Mechanism of Transmission
Trans-stadial transmission (from one stage of development to the next) as well as transovarian transmission (from fertilized female to its egg) of TBE virus in Ixodes ricinus is possible. Usually larvae and nymphs become infected by feeding on viremic hosts and pass on the virus as nymphs or imagos to other warm-blooded vertebrates; ticks themselves do not develop the disease. The virus hibernates in ticks. Once a tick is infected, it carries the virus for life. In the period that precedes moulting, the virus multiplies in the tick and invades nearly all its organs. Female ticks usually transmit the virus to a single host only. Male ticks feed more often and, in this way, may infect several vertebrates.
After attachment to the host, twelve hours may pass until the tick starts feeding. On humans, ticks prefer to attach themselves to the hair-covered portion of the head, behind the ears, to the elbows and backs of knees, hands and feet. Owing to the anesthetizing effect of the tick’s saliva, which contains analgesic, anti-inflammatory and coagulation-inhibiting substances, the procedure causes no pain and often passes unnoticed by the host.
TBE virus is transferred to the host with the saliva of an infected tick; at the same time TBE virus contained in host tissue enters the tick’s intestine with blood feeding. A tick doesn’t need to feed for a longer period of time to pass on the infection. A short feeding of tissue fluid by a male tick may suffice to transmit the virus. The fact that tick bites are often not even noticed may be the main reason why persons with manifest TBE frequently cannot remember having been bitten by a tick.