TBE Natural Foci
A natural focus, as defined by Pawlowsky, is a “region of distinct geographic features and ecological settings where by way of evolution a certain interrelationship between the species has developed, i.e. by the pathogen (microorganism) on the one hand and its carrier (vector) on the other. The latter transmits the pathogen from a vertebrate host, acting as the donor of infection, to another – recipient – host under environmental conditions that are either conducive or adverse to further circulation of the agent in such biozoonoses”.
The continental distribution of TBE in Europe is statistically associated with a specific pattern of the seasonal dynamics of Ixodes ricinus, and a particular characteristic of the seasonal land surface temperature profile. The development of a TBE natural focus also depends on the coincidence of other factors.
The circulation of TBE virus is also dependent on a certain population density of ticks and their hosts. Virus prevalence in the tick population within TBE foci is determined by the duration of viremia in hosts, because the virus is mostly ingested by ticks while engorging on a viremic host. Virus circulation in nature is also influenced by the percentage of immune hosts in a certain region.
The properties of the biotope also play a role in the development of TBE foci. In Austria, more than 90% of all natural foci are within the 7°C annual isotherm. Rare isolated natural foci have been observed up to a height of 1,300 meters above sea level.
The climate is one of the above mentioned factors, which influence tick-borne disease dynamics. Even if the major discontinuities in the TBE incidence cannot be explained satisfactorily by the recorded temperature increases, nevertheless a seasonal shift in reported cases of TBE in Central and Northeast Europe suggest that TBE virus transmission dynamics have changed somewhat – perhaps as a result of warmer temperatures. Although the dependence of TBE on temperature is not a direct one and various factors could be involved, an impact of climate warming on the vertical disease distribution in Central Europe is evident.
Apart from the temperature, tick activity is dependent on soil humidity and relative humidity. The critical water equilibrium for Ixodes ricinus is at 92% relative humidity. Without blood feeding, individual ticks can survive at a higher relative humidity for several months, but they soon perish at levels below 92% relative humidity.