HISTOLOGICAL OBSERVATIONS ON BORRELIA-BURGDORFERI GROWTH IN NATURALLYINFECTED FEMALE IXODES-RICINUS

Borrelia burgdorferi in naturally infected, flat, blood-feeding and re plete females was visualized by direct immunofluorescence assay, silve r stain and electron microscopy. In the majority of flat females, borr eliae were detected in the midget only. During blood-feeding, midgut l umen infection was only present in females fed for 0.5 and 1 day on ra bbits. All infected females fed for 2 days or more, including engorged ticks, contained bacteria within all tissues other than midgut lumen. These phenomena indicate that, if regurgitation can occur as a mode o f B. burgdorferi transmission, it must probably takes place at the beg inning of blood-feeding. The demonstration of numerous spirochaetes in the tissue of acini and ducts of salivary glands in all systemically infected blood-feeding females, provides further support for the saliv ary transmission of the bacterium. Although systemic borreliae could b e found intracellularly within different tick tissues, they were predo minantly situated in the extracellular site. Penetration of host cells by B. burgdorferi was often detected in the basal region of the hypod ermis and midgut and ovarian epithelium. The results indicate that B. burgdorferi multiplies in various tick tissues during and after blood- feeding.