Acquired Myasthenia Gravis in a Juvenile Australian Shepherd Dog and Efficacy of Leflunomide Treatment.

Myasthenia gravis is a disorder of neuromuscular transmission characterized by muscle weakness and exercise intolerance that is rarely diagnosed in dogs younger than 6 months. We report a case of juvenile-onset myasthenia gravis in a 5-month-old, intact male Australian Shepherd with onset of clinical signs at 4 months of age. The dog presented with a 4-week history of progressive weakness and regurgitation. Diagnosis was confirmed by an elevated serum acetylcholine receptor antibody concentration. Treatment with pyridostigmine provided only partial improvement. Leflunomide was added to achieve immunosuppression while avoiding the side effects of corticosteroids, such as muscle weakness. This treatment combination was well-tolerated; the dog went into clinical remission, including resolution of megaesophagus, and progressive reduction in acetylcholine receptor antibody concentration was noted. This report highlights the importance of considering myasthenia gravis in juvenile dogs and emphasizes the utility of measuring acetylcholine receptor antibodies in suspected cases regardless of age. The safe and possibly efficacious use of leflunomide in a dog with this disease is also demonstrated. Further studies are needed to characterize the juvenile presentation of this disease and to evaluate the efficacy of treatment with leflunomide as the sole immunosuppressant medication in a larger canine population.