was 29.0% and 22.3% for em A. are present. It has been estimated that up to 30-40% of horses Daurinoline in an endemic area are seropositive for em B. burgdorferi /em s. l. [1]. Most of these horses will remain asymptomatic, while 5-10% of them are likely of developing clinical signs [1-4]. Comparable, a previous study on em A. phagocytophilum /em , have indicated that up to 50% of seropositive horses in endemic areas undergo a subclinical contamination Daurinoline [5]. It is assumed that clinical equine granulocytic anaplasmosis is an overseen condition in most of Europe, as most horses recover spontaneously and therefore do not appeal to the attention of clinicians [6]. In Denmark, IL5RA em B. burgdorferi /em s. l. and em A. phagocytophilum /em are transferred by the tick em Ixodes ricinus /em . A Danish study from 2005 revealed that this tick density varies substantially between different regions of Denmark with highest density on the island of Bornholm (0.5-1.0 ticks/min. flagging) followed by Zealand, Funen, Middle-Jutland and East-Jutland (0.25-0.5 ticks/min. flagging), South-Jutland (0.15-0.25 ticks/min. flagging), and West and North-Jutland (0-0.15 ticks/min. flagging) [7]. An apparent increase in the tick density from 1984 to 1998 [8] fits well with a positively correlated relationship between warmer winters and longer spring and autumn periods and the density of em I. ricinus /em [9,10]. The seroprevalence of em B. burgdorferi /em s. l. and em A. phagocytophilum /em among horses in Denmark has to our knowledge never been evaluated. Recent European studies around the seroprevalence of em B. burgdorferi /em s. l. shows a prevalence of 47.8% seropositive horses in Slovakia [11], 25.6% in Poland [12], 16.8% in Sweden [13], 16.1% in Germany [14] and 6.3% in Turkey [15]. The seroprevalence of em A. phagocytophilum /em in Europe varies from 83.3% in Holland [6], 16.7% in Sweden [13], 11.3% in France [16], 8.1% in Italy [17,18] to 6.5% in Spain [19]. Furthermore, a Swedish study reported that 4.5% of the examined horses were seropositive for both em B. burgdorferi /em s. l. and em A. phagocytophilum /em [13]. In 2005, Danish researchers made a seroprevalence study around the distribution of em B. burgdorferi /em s. l. and em A. phagocytophilum /em antibodies in Danish deer. The overall seroprevalence was 36.6% for em B. burgdorferi /em s. l. Significant regional differences were found when Jutland was compared with the islands (Funen, Zealand, Lolland, Falster and Bornholm) with a seroprevalence of 27.1% versus 46.7%, respectively. In the case of em A. phagocytophilum /em , all tested deer in three districts of North-Jutland were antibody unfavorable although the average seroprevalence was found to be 42.6% ranging from 39.6% in Jutland to 47.6% of the islands [7]. The aim of the present study was to evaluate the seroprevalence of em B. burgdorferi /em s. l. and em A. phagocytophilum /em in Danish horses. Materials and methods Sample size The sample size was calculated using the following formulas: where em n /em is the sample size, em E /em is the margin of error, em r /em is the fraction of response that we are interested in and is the critical value for the confidence level of em c /em . As the prevalence of horses seropositive for em B. burgdorferi /em s. l. and em A. phagocytophilum /em vary considerably in areas comparable to Denmark [11-14], calculations were based on a situation where the test results from each individual horse could have 50%-50% test outcome ( em r = /em 50%). A difference of more Daurinoline than 5% ( em E /em ) to the expected 50%-50% outcome with a confidence of 95% ( em c /em ) should be detected. Thus, with an estimated population of 200,000 horses ( em N /em ) in Denmark, a sample size including 384 individuals was proposed. As the test kits came as pre-packed batches, 390 individual blood samples were taken. Selection of horses No formal randomized selection of horses was applied but an even representation of samples from all regions in Denmark was aimed at. Denmark is usually officially divided into five administrative regions including the Capital Region (C), Region Zealand (Z), Region South-Denmark (SD), Region Central-Jutland (CJ) and Region North-Jutland Daurinoline (NJ) (Physique ?(Figure1).1). Assuming that.