Tick-Borne Encephalitis: From Microfocus to Human Disease

Abstract

Ticks transmit a number of pathogens to humans and animals. Among them, the most important arboviral human disease in Central Europe and Northern Asia is tick-borne encephalitis (TBE). The Western subtype of TBE virus (TBEV) in Central Europe is mainly transmitted by the tick Ixodes ricinus. The incidence and the numbers of human cases are thought to be correlated to tick activity. Two different, but closely located TBEV endemic foci in South Eastern Germany were studied. The results of our longitudinal studies in both foci showed that the areas, where positive ticks could be repeatedly detected, were relatively small in comparison to earlier descriptions. The data of two endemic foci of TBEV imply that the natural circulation of TBEV between ticks and rodents or other small mammals occurs in rather small areas, named microfoci. From these microfoci, TBEV-bearing ticks are dispersed eventually, probably by larger forest animals with a greater radius of activity than rodents. Human infection occurs if humans enter the microfocus area or if infected ticks are dispersed and occasionally come into contact with humans, for example in gardens near forests or on forest ways within the area of activity of the larger forest animals, named macrofocus or endemic area. Further studies are needed to show whether this concept of TBEV microfocus and TBEV macrofocus will also apply to other endemic areas such as for example in Southwestern Germany.