TF and atrial fibrillation: Colonies of A. marginale containing RFs or DFs were easily recognizable with light microscopy (Fig. 1) The main site of A. marginale infection in ticks after AF was in the gut and Malpighian tubule cells (Figs. 1 and 2), while after TF colonies were also seen in gut muscle, salivary gland acinar and fat body cells (Fig. 2).