Ticks are obligate hematophagous ectoparasites and important vectors of infectious diseases transmitting parasites of livestock and humans, e.g. the etiologic agents of babesiosis, theileriosis or anaplasmosis and of human tick-borne encephalitis and Lyme disease [1, 2]. The sheep tick Ixodes ricinus is the most common tick species and principal vector for various infectious diseases in Europe and some regions of Asia and North Africa . The distribution and abundance of I. ricinus depends on various abiotic and biotic factors such as the microclimate, habitat (vegetation) and host cenosis . The host-seeking activity of I. ricinus is favored by air temperatures between 7°C and 24°C and relative humidities of 45-100% due to the risk of desiccation [5, 6]. Ecosystems that have a strong buffering capacity, for example, for humidity, such as mixed deciduous and coniferous forests with well-developed leaf and shrub layers are preferred tick habitats [7, 8]. However, also forest biotopes differ in the abundances of I. ricinus, presumably correlated to the water content of the soil [7, 8].
Since the development and survival of ticks strongly depends on climatic conditions, the distribution and abundance of ticks might also be influenced by global warming. In Sweden, the distribution of I. ricinus extended towards the north, and this was suggested to be caused by increased air temperatures which favored the survival, activity and development of ticks . Similarly, in the Czech Republic I. ricinus spread towards higher altitudes, from 700m to 1100m above the sea level within the last 20 years .
In addition to climatic factors, the host cenosis also affects the distribution and abundance pattern of ixodid ticks. I. ricinus is an euryphage species that has a broad host spectrum and thus feeds on more than 300 vertebrate species . It predominantly infests small rodents (mice), passerine birds and larger mammals such as hedgehogs, hares, squirrels, wild boar and roe deer . Increased population densities of these hosts induce an increase in the densities of ticks (summarized by ). In addition, also the anthropogenic impact on habitat changes the temporal and spatial pattern of tick populations .
An increase in the abundance of Ixodes can increase the transmission risk of diseases, e.g. of the spirochete bacterium Borrelia burgdorferi sensu lato, the etiologic agents of Lyme disease which is endemic in Europe . Prevalences of B. burgdorferi s.l. in I. ricinus range up to 11%, 43% and 58% in larvae, nymphs and adults, respectively . During the last decades, Borrelia prevalences have increased in different regions in Europe, such as in Denmark and Germany [15, 16]. Infection prevalences differ between different regions in Europe because the transmission of Borrelia depends on a complex zoonotic cycle between their reservoir hosts and their tick vectors. More than 50 avian and mammalian hosts are reservoir hosts for B. burgdorferi s.l. in Europe .
Different genospecies have been identified in the B. burgdorferi s.l. complex. In Europe, seven genospecies are prevalent, B. burgdorferi sensu stricto, B. afzelii, B. garinii, B. valaisiana, B. bissettii, B. spielmanii, B. lusitaniae[18, 19], and B. bavariensis, a recently classified genospecies  that was previously described as the rodent-associated B garinii OspA serotype 4 . Usually B. burgdorferi s.s., B. afzelii and B. garinii (including B. bavariensis) are present in tissues of Lyme disease patients [22, 23]. Spirochetes of B. valaisiana were isolated from a few patients who showed erythema migrans or acrodermatitis chronica atrophicans manifestations and an old man who showed strong clinical evidence of neuroborreliosis [23, 24]. B. bissettii was detected in tissues from a few patients suffering from Lyme borreliosis in Slovenia and in the Czech Republic [25, 26]. B. spielmanii was present in the skin of a few patients with erythema migrans . Only once, B. lusitaniae was identified in a patient, but he showed symptoms that are untypical for the clinical manifestations of Lyme disease . In I. ricinus from Slovakia, Latvia, Germany, Portugal and the United Kingdom, the most prevalent Borrelia genospecies were B. afzelii, B. garinii (including B. bavariensis) and B. valaisiana with overall prevalences of 39.3%, 21.2% and 12.8%, respectively .
Prevalence and the distribution of Borrelia genospecies strongly depend on the local host cenosis due to the host′s reservoir competence . B. afzelii is mainly associated with rodents and B. garinii and B. valaisiana with birds . Both, rodents and birds are competent reservoir hosts of Borrelia burgdorferi s.s. . Rodents do also serve as reservoir hosts of B. bissettii and B. bavariensis[21, 33] and the garden dormouse seems to be the main reservoir host of B. spielmanii in Central Europe . B. lusitaniae is associated with birds and lizards [35, 36].
In the present investigation, we determined the abundance of I. ricinus and the prevalence of Borrelia and of the different genospecies in ticks in the Siebengebirge, a nature reserve and well visited local recreation area near Bonn, Germany. In 2007 and 2008, ticks were collected at three sites that represented different plant communities and possessed different population densities of I. ricinus. These sites have been already examined in previous investigations in the years 1987, 1989, 2001 and 2003 [8, 15, 37]. Thus, abundance and prevalence were compared with previous investigations.