BIOLOGY: The Tick: Life Cycle The following is a simplified account of the life cycle of the tick, Ixodes ricinus. The life cycle of the other important old world vector, I. persulcatus, is broadly similar. 
Ixodes ricinus, the castor bean or sheep tick, is the commonest tick in northern and central Europe. The life cycle takes approximately 3 years to complete, with each life cycle stage (larva, nymph and adult) taking one year – but the life cycle can vary in length between 2 and 6 years (Gray, JS. 1991. Rev Med Vet Entomol. 79:323-333). Blood feeding occurs once in each stage and for a period of only a few days. Digestion of the blood meal and development to the next stage occurs whilst hidden deep in the vegetation. The larvae hatch from an egg batch of about 2000 and after a few days are ready to feed. The six-legged larvae, which are just visible to the naked eye, climb the vegetation and wait for a passing host, usually a mouse or vole. They climb on when the host animal brushes past and then attach to the skin with their specialised mouthparts. After 2 or 3 days feeding, during which they increase their weight 10–20 times, they drop off into the vegetation and commence development. After several months the fed larva moults to an 8-legged 1.5–2 mm nymph that usually feeds in the following year for 4–5 days on a larger animal such as a bird or squirrel. Finally, the adult female tick, about 4 mm long when unfed, parasitises a large animal such as deer or livestock on which they feed for about 7 days, taking up to 5 ml of blood and growing to the size of a small bean. The male tick stays on the host for longer periods in order to mate with females and may take sporadic small meals. The larvae and nymphs can in fact parasitise any animal, including large animals, such as deer but tend to make most contact with hosts that move within the vegetation cover, whereas the adults climb higher in the vegetation and usually only attack large animals from the size of a hare upwards (but also hedgehogs). All 3 stages are known to bite humans, but nymphs are most commonly involved.
In most of its geographical range I. ricinus feeds on animals from March to October and usually the main peak of host-seeking activity is in spring and early summer. In some areas another smaller peak occurs in late summer and autumn (see Seasonality). Since the unfed ticks can survive for several weeks where there is sufficient humidity at the base of the vegetation, disease transmission can occur at any time during the warmer months of the year, though disease seasonality is strongly influenced by human intrusion into tick habitat.
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