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

Abstract

Lyme disease has increased both in incidence and geographic extent in the United States and Canada over the past two decades. One of the underlying causes is changes during the same time period in the distribution and abundance of the primary vectors: Ixodes scapularis Say and Ixodes pacificus Cooley and Kohls in eastern and western North America, respectively. Aside from short periods of time when they are feeding on hosts, these ticks exist in the environment where temperature and relative humidity directly affect their development, survival, and host-seeking behavior. Other important factors that strongly influence tick abundance as well as the proportion of ticks infected with the Lyme disease spirochete, Borrelia burgdorferi, include the abundance of hosts for the ticks and the capacity of tick hosts to serve as B. burgdorferi reservoirs. Here, we explore the linkages between climate variation and: 1) duration of the seasonal period and the timing of peak activity; 2) geographic tick distributions and local abundance; 3) enzootic B. burgdorferi transmission cycles; and 4) Lyme disease cases. We conclude that meteorological variables are most influential in determining host-seeking phenology and development, but, while remaining important cofactors, additional variables become critical when exploring geographic distribution and local abundance of ticks, enzootic transmission of B. burgdorferi, and Lyme disease case occurrence. Finally, we review climate change-driven projections for future impact on vector ticks and Lyme disease and discuss knowledge gaps and research needs.

The impact of temperature and precipitation on blacklegged tick activity and Lyme disease incidence in endemic and emerging regions

“…Correlative models have explored the effect of climatic factors on Lyme disease incidence averaged across expansive geographical regions, most commonly states in the U.S.A. [7, 8]. Unfortunately, there are large variations in incidence within many states [9], and the factors that drive inter-annual variation in Lyme disease incidence vary spatially [10]. Specifically, underlying factors such as physician and public awareness have a strong impact on the reporting of Lyme disease cases in emerging areas [11, 12], and data collected at a coarse scale (State) may not reveal patterns that exist at finer scales (County).…”

“…However, few studies have attempted to connect these weather and climate effects to patterns in the human incidence of Lyme disease [10]. There is strong evidence that Lyme disease and tick populations are spreading from two epicenters in the United States, one in the upper Midwest, and another the Northeast [14, 15].…”

Host-Seeking Phenology ofIxodes pacificus(Acari: Ixodidae) Nymphs in Northwestern California in Relation to Calendar Week, Woodland Type, and Weather Conditions

“…Host-seeking activity commences when diapause (behavioral or morphogenetic) is broken, and ceases when it is induced. Both the induction and cessation of diapause are linked with day length and temperature (Padgett and Lane 2001;Belozerov 2009;Eisen et al 2016;Gray et al 2016). These factors were captured directly (GDD) or indirectly (week is correlated with day length) in our models.…”

“…A TDD was defined similarly, but values of TDD below 10 C are not set to zero. Therefore, GDDs provide a measure of warmth above a biologically relevant threshold of 10 C (Eisen et al 2016), whereas TDDs do the same, but also account for the effects of cold temperatures below the threshold. The model was also fit with GDD and TDD with a threshold of 0 C in order to test the sensitivity of the variables' classification.…”

“…Previous studies in North America have shown that the host-seeking phenologies of the primary vectors of Lyme disease spirochetes-the nymphal stages of Ixodes scapularis Say in the east and Ixodes pacificus Cooley and Kohls in the far west-are strongly associated with environmental variables (Lane et al 1995;Vail and Smith 1998;Eisen et al 2002Eisen et al , 2003Eisen et al , 2016Diuk-Wasser et al 2006;Gatewood et al 2009). …”