Serial propagation of Ehrlichia canis in primary canine peripheral blood monocyte cultures

Cultures of primary canine monocytes were prepared from blood to which dextran (3%) had been added to enhance sedimentation of the erythrocytes. Homogeneous populations of monocytes were observed by 5 days after seeding, and some cultures were maintained as long as 30 days. The monocyte cultures were used successfully for the serial passage of Ehrlichia canis. After the 3rd passage, small intracytoplasmic inclusions of E canis were detected by 60 hrs after inoculation and greater than 90% of the monocytes were infected by 7 days. In vitro propagated ehrlichiae were preserved at -80 C for at least 31 months with minimal loss of infectivity. Following 125 in vitro passages E canis induced signs of ehrlichiosis in experimentally infected dogs.