In Europe, TBE – a classical viral zoonosis - is the most important virus disease transmitted by ticks. With the exception of the United Kingdom and the Iberian Peninsula, TBE occurs autochthonously in all European countries. Its incidence varies considerably between the individual risk areas. Its causative agent is a human pathogenic flavivirus, in most cases transmitted by ticks. Other flaviviruses, e.g. the yellow fever virus, also are transmitted by mosquitoes.
In Germany, TBE is a notifiable disease. Approximately 250 TBE patients are registered annually (with considerable fluctuations from year to year), approximately 30% show a severe clinical course. In these cases the first influenza-like clinical course progresses to a second stage with neurological symptoms following some afebrile days. Meningitis, encephalitis, and radiculitis or clinical mixed forms of these are the consequence. So far, most clinical human cases have occurred in Bavaria and Baden-Württemberg. However, single autochthonous cases are seen in many other federal states. Every year the Robert Koch-Institut publishes a map of current TBE risk areas in Germany, based on human cases.
In veterinary medicine, clinical cases of TBE with neurological symptoms have been known in dogs for more than 30 years. In dogs, TBE is relatively rare. The clinical course may vary considerably; thus, a subclinical or a hyperacute lethal course is possible. TBE in dogs has been observed in most areas where TBE is endemic, including Germany. There is no approved vaccine for administration to dogs. Avoiding tick bites is the most important prophylactic method.
Rarely, TBE cases have been described in horses, also in sheep, goats, mouflons (Ovis ammon musimon) and monkeys (Macaca sylvanus) when exposed to tick bites.
TBE virus circulates between the virus-competent tick and the competent host in so-called natural foci or in risk areas, whose geographical extension as a rule is strictly limited and can be very small. This is in contrast to Borrelia burgdorferi s.l., the causative agent of Lyme disease, which in Germany is endemic in areas where Ixodes ricinus occurs.
Small mammals, e.g. various mouse populations (Apodemus (A.) flavicollis, A. sylvaticus, Clethrionomys glareolus), but also hedgehogs (Erinaceus europaeus) and moles (Talpa europea), which due to their distinct viraemic stage often contribute to virus transmission to the tick during the blood meal and, thus, keep the virus within the habitat, are competent hosts or represent the virus reservoir in the natural foci. Other contributing factors are transovarial transmission and the so-called co-feeding phenomenon. For the above-mentioned hosts the TBE virus is low- or non-pathogenic. During co-feeding infected and non-infected ticks take a blood meal on the same host at the same time and closely to each other. TBEV is transmitted to the non-infected ticks without viraemia of the host.
Large domestic animals such as goats, sheep and cattle are the appropriate hosts for adult stages of I. ricinus. It is assumed, that for transmission of the virus to the tick they are of minor importance. Especially goats and sheep, more rarely cattle, are of importance for the so-called alimentary TBE. During the viraemic stage the virus is excreted in milk and can then be ingested orally by consumption of non-pasteurized milk or of cream cheeses produced from raw milk. The occurrence of individual cases or of a small number of cases in humans may be the consequence. In Slovakia, Lithuania, Latvia, Poland, Russia, Hungary, and Albania alimentary TBE cases in humans caused by virus-infected milk has occurred in the past few years. After decades without alimentary TBE, in the last years alimentary TBE also occurred in Germany in some single cases.
Grazing animals (goats, sheep, cattle, horses) and wildlife animals (foxes, various small mammals) have been used as sentinels for the characterization of TBE risk areas by means of serosurveillance tests or virus detection. This is an additional tool to characterize natural TBEV foci, as in many cases TBEV prevalence in ticks is too low to identify a focus only via TBEV detection in ticks.
The development of TBE can easily be avoided by vaccination of persons who live in or travel to risk areas and are exposed to tick bites. Vaccines are available for adults and children. It is recommended to contact the family doctor for individual consultation.
However, to remove ticks as soon as possible after leaving tick habitats and to use tick repellants can reduce an infection risk.