Acute myeloid leukemia-M1 in a horse with neurologic signs and necrotizing enterocolitis.

The research article discusses a case of acute myeloid leukemia-M1 (AML-M1) in an 18-year-old American Saddlebred mare horse, which presented unusual neurologic signs, and was further complicated by necrotizing enterocolitis.

Introduction and Clinical Presentation

• The horse was admitted with a fever and acute onset of neurological symptoms including: grade 3 of 5 ataxia (difficulty in movement and coordination), difficulty in prehension (difficulty in grasping), and dull mentation (reduced mental activity).
• Due to financial constraints, comprehensive testing couldn’t be done, and the horse’s condition worsened despite supportive treatment, leading to a decision of euthanasia.

Postmortem Investigation

• Postmortem examination revealed petechiae (small red or purple spots caused by bleeding) and ecchymoses (caused by leaking blood vessels) in the meninges (protective membranes covering the brain and spinal cord) and brain neuroparenchyma (functional tissue in the brain).
• Neoplastic round cells, indicative of a cancerous condition, were identified within the vasculature and areas of hemorrhage in the neuroparenchyma, as well as in other parts of the body including the intestinal submucosa, liver, kidney, lungs, and the mesenteric lymph node.
• The horse was also found to have necrotizing enterocolitis (damaged and inflamed intestinal tissue) and acute fibrinonecrotizing bacterial pneumonia (severe lung inflammation and damage).

Laboratory Findings and Final Diagnosis

• Of the abnormal round cells in the brain, over 70% expressed ionized calcium-binding adapter molecule 1 (Iba1, a macrophage marker), 10-20% expressed myeloperoxidase (MPO, a marker for myeloid cells) and less than 10% expressed each of these – PAX5, CD3, CD20, CD79a, or MUM1, which are markers typically used in identifying hematological malignancies.
• The bone marrow was found to be largely replaced by cancerous round cells expressing Iba1 and approximately 70% of these cells expressed MPO without any expression of CD3 or CD20.
• The abnormal cells also showed expression of CD172a, a marker associated with myeloid lineage.
• These findings led to a final diagnosis of acute myeloid leukemia-M1 (AML-M1), a form of blood and bone marrow cancer, with a rare neurological presentation.