The Ixodes ricinus complex life cycle comprises four life stages (egg, larva, nymph and adult), where each post-egg stage needs a blood meal to develop into the next stage or reproduce. Larvae and nymphs feed on many vertebrate species, including mammals, songbirds and lizards, whereas adults feed on a narrower range of hosts, most commonly deer. Lyme borreliae are not passed from infected female ticks to their offspring; thus, the bacteria are maintained in an enzootic cycle between immature ticks (larvae and nymphs) and reservoir hosts, typically small mammals and songbirds. For illustrative purposes, this figure typifies the life cycle of Ixodes scapularis in the northeastern USA. Eggs are laid in the late spring; larvae will hatch in mid-summer and then seek their first host in late summer. Engorged larvae will detach, drop off the host and moult into nymphs. Unfed nymphs emerge and quest for their second host the following late spring through summer, which is therefore the epidemiologically most risky time of the year. Engorged nymphs detach, drop off the host and moult into adults in the leaf litter. Beginning in the fall, adult ticks quest for a third host, most commonly deer. Engorged females detach from the host and drop into the leaf litter, where they overwinter. Adult ticks that are unable to locate hosts in the fall will also overwinter and resume questing in early spring. Fed I. scapularis females will oviposit an egg mass (up to 1,500–2,000 eggs), and then die. Although both nymphs and adult females may be infected, because of their small size (about the size of a poppy seed), their large numbers, and a spring/summer activity period that coincides with increased outdoor human activity, nymphs are responsible for most human infections. Because they are larger (about the size of a sesame seed) and have red coloration, adult female ticks are easier to notice and remove before the bacteria can be transmitted
Lyme Borreliosis
Nature Reviews Disease Primers
pubmed.ncbi.nlm.nih.gov/41888159/
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