A grade IV glioblastoma with an oligodendroglial component (GBM-O) in a horse.

A 4-year-old horse diagnosed with a tumour in its brain was examined in this study. The investigation revealed the presence of a rare form of cancer, a grade IV glioblastoma with an oligodendroglial component (GBM-O), marking this as the first known report of such a case in a horse.

Presentation of the Horse

• The study involved a 4-year-old Dutch warmblood mare that had been exhibiting signs of ataxia (a neurological disorder affecting balance, coordination, and speech) and proprioceptive deficits (problems with the sense of self-movement and body position) for a period of ten months.
• An initial scan conducted using computed tomography revealed the presence of a large, non-contrast enhancing mass in the left cerebral hemisphere of the horse’s brain.

Necropsy Examination

• The horse was later necropsied and a detailed examination of the brain revealed a tumor that had considerably impacted and reshaped the piriform and temporal lobes.
• Under microscopic examination, the tumor was identified as a grade IV glioblastoma with an oligodendroglial component (GBM-O), which is a very aggressive form of brain cancer.
• This type of tumor was composed of a wide variety of cell types, organised in different patterns within a fibrous connective tissue.
• Multiple foci of necrosis (cell death) were observed in the tumor.

Microscopic and Immunohistochemical Findings

• Around the boundaries of the tumor, neoplastic oligodendroglioma-like cells (cancerous cells that resemble those in a specific type of brain tumor) were found in an extracellular mucinous matrix (a compound that helps maintain the structure of tissues).
• Most cells in the tumor were strongly reactive to glial fibrillary acidic protein (GFAP), a type of protein produced in the brain and spinal cord. However, it was noted that the oligodendroglioma cells did not express this marker.
• The cells forming the microvascular proliferations (outgrowths of abnormal blood vessels that feed the tumor) were positively labeled for expression of factor VIII and smooth muscle actin, indicating their role in blood vessel formation.
• All the tumor cells tested negative for Neu-N and synaptophysin, markers usually present in neuronal cells and synapses respectively.
• The cell proliferation index, or the percentage of cells in the process of dividing, was up to 5%.
• Both the tumor cells and the normal cells in the horse’s brain did not express epidermal growth factor receptor (EGFR), a protein that, when mutated, can promote cancerous growth. None of the cells expressed EGFR vIII mutant or the phosphatase and tensin homologue (PTEN), a tumor suppressor gene. These results were confirmed through the comparison with a positive control from human GBM tissue.

Conclusion

• This study marked the first recorded case of GBM-O in a horse.