Seasonal Dynamics of Ixodes ricinus in a 3-Year Period in Northern Spain: First Survey on the Presence of Tick-Borne Encephalitis Virus

Abstract

No cases of tick-borne encephalitis (TBE) in humans have ever been diagnosed in Spain. However, it is not clear if the necessary conditions for TBE virus (TBEV) persistence and transmission exist there, particularly synchrony in larval and nymphal activity, which has been related to the appearance of TBE in several countries. This synchrony allows ticks to cofeed on the same rodent host, facilitating transmission of TBEV between tick stages. From March 2006 to October 2008, 182 tick samplings were carried out on a monthly basis in six field sites in northern Spain using the blanket dragging technique. A total of 964 tick adults, 10,117 nymphs, and 73,534 larvae were counted. Ixodes ricinus was the predominant tick species. More number of adults, nymphs, and larvae were captured during the first year (2006) compared with the second or third years (p<0.05). Adults and nymphs were more active between April and June, whereas the peak of larvae appeared between May and September. Good synchrony between larvae and nymphs was observed only in two sampling sites, where mean maximum temperatures were below 10 °C from November 2006 to March 2007, but this synchrony was not maintained during the third year. Significant associations were found between abundance of I. ricinus larvae and temperature. Presence of TBEV was investigated by real-time reverse transcriptase polymerase chain reaction in 1800 nymphs analyzed in pools of 5 and in 630 adults processed individually, and all of them were negative. We therefore concluded that TBEV prevalence is either very low or absent in the investigated regions in northern Spain and current conditions there do not seem to favor TBEV maintenance. However, active and extensive surveillance based on epidemiological data collection, including data on changes in livestock, temporal fluctuations in density of small mammals, as well as tick seasonal population dynamics, is indispensable to predict the appearance of new TBE foci and recommend preventive measures.