Ehrlichia ruminantium grows in cell lines from four ixodid tick genera

Continuous cell lines from the ticks Amblyomma variegatum, Boophilus decoloratus, Boophilus microplus, Hyalomma anatolicum anatolicum, Ixodes scapularis, Ixodes ricinus and Rhipicephalus appendiculatus were tested for ability to support growth of the rickettsial pathogen Ehrlichia (previously Cowdria) ruminantium. Five E.ruminantium isolates, from West Africa, South Africa and the French West Indies, were used. Twelve tick cell lines were inoculated with E.ruminantium derived either from cultures of a bovine endothelial cell strain designated BPC or from other tick cell lines. Successful infection resulted in either continuous growth (in which the pathogen/cell line system could be perpetuated through regular subculture on fresh, uninfected cells for many months or years) or finite growth (in which the pathogen disappeared after one or a few subcultures). Infection with E.ruminantium from BPC was established in I. scapularis, I. ricinus and A. variegatum cell lines; E. ruminantium was transferred from these infected cell lines to B. decoloralus, B. microplus and R. appendiculatus cell lines. H. a. anatolicum cells could not be infected with E.ruminantium by any procedure. All five E.ruminantium isolates grew continuously in at least one tick cell line at temperatures between 28 degreesC and 37 degreesC; three of the isolates were successfully re-established in BPC following prolonged maintenance in tick cells. This study demonstrates that E.ruminantium is not intrinsically restricted to growth in cells from ticks of the natural vector genus Amblyomma.