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New research findings confirm: remove ticks as quickly as possible otherwise the risk of TBE increases

06/2003, 13.03.2003

Tick-borne encephalitis - TBE (also known as spring-summer encephalitis) is the most prevalent tick-borne viral disease in Europe. In Germany alone around 250 people contract tick-borne encephalitis every year, around 30% of them a severe form. 10% of all patients show lasting and, in some cases, very severe neurological damage. 2% of patients die. There are risk areas for tick-borne encephalitis in Bavaria, Baden-Württemberg and, to a growing degree, in Odenwald. One rural district in the Rhineland-Palatinate has been classified as a low-risk area and TBE has now been detected in three rural districts of Thuringia. New research findings indicate that, similar to the situation with Lyme borreliosis, the risk of infection with TBE increases parallel to the length of the blood meal of the tick in man. These research findings, along with many others, will be presented today and tomorrow at the VII^th International Potsdam Symposium on Tick-borne Diseases in the Federal Institute for Risk Assessment (BfR). 200 scientists from 26 countries are attending the meeting.

In order to determine the degree of contamination of ticks with TBE viruses and specify corresponding risk areas, ticks are being examined from selected areas for these viruses. This is done in the Reference Laboratory for Tick-borne Diseases attached to the Federal Institute for Risk Assessment (BfR) in Berlin. In recent years scientists have observed a drop in the contamination rates of ticks: Whereas in 1997 TBE viruses could still be detected in around 5% of "adult" ticks in the Black Forest and around Passau, the values in recent years were only around 1%. The contamination rate of the nymphs ("young" ticks) was even lower.

The results of a new study by the Reference Laboratory attached to BfR in cooperation with a laboratory in Passau are, therefore, all the more surprising. According to it, the contamination level of adult ticks, which have sucked human blood, is almost 10% in and around Passau. The virus burden of adult ticks collected outdoors and examined in this region which not yet sucked human blood, was, by contrast, at the expected level of around 1%. Similar results were obtained for the nymphs (the "young" ticks). Whereas 0.4 % of the free living nymphs in and around Passau, which had not yet sucked human blood, carried the viruses, the values for those which had, was 7%. All contamination rates were determined using molecular-biological methods and validated through nucleic acid sequence data.

It is still largely unclear why TBE viruses in ticks which have "sucked" blood can be detected far more frequently than in ticks which have not yet had a blood meal. One possible explanation could be major virus multiplication during blood intake. In the past, ticks were possibly classified as "virus free" because their "virus burden" was so low that it could not be detected with the methods available at that time.

Since the entry into force of the new Protection against Infection Act (IfSG) in 2001, all cases of TBE must be notified to the Robert Koch Institute (RKI) in Berlin. The data obtained in this way make possible a reliable assessment of the scale of the disease. Individual, imported cases only play a subordinate role in this context. For 2001 a total of 254 cases of TBE were notified to the RKI on the basis of the IfSG (cf. Epid. Bull. 17/2002), including 108 cases from Bavaria and 117 from Baden-Württemberg and for 2002 a total of 239 cases of disease, including 78 from Bavaria and 114 from Baden-Württemberg. Further federal states with individual cases of disease were Thuringia (3 cases in 2001 and 8 cases in 2002) and Hesse (10 cases in 2001 and 13 cases in 2002).

BfR explicitly points out once again that only active vaccination offers reliable protection against infection in TBE risk areas. At present, two well-tolerated and highly effective vaccines for adults and one vaccine for children aged one and upwards are authorised in Germany.

The results of the symposium will be published. The abstracts are available on the Institute's homepage (www.bfr.bund.de) under "Events".

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