In 2017, the Dutch Wildlife Health Centre (DWHC) and the Dutch Institute for Public Health and the Environment (RIVM) renewed testing roe deer serum samples for antibodies against tick-borne encephalitis virus (TBEV). The objective of this study was to locate potential TBEV foci and identify signs of increased TBEV circulation the Netherlands.
Other countries observed that TBEV does not spread homogeneously across large areas, it is often present in small areas, also known as foci. In these foci, virus infected ticks will be present. To find the smaller areas and consecutively look for ticks in these foci, roe deer are used as sentinel animals. Roe deer can become infected with the TBEV, develop antibodies against the virus, but do not die from the infection. Roe deer is a good sentinel to assess TBEV-infected areas: they have rather small home ranges, are widely distributed, live in heavily tick infested areas and are often infested with ticks. In large parts of the Netherlands roe deer are often hunted for management purposes and therefore no additional hunting or capture efforts were needed to collect samples from this species.
Roe deer serum samples collected in 2009-2010 for another DWHC research project was tested for TBEV antibodies by RIVM in 2016. Six of the 297 tested serum samples tested positive, with five of the samples obtained from roe deer at the Sallandse Heuvelrug (province of Overijssel, eastern part of the Netherlands) and one of the samples was obtained in the province of Noord-Brabant (south – central part of the Netherlands). Follow-up tick collection in 2016 at the Sallandse Heuvelrug also revealed TBEV infected ticks. The Dutch Ministry of Public Health, Welfare and Sport (WVS) decided to repeat deer surveillance to get a better idea about the current distribution of the virus, because the tested serum samples were seven years old.
In 2017, over 600 serum samples from roe deer were tested for TBEV antibodies with 22 samples testing positive. In this recent study, a TBEV focus was again located on the Sallandse Heuvelrug. In addition, positive samples were collected from roe deer in the province of Overijssel and the province of Gelderland along the German border, as well as individual observations in the provinces Noord-Brabant and Limburg.
The recent results indicate that the TBEV is now present at several locations in the Netherlands. To confirm the TBEV is truly present at these locations, actual virus has to be detected in ticks. Antibodies against the virus show that an animal has been exposed at some point during their life. At the moment statistical analyses are being completed to assess if there is indeed an increased geographical range of TBEV or if the increase in positive observations is merely due to the greater number of samples tested.
In general, nothing changes for the fieldworker. The results do emphasize the importance of employing strategies to prevent tick bites and completing a tick-check after a day in the field. Ticks can transmit several pathogens that can cause disease of which TBEV is one. For the Sallandse and Utrechtse Heuvelrug it was reasonable well known that TBEV was present. In 2016, the RIVM concluded that people had contracted tick-borne encephalitis from those areas. This study illustrates that one should also be aware that one could contract TBEV in other areas than the Sallandse and Utrechtse Heuvelrug.
Tick-Borne Encephalitis Virus Antibodies in Roe Deer, the Netherlands. Rijks, J. M., M.G.E. Montizaan, N. Bakker, A. de Vries, S. van Gucht, C. Swaan, J. van den Broek, A. Gröne. & H. Sprong (2019). Emerging Infectious Diseases. 25, 2, p. 342-345 4 p..