Wright's stain, 500x

Haemobartonellosis may be very difficult to diagnose based on examination of a blood smear given the fluctuation that may occur in the number of parasites in the patient’s bloodstream. Parasitemia can in effect vary greatly from day to day. Parasitemia must be marked in order for the infection to be detected. Haemobartonella  felis will appear as small, short, rounded rods alone or in chains on the surface of red blood cells. H. felis  can form rings or chains on the cell surface, but smears must be prepared quickly, as the bacteria will detach from the red blood cells in vitro. Wright’s (or a Romanowsky-based) stain is the desirable staining agent to enable proper differentiation of the microorganisms on the surface of the red blood cells. Care must be taken to avoid interpreting stain precipitate as microorganisms.  Haemobartonella canis infects dogs and will have a similar appearance.