DIFFERENCES IN BORRELIA INFECTIONS IN ADULT IXODES-PERSULCATUS AND IXODES-RICINUS TICKS (ACARI, IXODIDAE) IN POPULATIONS OF NORTH-WESTERN RUSSIA

Dark-held microscopy was used to determine the number of Borrelia spir ochetes in 630 standard preparations obtained from adult ixodid ticks (344 Ixodes persulcatus and 286 I. ricinus) collected in 1989-92 in th e Leningrad region of Russia. The average numbers of Borrelia in I. pe rsulcatus and I. ricinus preparations were 34.7 and 23.3 per 100 micro scopic fields, respectively. The maximal individual values registered each year for ticks of both species were several hundred times greater than the minimal values. Ticks carrying relatively small numbers of B orrelia generally predominated. Proportions of more heavily infected t icks varied considerably from year to year. These parameters were sign ificantly higher in foci with predominance of I. persulcatus ticks. As a consequence, risk to acquire Lyme borreliosis in such foci is consi dered greater than in foci where I. ricinus predominates.