The relationship between Cowdria and Ehrlichia: change in the behaviour of ehrlichial agents passaged through...

Following an earlier report that an Ehrlichia-like agent isolated from an adult Hyalomma truncatum female became more pathogenic and elicited a disease in sheep indistinguishable from heartwater after having been passaged through Amblyomma hebraeum, a similar phenomenon is herewith recorded. An ehrlichial agent demonstrated in the blood smear of a serologically positive, naturally infected lamb, changed in behaviour and assumed the characteristics of Cowdria after passage through A. hebraeum. Cross-immunity between sheep that had recovered from infection with the transformed agent and several stocks of Cowdria ruminantium confirmed the close relationship between the putative ehrlichial agent and Cowdria. Seven other sheep/tick passage lines resulted in high antibody titres and resistance to challenge with C. ruminantium in the sheep in the case of 2 of them, suggesting a similar change in behaviour of the agents passaged through Amblyomma.