Mapping of Dermacentor reticulatus expansion in Poland in 2012-2014.

Rapid expansion of the tick Dermacentor reticulatus (Fabricius) has been reported in many European countries. In Poland its range was limited to the area on the eastern side of the Vistula River up until the 1990s. However, new foci were recently discovered, while the centre of the country and mountain regions are believed to be free of this tick and are known as "the gap". A few unconfirmed factors, like unfavourable weather conditions, changes in land cover and absence of competent hosts have previously been reported as possible reasons for the absence of D. reticulatus in this area. Since D. reticulatus plays an important role for the maintenance and the circulation of tick-borne pathogens, we (1) determined its actual range in Poland, (2) monitored its expansion in 2012-2014 and (3) correlated abiotic conditions on its known range. Dragging was conducted in the area between the Vistula River and the western border of Poland in 2012-2014, along the three major Polish rivers and their tributaries. Temperature and humidity on the ground were recorded 4 times a day at a total of 32 sites. D. reticulatus was found in 21 new locations on the western side of the Vistula River and in 22 locations in western Poland. The presence of this tick species was confirmed for the first time in Wielkopolskie, Kujawsko-Pomorskie and Łódzkie voivodeships. Existence of 'the gap' was confirmed on a strip of land along the northwest-southeast axis (an area of approximately 151000km(2)), that extends between Zachodniopomorskie and Pomorskie in the North, and the mountain areas with foothills in the South, in Opolskie, Śląskie, Małopolskie and Podkarpackie voivodeships. This gap divides the tick population in Poland into two separated populations - Western and Eastern. The lowest abundance of ticks was found in the newly established foci in central and western Poland. Our study showed the need for the monitoring of the D. reticulatus expansion. Early detection of new foci is crucial for taking proper prevention measures.