Histopathological characteristics of an ossifying fibroma formed in the maxilla of a racehorse.

This research article describes the case of a racehorse in Japan who developed an unusual growth, known as an ossifying fibroma, in its maxilla (upper jaw). Despite two operational attempts to reduce the mass, it continued to grow, ultimately having to euthanize because of complications with its breathing. This case represents the first reported instance of equine ossifying fibroma in Japan.

Case Presentation and Initial Findings

• The study focuses on a case of a 1-year-old male thoroughbred racehorse, who exhibited swelling on his left upper lip.
• The swelling was a result of an enlargement around his incisive (biting) bone situated in the left maxilla, a part of the skull located in the upper jaw region.
• Despite two surgeries aiming to reduce the size of this enlargement, the mass continued to grow.

Deterioration and Resulting Action

• The growth of the mass caused a narrowing of the nasal cavity, known as stenosis, which ultimately led to the horse having difficulties with breathing (dyspnea).
• This complication forced the veterinary team to perform euthanasia on the horse.
• After euthanasia, a post-mortem pathological examination of the mass was performed to ascertain its nature and cause.

Pathological Findings and Diagnosis

• A macroscopic examination (visible to naked eye) was conducted on a section of the mass. It revealed the formation of multiple solid fibrous tissues, and network of bone tissue (trabeculae) within the incisive bone which had displaced the cortical bone.
• A histological analysis (study of tissue under a microscope) of the mass showed it to be composed of bone-like structures formed from the proliferation (increase) and aggregation (collectively grouping) of fibroblasts, which are cells that produce collagen and other fibers in the body.
• Based on these microscopic findings, the mass was diagnosed as an ossifying fibroma, a benign tumor originating from the periodontal ligament, which behaves like bone tissue.

Significance of the Study and Findings

• While ossifying fibroma in horses is usually found in the mandible (lower jaw), it was located in the maxilla (upper jaw) in this particular case, making it an unusual occurrence.
• This case marked the first reported instance of an ossifying fibroma affecting a horse in Japan.
• The study thus represents significant case documentation in the field of equine medicine in Japan, potentially contributing to our understanding and knowledge of equine pathological conditions.