Clinical History
A 4 month old female Cavalier King Charles Spaniel (Figure 1) presented with a 10 day history of episodic facial swelling, progressive inability to open the jaw, and exopthalmia. During these episodes, the masticatory muscles appeared to swell and became firm and painful. On each occasion the swelling quickly decreased in response to corticosteroids (Dexamethasone 0.25mg/kg). However, clinical signs persisted and dysphagia resulted in weight loss. The dog had no prior history of illness, was fully vaccinated, wormed and had no history of travel outside the UK.

Physical and Neurological Examination
At presentation the dog was bright, alert and in fair body condition (weight 5.25Kg, condition score 2/5). Clinical examination revealed bilateral swelling of the masseter muscles, slight exophthalmia and discomfort on palpation of the masticatory muscle group and on retropulsion of both eyes. Jaw opening was limited to 15mm (Figure 2). Neurological examination was unremarkable.

Diagnostic Testing

Haematology – No significant abnormality detected
Serum biochemistry – Moderate elevations in CK and AST
Urinalysis - Unremarkable
Toxoplasma serology – IgG <50 and IgM <20, suggesting no active infection
Neospora serology (IFAT) – Positive 1:1600 (Normal <50, Exposure <300)
Serum 2M Antibody titer (ELISA) – Positive 1:500 (reference <1:100)

Radiographs – Skull radiographs were unremarkable, the temporomandibular joints appeared normal. Thoracic radiographs were also unremarkable.

Electromyography – Fibrillation potentials were found in the masticatory muscles. All other muscle groups were unremarkable.

Muscle biopsies – Biopsies were taken from the masseter and temporalis muscles. These revealed multifocal areas of mononuclear cell  infiltration composed of scattered macrophages and clusters of lymphocytes, generalized myofiber loss and atrophy, fibrosis and edema (Figure 3).

Clinical Course and Treatment
The dog recovered well from the procedure and was discharged 24 hours later with clindamycin (11mg/kg/BID) and carprofen (1mg/kg/BID). At this stage the dog was eating well with syringe assisted feeding. Three days later the dog represented with a sudden onset of dullness and a head tilt. Clinical examination was again unremarkable and the biopsy sites were healing well. Neurological examination revealed a marked head tilt to the left, an absent menace response in the left eye, and bilateral horizontal nystagmus. This localised a lesion to the left cranial nerves VII and VIII. Further investigations (MRI scan and CSF analysis) were discussed with the owner but were declined at this stage for financial reasons. Treatment for Neospora was pursued as it was felt this was likely to underlie the deterioration and neurological signs.

Diagnosis and Further Treatment

Supportive care with intravenous fluids and syringe feeding was initiated and potentiated sulphonamides were also added (15mg/kg/BID). Ocular lubricants were used to prevent exposure keratitis in the left eye (Lacrilube overnight and Viscotears q4 hours in the day). Over the next 4 days the dog’s demeanor improved, the nystamus resolved and the head tilt reduced in severity. The dog was discharged to her owner at this point.  

Although the positive 2M antibody titre confirms a diagnosis of masticatory muscle myositis (MMM), the positive Neospora IFAT also indicates an active Neospora infection. The development of acute onset of neurological signs, lead us to believe there was active Neospora infection within the central nervous system. It was felt that there may also be Neospora infection within the muscles, however review of the muscle biopsies in light of this did not reveal any Neospora cysts or tachyzoites; antibody staining for Neospora was also negative. As masticatory muscle myositis is an immune mediated condition it is possible that the Neospora infection acted as a trigger for the development of 2M auto-antibodies.  

Masticatory muscle myositis can lead to the rapid destruction of myofibers and fibrosis, and immunosuppressive therapy with corticosteroids is indicated. In this case steroids were not added initially due to the concern of worsening Neospora infection. Six weeks of clindamycin and potentiated sulphonamides were prescribed, and after 3 weeks, anti-inflammatory doses of corticosteroids were added. The dosage was incrementally increased to an immunosuppressive dose (1mg/kg/BID) whilst watching for worsening of the neurological signs. The dog remained stable and the jaw movement increased rapidly over the next 2 weeks. The prednisolone dose was then gradually reduced over the next 6 months.

Outcome

The dog has now completed the 6 month tapering prednisolone course, and although a slight head tilt and the absence menace response are still present, the dog has a very good quality of life. The dog’s jaw opens close to a normal range with normal prehension and chewing.  

A severe and early onset form of masticatory myositis is seen in Cavalier King Charles Spaniels, although this is yet to be fully characterized (see case of the month July 2005, http://medicine.ucsd.edu/vet_neuromuscular/cases/2005/jul05.html). With respect to the Neospora infection, this dog had never been outside the owners’ garden, thus vertical transmission was suspected. The dog had been bred by the owner and the bitch was frequently walked on dairy farm land, a known risk factor for Neospora infection (Kramer et al, 2004); Neospora titres in the bitch did not indicate exposure to Neospora (IFAT <1:100 ), and there were no clinical signs of infection.

References

Kramer L, De Riso L, Tranquillo V M, Magnino S & Genchi C. Analysis of risk factors associated with seropositivity to Neospora caninum in dogs. Veterinary Record (2004) 154,692,693

Ruehlmann D, Podell M, Oglesbee M, Dubey J P. Canine Neosporosis: A case report and literature review. Journal of the American Hospital Association (1995) 31,174-183

Melmed C, Barton CL, Bergman R, Shelton G D. Masticatory muscle myositis: Pathogenesis, Diagnosis, and Treatment. Comp Continuing Ed Small Animal. August 2004

http://medicine.ucsd.edu/vet_neuromuscular/publications/masticat.pdf