Masseter myodegeneration as a cause of trismus or dysphagia in adult horses.

This study examines the medical records of eight horses that showed myodegeneration in their masseter muscles, identified through symptoms such as difficulty in eating or opening their mouths, weight loss, difficulty in moving, and noticeable atrophy of the masseter muscles, along with high muscle enzyme levels and varying degrees of muscle tissue damage.

Research Subjects and Symptoms

• The research involved eight horses that were experiencing various symptoms, including difficulty in functioning normally and loss of weight due to problems in eating or opening their mouths.
• The common physical manifestation of their ailment was the atrophy or degeneration of masseter muscles which affected their ability to move or eat.

Enzyme Activity and Nutrient Concentration

• The serum activities of muscle enzymes in these horses were abnormally high. These enzymes indicate muscle damage and their high activities suggest high degrees of muscle tissue damage.
• Some of the horses also had lesser than normal levels of selenium and vitamin E in their blood and liver. Both selenium and vitamin E play crucial roles in maintaining muscle health in horses. Their deficiency, therefore, might be linked to muscle degeneration.

Disease Stage and Lesions

• The lesions in the horses’ masseter muscles varied according to the stage of the disease – from swelling and discoloration at the initial stages, progressing to muscle atrophy and fibrosis as the disease advanced.
• The muscle tissue showed various degrees of damage – ranging from acute to subacute degeneration, with signs of efforts by the body to regenerate the tissue, eventually leading to chronic degeneration and replacement of the muscle tissue by fibrous tissue.

Widespread Muscular Damage

• While all the horses showed changes in the masseter muscle, some had widespread lesions in skeletal muscles, implying the condition might affect muscles apart from masseter.
• A few horses also had myocardial lesions, indicating potential heart muscle involvement in this ailment.