Giemsa or acridine orange stained blood smears may reveal the organisms as small round dots (perhaps in chains), rings or rods, adherent to red blood cells. Experience is required to accurately interpret smears, and, because of the cyclic nature of the infection, several samples taken over a period of several weeks may have to be examined to detect the organisms. Note: smears will almost certainly be negative for H. felis if antibiotics, particularly tetracyclines, have been given.

The anaemia is regenerative, with polychromasia and increased numbers of reticulocytes. Nucleated red blood cells may be present in smears. Spherocytes may be detected but should not be interpreted as evidence of regeneration in the absence of reticulocytosis.

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