The Distribution of TBE natural foci
The distribution of the TBE virus covers almost the entire southern part of the nontropical Eurasian forest belt, from Alsace-Lorraine in the west to Vladivostok and the northern and eastern regions of China in the east. The true extent of TBE infections has only been revealed during the past few years. Little is known about the rate of infection in China. Recently an isolated endemic area was identified in Hokkaido/Japan. The number of infected ticks in known natural foci may vary from year to year.
Widely differing figures are also given for TBE virus prevalence in the tick populations in endemic areas of various European countries. Usually, questing ticks are collected by flagging in special tick monitoring sites, and TBEV prevalence is investigated by examining individual ticks or by pooling ticks. When investigations are carried out on ticks removed from humans, the TBEV prevalence could be up to ten times higher. The techniques of investigating the TBEV prevalence in unfed versus partially engorged ticks are not standardised. Therefore, the prevalence values canít be compared across Europe.
As ticks are usually infected for life, it follows that the degree of virus prevalence increases during their development from the egg to the adult arthropod. Compared to nymphs, three to five times more adult ticks are infected with TBE virus.
TBE virus prevalence is much higher in these hosts than in the tick populations. On account of their longer life span, large mammals can be repeatedly infested by infectious ticks, and due to their size they often serve as feeding targets for several ticks at a time, either factor being conducive to the transmission of TBE virus.
The risk of contracting TBE in the most affected countries increased considerably between 1974 and 2003. In Lithuania, where incidence increased by 1,033%, and in Germany (with an increase of 574%), TBE came to be a serious problem. In addition to the already known risk areas, new risk areas formed in Norway, and possibly in the southern part of Sweden. The only exception to this trend is Austria, where national campaigns leading to consistent immunization reduced the number of new infections from 600 to about 60.
The noted changes in the frequency of hospitalization are based on a multitude of factors. Evidently, there are various parallel factors working alongside, making interpretation of strongly differing annual hospitalization and/or virus prevalence patterns difficult. A possible warming of the climate is discussed as one of the reasons for an increased prevalence of ticks.