Fibrosis of the masseter leading to trismus and dysphagia in a mare.

The article discusses a case of a horse that developed trismus, the inability to open the mouth fully, due to fibrosis in the masseter (jaw muscle). The fibrosis and ensuing trismus originated from an abscess in the same muscle, and despite treatments, the horse remained unable to open its mouth normally.

Background

• The research lays out the various conditions that can cause trismus in a horse — tetanus, joint disease, mandibular fracture, eclampsia, and neurological disease.
• In this case, however, the trismus resulted from masseter fibrosis developed secondary to abscessation.

Case Presentation

• A seven-year-old Quarter horse mare displayed symptoms of dysphagia (difficulty swallowing), trismus, severe weight loss, and inability to open its mouth.
• The horse had a history of a large abscess in the right masseter muscle that had been drained and treated with systemic antibiotic treatment.
• The horse still developed an inability to open its mouth over the subsequent two weeks despite treatment.

Findings and Diagnosis

• The horse was examined at the Koret School of Veterinary Medicine-Veterinary Teaching Hospital (KSVM-VTH), where it presented thin and weak.
• Despite normal dentition and clear neurological and blood exams, they found that the horse could only open its mouth by 1 to 2 cm, even under sedation and general anaesthesia.
• An ultrasonography of the masseter muscles showed reduced echogenicity of the muscle adjacent to the right mandible, suggesting a disruption to its normal structure, possibly caused by granulation or early fibrous tissue.
• Electromyography performed on the right masseter muscle recorded decreased insertion activity.

Conclusion

• By examining this case, the report shows that trismus can be caused in horses due to fibrosis resulting from a masseter muscle abscess.
• Despite interventions to treat the abscess and suspected myositis inflammation, the horse was left suffering from trismus and dysphagia, indicating a need for more research to understand and intervene effectively in such cases.