerable attention has been devoted to the possibility that climate change will exacerbate the burden of mosquito-borne diseases such as malaria and dengue, with important impacts on public health (Githeko et al. 2000). Early assessments of the effects of climate change on malaria and dengue used simplistic models to assess possible effects of climate change on their basic reproductive numbers ([R.sub.0], the universally recognized metric of the capacity of a parasite or pathogen to reproduce given particular environmental conditions) (Martens et al. 1995; Patz et al. 1998). However, these assessments were criticized for giving weight to future increases in [R.sub.0] whether or not such increases resulted in [R.sub.0] rising above the critical threshold of > 1 for disease persistence (Rogers and Randolph 2000) and for being oversimplistic by only accounting for climate effects rather than the full range of nonclimatic factors that impact the occurrence of these diseases (Reiter 2001; Rogers and Randolph 2000). Any impact of climate on [R.sub.0] of malaria and dengue is limited by the effects of variations in human host density, mosquito control, infection prevention and treatment in humans, and human management of the environment (e.g., agriculture, forest management, logging) that affect the ecology and epidemiology of the vectors, pathogens, and diseases (Githeko et al. 2012). Consequently, the strength of evidence for recent climate warming effects on malaria risk has been questioned and much debated (Reiter et al. 2004; Tanser et al. 2003).

Many vector-borne diseases of public health significance (e.g., Lyme disease, West Nile virus) are, however, maintained in transmission cycles that involve wild animal hosts. These cycles are independent of human cases, and the spatio-temporal risk of human disease is less dependent on the direct effects of human activities than is the risk from malaria and dengue. Nevertheless, despite some assessments (Gubler et al. 2001), the effects of climate change on vector-borne zoonoses have also been downplayed mostly on the basis of limited evidence for recent effects of climate change (Kilpatrick and Randolph 2012). Lyme disease emerged (or likely reemerged) in the northeastern United States in the late 1970s due to the expansion of tick populations, which was generally thought to have been associated with changes in land use over some decades that resulted in reforestation and expansion of the population of the deer that are key hosts for the ticks (Wood and Lafferty 2013). Lyme disease is now emerging in Canada and some northern U.S. states due to the northward expansion of the geographic range of the tick vector Ixodes scapularis (I. scapularis) (Hamer et al. 2010; Ogden et al. 2009), which is dispersed from source populations by migratory birds and terrestrial hosts (Leighton et al. 2012).

A mechanistic simulation model of the I. scapularis life cycle has identified temperature effects on I. scapularis population survival in order to assist in assessment of current and future on-the-ground Lyme disease risk in Canada (Ogden et al. 2005, 2006b). Prospective field studies and retrospective analyses of surveillance data on tick and pathogen emergence in southeastern Canada validated the model findings and identified temperature as a statistically significant determinant and possible driver of emergence of the tick in Canada (Bouchard et al. 2013a, 2013b; Leighton et al. 2012; Ogden et al. 2008, 2010). The I. scapularis model was modified to permit the direct calculation of [R.sub.0] for I. scapularis via the next generation operator approach (Wu et al. 2013), which, given the universal use of [R.sub.0]0 and its estimation for a wide range of parasites and pathogens under many different conditions, allowed comparison of [R.sub.0] variations in the present study with observed variations for other parasites and pathogens.

Here, we have estimated projected effects of climate change on [R. …

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