Effects of Borrelia burgdorferi (sensu stricto) exposure and Rickettsia buchneri load on larval feeding kinetics and survival dynamics in Ixodes scapularis

Document Type

Presentation

Date of Original Version

3-27-2026

Abstract

The microbiome of blacklegged tick (Ixodes scapularis) larvae includes both vertically and horizontally transmitted bacteria. Vertically transmitted symbionts include obligate microbes such as the putative nutritional endosymbiont Rickettsia buchneri, whereas horizontally transmitted microbes include pathogens that cycle between ticks and vertebrate hosts, such as Borrelia burgdorferi (sensu stricto). Although B. burgdorferi is widely regarded as a pathogen due to its role as the etiologic agent of Lyme borreliosis, it does not appear to cause measurable harm to its tick vector. We therefore hypothesize that within I. scapularis larvae the Lyme spirochete may function not as a pathogen but as a facultative symbiont during development, potentially influencing feeding performance and survival. To test this hypothesis, we experimentally evaluated how B. burgdorferi exposure and/or acquisition, together with maternally acquired R. buchneri, individually and jointly influence larval feeding kinetics and off-host survivorship of I. scapularis. To characterize symbiont load, R. buchneri levels were quantified by qPCR in a subset of larvae from each clutch, allowing clutches to be classified into high, medium, or low R. buchneri cohorts. Approximately 500 unfed larvae from each cohort were then placed on B. burgdorferi–infected or uninfected white-footed mice (Peromyscus leucopus). Larvae were removed from mice at 24, 48, 72, and 96 hours post-attachment, and feeding progression was quantified using the scutal index, a widely used morphometric method for blood-meal intake and engorgement level in hard ticks. Following detachment and scutal index measurements, larvae were housed individually under controlled environmental conditions (23 °C, 95% RH, 16L:8D photoperiod) and monitored daily for survival until either death or successful molt to the nymphal stage. Feeding progression differed among treatment groups, suggesting that microbial status may influence larval feeding dynamics. Larvae derived from lineages with high R. buchneri levels exhibited significantly greater survivorship (median 94 days) than those from lineages with low R. buchneri (median 85 days). Similarly, larvae that fed on B. burgdorferi–infected mice survived significantly longer (median 98 days) than unexposed controls (median 82 days), with Kaplan–Meier survival curves diverging markedly after day 30. When stratified by feeding duration, early-detached larvae (24–48 h) exhibited rapid mortality and showed no exposure-dependent differences, likely a result of minimal or absent B. burgdorferi acquisition. Together, these results support the hypothesis that B. burgdorferi may function as a facultative symbiont of I. scapularis. Our findings further suggest that R. buchneri provides important ecological context for the tick–B. burgdorferi interaction and may modulate the effects of spirochete exposure on larval feeding kinetics and survival trajectories. Additional work is needed to elucidate the mechanisms underlying these microbial interactions and to determine how microbiome-mediated effects on tick development ultimately influence lifetime fitness and I. scapularis vectorial capacity.

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