Studies on the non-viraemic co-feeding transmission of tick-borne viruses indicate a complex interaction between viruses, ticks and hosts at the tick – vertebrate host interface. Cellular involvement at the site of tick feeding has been clearly demonstrated when tick-borne encephalitis (TBE) virus and selected natural rodent hosts were used in the experiments. Very efficient virus transmission during co-feeding of infected and uninfected ticks on the same host, irrespective of the levels of viraemia was accompanied by virus infection in the skin feeding sites of both infected and uninfected ticks. Viraemia did not induce generalized infection of the skin. The results indicate that the local skin tick feeding site is an important location for primary virus replication in the host. Virus susceptibility of host skin-associated cells seems to be a prerequisite for subsequent transmission between co-feeding ticks. Titration of virus in skin homogenates and of virus produced by skin explants was used to assess the ability of the host to support transmission of TBE virus. Results showed the most efficient amplifying host among the tested species was Apodemus flavicollis. A host species susceptible to generalized infection and developing high viraemia was not necessarily very efficient in supporting TBE virus transmission, as observed with Pitymys subterraneus. The possible explanation is that the tick associated skin infection of the given host species does not favour virus transmission between co-feeding ticks. Among the cells recruited into the tick feeding site in the skin are Langerhans cells, very potent antigen presenting cells, and these were infected with TBE virus. It is reasonable to assume they carry the infection into the draining lymph nodes. This hypothesis is being tested experimentally by co-culture of uninfected lymph nodes with infected skin explants. Immunomodulatory activities in tick saliva, like the observed suppression of IFN? and NK cells can also be host specific and may be important in supporting non-viraemic transmission.

Index terms: tick, non-viraemic transmission, skin infection, Langerhans cells.

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