Although I. ricinus ticks can be routinely maintained in laboratories, SIB TBEV is normally associated with transmission by I. persulcatus, which is not as readily available for laboratory experiments. Nevertheless, WE TBEV strains show ,100% transmission efficiency in their natural tick vector I. ricinus (manuscript in preparation). Thus, logically, Yar viruses (i.e. SIB TBEV strains) should be transmitted efficiently in their natural vector I. persulcatus. Geographically, I. ricinus and I. persulcatus overlap in Europe and numerous reports describe the isolation of Siberian strains from I. ricinus which is now recognised as a second vector for SIB TBEVs [11, 34] . We therefore propose that the driving force behind the westward dispersal of these HA-deficient strains is their adaptation to newly-encountered European I. ricinus ticks. This hypothesis is also supported by our analysis of E protein comparative alignments between FE, SIB and WE subtypes that revealed more hydrophobic ''ricinus'' amino acid patterns, compared with ''persulcatus'', in correspondence with tick preference of WE or FE/SIB strains respectively. Louping ill virus, which is transmitted by I. ricinus in the UK, shares this ''more hydrophobic'' pattern with the WE TBEVs.