-1 Physiological age of field-collected female taiga ticks, Ixodes persulcatus (Acari: Ixodidae), and their infection with Borrelia burgdorferi sensu lato IGOR USPENSKY 1, *, YURI V. KOVALEVSKII 2 and EDWARD I. KORENBERG 2 1 Department of Biological Chemistry, A. Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; 2 Department of Infections with Natural Focality, Gamaleya Institute for Epidemiology and Microbiology, Russian Academy of Medical Sciences, Moscow 123098, Russia; *Author for correspondence (e-mail: igorusp@vms.huji.ac.il; phone: +972-2-645- 0056; fax: +972-2-658-5458) Received 30 September 2005; accepted in revised form 26 January 2006 Key words: Age determination, Borrelia burgdorferi s.l., Intensity of infection, Ixodes persulcatus, Prevalence of infection (infection rate), Tick physiological age Abstract. In some studies the prevalence of tick infection (infection rate) and the intensity of infection are negatively correlated with unfed tick age (in the broad sense of this term). However, no special research has been carried out to consider the phenomenon thoroughly. The infection indices of the female taiga ticks, Ixodes persulcatus, infected with Borrelia burgdorferi s.l. were related to tick physiological age, an index that more precisely reflects tick physiological state than the time of tick collection in the field or the duration of tick survival under laboratory conditions. A novel quantitative technique of physiological age determination based on the evaluation of the ratios between sizes of the stable (scutum) and the changing (alloscutum) structures of the tick body was used. The age was estimated in accordance with the classical age-grade scale introduced by Balashov and a more fractional scale determined by the new technique. In total, 131 female ticks were examined for their infection and physiological age, 46 of which were infected with B. burg- dorferi s.l. (mean infection rate 35.1%). The minimal intensity of infection was 0.4 bacterial cells per 100 fields of view whereas the maximal infection was 172 cells. There was no difference between the prevalence of infection in ticks of different physiological age. The intensity of infection obviously differed between ticks of different age groups in the scale introduced by Balashov but did not significantly differ between ticks of different age groups according to the fractional age-grade scale. The data concerning the relationships between Borrelia burgdorferi and unfed Ixodes ticks are considered. Introduction The fate of pathogens in the vector body is a question of great interest. The ixodid ticks are the best model for such studies as compared with other arthropod vectors because of their single feeding at each active stage which excludes the influence of repeated feedings on the fate of a pathogen. Some observations on unfed ticks infected with different pathogens showed that the prevalence and/or the intensity of tick infection by pathogens are negatively correlated with the tick age (in the broad sense of the term) [reviewed by Balashov (1995, 1998) and Uspensky (1995)]. These observations were made Experimental and Applied Acarology (2006) 38:201–209 Ó Springer 2006 DOI 10.1007/s10493-006-0006-2