INFECTION-RATES OF BORRELIA-BURGDORFERI IN DIFFERENT INSTARS OF IXODES-RICINUS TICKS FROM THE DUTCH NORTH-SEA ISLAND OF AMELAND

Between 1988 and 1993, a total of 7173 I. ricinus ticks, predominantly nymphs, were collected from the vegetation on the Dutch North Sea Isl and of Ameland. A proportion of the ticks (n=547) was screened for the presence of Borrelia by immunofluorescence. Infection rates of Borrel ia varied, in nymphs (n=347) from 13% to 46% and in adults, (n=122) fr om 20% to 43%. The infection rate in larvae (n=84) collected in 1993 w as 21%, showing that transovarial transmission of B. burgdorferi occur s in the I. ricinus population on Ameland. Two tick-naive sheep seroco nverted for B. burgdorferi after field-collected adult or nymphal I. r icinus were allowed to feed on them. Larval progeny (n=168) of 15 fema le adult ticks fed on one of these sheep were free from B. burgdorferi . B. burgdorferi was isolated in culture from field-collected adult ti cks. Serotyping using monoclonal antibodies against outer surface prot eins A and C indicated that both isolates belonged to genospecies B. g arinii, and this was confirmed by DraI restriction analysis of the var iable DNA sequence between the 5S and 23S rRNA genes.