FIG. 2. The tick's life cycle in relation to transmission of tick-borne encephalitis virus (TBEV). The approximate timing of tick metamorphosis is illustrated in relation to potential virus transmissibility between ticks, reservoir, and host animals. The figure shows a generalized 2-yr life cycle, although the average life cycle is 2-3 yr, with one year for each blood-sucking development stage. The concentration of TBEV in ticks in endemic regions varies between <1% and 4%, depending on geographical location and tick stage (WHO meeting 1986). The adult female is more frequently infected than the nymphs and especially the larvae, but nymphs are more abundant and contribute grossly to transmission of the disease to humans. TBE virus transmission in nature is mainly maintained between I. ricinus and small rodents like the Scandinavian long-tailed wood and field mice, which are effective blood reservoirs for the virus, due to a high reproductive rate and a prolonged viremia. Small vertebrates are preferred hosts of larvae and nymphs. Larger mammals like cattle and roe deer serve as blood meals for the adult female tick, in particular, but also for the nymphs (Jaenson et al. 1994). These animals are poor TBE blood reservoirs, as the period of viremia is too short. In some cases, the virus may be transferred from an infected tick that is co-feeding on the same host animal as an uninfected tick (Randolph et al. 1996). Humans are accidental dead-end hosts. The TBE virus hibernates mainly in ticks and, to a lesser extent, in hedgehogs, bats, and other small vertebrates. A mild winter may allow more ticks, as well as host and reservoir animals, to survive. This means easier access to blood meals for the ticks, increased tick population abundance, and a faster pathogen transmission rate. With an early arrival of the spring and /or a late arrival of the next winter, the ticks will be active for an extended period. In late autumn, the population of the older, more cold-resistant tick stages will be large. This will increase the chance of survival for another winter. If the following spring and fall seasons are prolonged due to a rise in daily minimum temperatures, the risk for a human to be bitten by an infected tick will increase.
