Acute myeloid leukaemia (M6B: pure acute erythroid leukaemia) in a Thoroughbred foal.

This is a case study about a 10-week old Thoroughbred filly diagnosed with pure acute erythroid leukaemia, a rare type of blood cancer previously undocumented in horses. The filly presented severe anaemia and panleucopenia, confirmed by a haematological and bone marrow examination.

Case Presentation and Initial Findings

• A 10-week-old Thoroughbred filly was brought in for consultation due to anaemia that had lasted 4 weeks.
• The haematological exam revealed severe anaemia and panleucopenia, which is a decrease in the number of all types of white blood cells.

Bone Marrow Examination

• The cytological examination of the bone marrow smears revealed a myeloid to erythroid ratio of less than 0.02:1, which is significantly lower than the reference range of 0.5-2.4 :1 . This low ratio indicates an abnormally high number of erythroid cells in comparison to myeloid cells.
• The erythroid cell population was composed of rubriblasts, prorubricytes, and rubricytes, with very few metarubricytes. This distribution suggests the presence of a significant number of immature erythroid cells.
• There were several mitotic erythroid cells, some of which were abnormal and megaloblastic. This observation is consistent with a diagnosis of acute erythroid leukaemia.

Diagnosis and Outcome

• No treatment was initiated for the filly, and she died three days after consultation. This case highlights the importance of examining both haematology and bone marrow findings to diagnose pure erythroid leukaemia.
• It is important to highlight that there are no known cases of acute myeloproliferative disease, a condition involving the overproduction of blood cells, in horses involving cells of the erythroid lineage. However, this case indicates that such a condition should be considered a differential diagnosis for horses presenting with anaemia.