Unusual case of muscle cramping in a 2-year-old male Labrador retriever

Clinical History

A 2-year-old male neutered Labrador retriever was presented for evaluation of muscle cramping and fasciculations. The dog was adopted as a puppy from a pet store and was otherwise health with no history of travel, kenneling or tick-exposure. Vaccinations were current and the dog takes heartworm preventative. At 6-months of age the dog developed episodic proximal pelvic limb stiffness with exercise that lasted about 1-2 minutes. This occurred with activity every 2-3 weeks. At 1 year of age the dog developed temporalis muscle cramping (see video clip) with activity such as ball playing. Recently, muscle cramping and fasciculations occur without activity. During these episodes the dog whines and is unable to open the jaw.

Physical and Neurological Examination

With the exception of intermittent spasms of the temporalis muscle with activity, no abnormalities were identified on physical examination. Gait evaluation was normal and mentation was appropriate. Spinal reflexes were normal. The neurolocalization was to the neuromuscular system with a primary myopathy or possibly motor neuron disease considered.

Diagnostic Tests

CBC and serum chemistry panel: Mildly elevated serum creatine kinase activity at rest and post-exercise
Electrodiagnostic testing: Normal electromyogram, normal motor nerve conduction velocity and repetitive nerve stimulation
Muscle biopsies (temporalis and vastus lateralis muscles):  Mild inflammatory myopathy/myositis was evident within the temporalis muscle. Although cellular infiltrates were not observed in the vastus lateralis muscle, fiber type grouping was identified consistent with denervation and reinnervation.

Serum antibodies against Toxoplasma, Neospora – Negative IgG and IgM for Toxoplasma, Neospora antibody titer > 1:100,000 consistent with Neosporosis.

Treatment and Outcome

Based on the diagnosis of Neosporosis, treatment was initiated with trimethoprim sulfa 180 mg twice daily for 1 month, clindamycin 500 mg twice daily for 2 weeks, and pyrimethamine 30 mg once daily for 2 weeks. On recheck examination 2 weeks later, the dog was doing well and the owner noted that since treatment was initiated, clinical signs of muscle cramping and fasciculation had resolved. Clinical signs recurred when pyrimethamine was discontinued, but resolved again after restarting pyrimethamine. Although thyroid was normal prior to treatment, the dog became hypothyroid after 6 weeks of therapy. It is not clear if hypothyroidism is a result of trimethoprim sulfa treatment or is disease related.