The focal distribution of tick-borne encephalitis virus (TBEV; Flaviviridae, 
Flavivirus) appears to depend mainly on cofeeding transmission between infected 
Ixodes ricinus L. nymphs and uninfected larvae. To better understand the role of 
cofeeding ticks in the transmission of TBEV, we investigated tick infestation of 
rodents and the influence of microclimate on the seasonality of questing I. 
ricinus ticks. A 3-yr study was carried out at four sites, including two 
confirmed TBEV foci. Free-living ticks and rodents were collected monthly, and 
microclimatic data were recorded. A decrease in questing nymph density was 
observed in 2007, associated with low relative humidity and high temperatures in 
spring. One site, Thun, did not show this decrease, probably because of 
microclimatic conditions in spring that favored the questing nymph population. 
During the same year, the proportion of rodents carrying cofeeding ticks was 
lower at sites where the questing nymph density decreased, although the 
proportion of infested hosts was similar among years. TBEV was detected in 0.1% 
of questing ticks, and in 8.6 and 50.0% of larval ticks feeding on two rodents. 
TBEV was detected at all but one site, where the proportion of hosts with 
cofeeding ticks was the lowest. The proportion of hosts with cofeeding ticks 
seemed to be one of the factors that distinguished a TBEV focus from a non-TBEV 
focus. The enzootic cycle of TBEV might be disrupted when dry and hot springs 
occur during consecutive years.