Virus tick-to-tick transmission experiments were carried out essentially as described previously [6] [7] [8] . Two of 45 infected adult female ticks were allowed to feed simultaneously on Balb/C mice for 3 days with 15 uninfected I. ricinus nymphs that were attached in close proximity (1-1.5 cm) to the feeding donor females. Surviving nymphs and salivary glands of donor females were used to determine the titres of infectious virus using plaque assays. The co-feeding transmission rate was estimated as the proportion of nymphs (%) that became infected.