Howell-Jolly bodies (also called micronuclei) are small fragments of non-functional nucleus which were not extruded as the erythrocyte left the bone marrow. Usually, but not always, the host erythrocyte contains only a single Howell-Jolly body, which appears as a blue, perfectly round inclusion. Occasionally, uneven staining of an Howell-Jolly body can result in a ring-like appearance which may mimic a parasite.
In dogs and ruminants, Howell-Jolly bodies are not routinely observed in peripheral blood because they are removed from red blood cells by the pitting function of the sinusoidal spleen.
Howell-Jolly bodies can be seen in the following situations:
In normal cats and horses (non-sinusoidal spleens): Howell-Jolly bodies are not removed as readily as in dogs or cattle, so low numbers may be seen.
Regenerative anemia: All species.
Compromised or absent splenic function: Corticosteroids, splenectomy.
Erythroid dysplasia: In non-anemic miniature and toy poodles with hereditary macrocytosis, increased numbers of Howell-Jolly bodies (some abnormal and fragmented) may be observed. Similarly, abnormal and increased Howell-Jolly bodies may be seen in myelodysplastic syndrome (primary myelodysplasia).