Epidermoid cyst of the brain in the horse.

The research article discusses a neurological condition observed in an older Connemara horse, characterized by episodic symptoms of dullness, head-pressing, a staggering gait, and hyperexcitability. The disease, which recurred every 3-4 days, was found to be caused by an epidermoid cyst on the horse’s brain, considered to be a congenital malformation.

Neurological Symptoms in the Affected Horse

• The Connemara mare presented signs of neurological illness over a span of 3 weeks. This included frequent episodes of dullness or lethargy, a habit of pressing its head against objects (head-pressing), a staggering gait (unsteady and uncoordinated movements), and states of hyperexcitability.
• These signs were recurring, observed at regular intervals of 3 to 4 days. Significantly, the horse displayed periods of apparent normality in between these episodes.

Post-mortem Examination and Findings

• An autopsy was performed after the horse’s death to determine the cause of its neurological symptoms.
• The post-mortem examination revealed an epidermoid cyst on the dorsal surface of the occipital lobe in the left cerebral hemisphere of the horse’s brain. An epidermoid cyst is a rare, non-cancerous type of cyst often filled with keratin, a protein found in skin, hair, and nails.
• This cyst was lined with stratified squamous epithelium, a type of tissue that often lines the outer layer of the skin, mouth, and throat. Inside the cyst, the researchers found desquamated keratinised material and blood.

Conclusion on the Cause of the Neurological Disease

• The lesion that was discovered in the horse’s brain is believed to be a congenital malformation, an anomaly present at birth. This suggests that the cyst was not acquired due to environmental factors or disease, but was present in the horse from birth.
• Over time, as the horse aged, the cyst increased in size, causing space-occupying signs. In other words, as the cyst enlarged, it started taking up more space in the horse’s brain, leading to pressure and displacement of the brain tissue. This displacement and pressure from the enlarging cyst likely resulted in the neurological symptoms exhibited by the horse.