Geographical and environmental factors driving the increase in the Lyme disease vector Ixodes scapularis

Abstract

The population densities of many organisms have changed dramatically in recent history. Increases in the population density of medically relevant organisms are of particular importance to public health as they are often correlated with the emergence of infectious diseases in human populations. Our aim is to delineate increases in density of a common disease vector in North America, the blacklegged tick, and to identify the environmental factors correlated with these population dynamics. Empirical data that capture the growth of a population are often necessary to identify environmental factors associated with these dynamics. We analyzed temporally- and spatially-structured field collected data in a geographical information systems framework to describe the population growth of blacklegged ticks (Ixodes scapularis) and to identify environmental and climatic factors correlated with these dynamics. The density of the ticks increased throughout the study’s temporal and spatial ranges. Tick density increases were positively correlated with mild temperatures, low precipitation, low forest cover, and high urbanization. Importantly, models that accounted for these environmental factors accurately forecast future tick densities across the region. Tick density increased annually along the south-to-north gradient. These trends parallel the increases in human incidences of diseases commonly vectored by I. scapularis. For example, I. scapularis densities are correlated with human Lyme disease incidence, albeit in a non-linear manner that disappears at low tick densities, potentially indicating that a threshold tick density is needed to support epidemiologically-relevant levels of the Lyme disease bacterium. Our results demonstrate a connection between the biogeography of this species and public health.

Modeling the Geographic Distribution ofIxodes scapularisandIxodes pacificus(Acari: Ixodidae) in the Contiguous United States

“…with reasonable accuracy based on current vector surveillance data, the expansion of tick populations into new areas depends on either dispersal of a gravid female, male and female adults, or several immature ticks that survive until reproduction (Springer et al 2015). Ticks are generally dispersed by the movement of hosts (Madhav et al 2004, Khatchikian and Prusinski 2012) and therefore, Ixodes spp. range expansion will also be affected by available host habitat or host movement, which was not explicitly included in our models.…”

Linkages of Weather and Climate WithIxodes scapularisandIxodes pacificus(Acari: Ixodidae), Enzootic Transmission ofBorrelia burgdorferi, and Lyme Disease in North America

“…Habitat suitability and host densities vary at a much more local scale, and studies conducted at State or sub-State levels frequently find that tick abundance and occurrence varies mostly with habitat, and climatic variables are weak predictors of tick abundance (Guerra et al 2002, Khatchikian et al 2012). Consistent with this, abundance of I. scapularis and I. pacificus can vary markedly over small geographic scales experiencing similar temperature and humidity conditions, further supporting the role of nonmeteorological factors, such as local host abundance, as important in determining local tick abundance (Eisen et al 2003, 2010; Diuk-Wasser et al 2006, 2010).…”

Tick-borne disease preventive practices and perceptions in an endemic area

“…Furthermore, the requirement to report cases of Lyme disease may contribute to an increased annual number of recognized cases (Connecticut Department of Public Health, 2009). However, multiple other measures support the overall increasing incidence of Lyme disease, including a substantial geographic expansion of counties at high risk for Lyme disease (Centers for Disease Control and Prevention, 2015a) and evidence of a paralleled increase in the population density of blacklegged ticks (Khatchikian et al, 2012). Thus, the need for awareness of targeted prevention strategies, as well as early disease recognition and treatment and a sustainable surveillance system, is underscored.…”