The common castor-bean tick acts as a parasite on more than 100 different species of mammals, reptiles, and birds.

Infection of Ixodes ricinus with TBE virus by a host harbouring the virus is only possible during the viremic stage in the host, provided the TBE virus titer in its blood is high enough to infect the blood-feeding vector.

For most hosts, TBE virus is apathogenic, i.e. they hardly ever develop the disease. An infected host develops specific antibodies to the TBE virus and then remains immune to re-infection for life. Under these aspects, TBE virus circulation in nature would soon come to an end; thus, virus persistence in a natural focus is dependent on the following conditions:
• a population of hosts with a sufficient
• duration of viremia and a high virus titer,
• a sufficient number of young animals susceptible to infection,
• different species of hosts,
• large vertebrate hosts serving as feeding targets for numerous ticks.

Duration of viremia: A long viremic stage (two to eight days) along with a high virus titer is likely to be observed in small mammals, such as the yellow-necked field mouse, the red-backed vole, the common vole, the hazel mouse etc. Therefore, ticks are most likely to become infected by feeding on these hosts, in which TBE virus can hibernate.

In large mammals (roe, goat) viremia is short-lived and only low virus titers are reached. However, recent findings suggest that, with several ticks feeding on large mammals, a non-viremic transmission of the virus to ticks may also play a role in the virus cycle. During the viremic stage, milk from goats, cows, and sheep contains the virus and may be a source of infection for man.
Birds only pass through a very short viremic stage and play no role as reservoirs of TBE virus. However, they often serve as hosts for the immature stages of Ixodes ricinus and may contribute to the spread of infected ticks.

As a source of sustenance and infection man is only of tangential importance for ticks. He is a dead-end host in the chain of virus transmission.

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