The biological cycle of the tick
Ixodes ricinus
Ixodes ricinus belongs to the family Ixodidae and ixodid ticks have four life cycle stages: egg, larvae, nymphs and adults (5). Ixodid ticks feed once in each active stage (larvae, nymphs and adult female). Larvae and nymphs require a blood meal to progress to the next stage of the life cycle (larvae to nymphs and nymphs to adults) and adult females require a blood meal before producing eggs (24).
This is generally true but a tick stage may occasionally take more than one blood meal from more than one host type (reviewed by (10)) . For example if a tick falls off its host during a feed, it will need to find a second host (L. Gilbert, personal communication.))
The immature stages (larvae and nymphs) usually feed upon small hosts, such as rodents and/or birds, while adults commonly feed upon large animals, including carnivores and deer, cattle or sheep (among others). However, this is not the rule for every species of tick, because some species are highly specific for a particular type of host (4). Humans are more likely to be bitten by nymphs.
Ixodes ricinus belongs to the group of “three-host ticks” (5). I. ricinus feed on a wide range of warm- and cold-blooded vertebrate hosts. Larvae and nymphs can feed on hosts of all sizes (deer, mice, voles, birds), while adult stages feed more exclusively on larger hosts such as sheep, cattle, deer or hares (24). Besides the importance of hosts, climate has a major impact on the abundance distribution of I. ricinus, its life-cycle processes, seasonal activity, population dynamics and individual tick behaviour (reviewed by (25)).
Larvae, nymphs and adults quest for hosts using an ‘ambush’ technique whereby they climb up vegetation to
differing heights and wait for a host to brush past. Larvae emerge from the eggs and quest near the ground, where
the relative humidity is higher, whilst waiting for a vertebrate host. Each stage (larvae, nymph and adult female)
feeds for a few days and then detaches and develops in the vegetation to the next stage in approximately 1 year (24). The speed of development between stages is faster in warmer
climates, so in cooler climates such as northern Scotland, it is likely that the tick will emerge as the next stage
in the following year, so egg to larva to nymph to adult could take 3 years.
Mating can occur on vegetation or on the host animal. If mating occurs on the host engorged females detach after mating, then fall on the ground, lay eggs and die (5).
Ixodid ticks spend a very short time on the host compared with the time they spend in the environment so their activity is highly seasonal (27, 28).
Temperature and relative humidity directly affect Ixodes ricinus questing, development and survival, and are thought to be the principal factors limiting the geographic range of this species (29).
Higher temperatures increase the development rate of ticks, extend questing season in cooler climates and also induce questing behaviour (30, 31). Activity is inhibited by cold temperatures and/or short day length in winter, inducing tick diapause (developmental inactivity) (32).
Field surveys in Scotland, Switzerland and northern Italy have estimated a weekly average maximum daily temperature of 7–8°C for I. ricinus nymphs to emerge from their over-wintering diapause and starting questing for hosts (reviewed by (33)).
Ticks are vulnerable to desiccation (drying out), which can be fatal, during the questing and development phases, when transforming to the next instar or, in the case of adult females when laying eggs. To survive desiccation, it is critical that the relative humidity of their micro-climate does not fall below 80%, a micro-climate found in areas with a good vegetation cover and a mat of decaying vegetation (24).
In Northern Europe ticks are active mainly between spring and autumn when temperatures are warm enough. In the study developed by Perret et al.(27), questing tick density showed a significant increase in March for both nymphs and adults. The peak of questing nymphs was reached at the end of April and decreased dramatically in June. Adult activity was prolonged until September (27).