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A 3-year-old Bernese Mountain Dog had a 1-week history of pain on opening its mouth and reluctance to eat. Swelling caudal to the left second maxillary molar had been found by the referring veterinarian. The swelling was incised and flushed. No foreign material was observed. After 1 week of supportive care, there was no improvement in clinical signs and the patient was referred. At that time, there was marked discomfort upon opening the mouth. No notable laboratory abnormalities were found. ImagingMagnetic resonance (MR) imaging of the head was performed using a 1.5 T instrument. Ã Transverse and dorsal T2-and proton density-weighted images, and postcontrast T1-weighted transverse images were obtained. There was an approximate 4 Â 4 Â 2 cm of signal abnormality affecting the medial aspect of the left temporalis muscle and lateral aspect of the left medial pterygoid muscle. This lesion was hyperintense to muscle on T2-weighted images (Fig. 1), and mostly hypointense with a hyperintense, presumably contrast enhancing, rim on postcontrast T1weighted images (Fig. 2). Milder T2 hyperintensity and less intense poorly defined presumed contrast enhancement extended diffusely into the surrounding muscles and also affected the left zygomatic salivary gland. These changes were consistent with an abscess in the left medial temporalis muscles with inflammation of the masticatory muscles and left zygomatic salivary gland. Using ultrasound-guidance, fluid was removed from the hypoechoic fluid pocket. Cytologically, the fluid was consistent with suppurative inflammation, but organisms were not isolated. The diagnosis was an abscess and an antibiotic and an analgesic were prescribed. Follow-upThe patient was reexamined 6 weeks later because the clinical signs had returned when the antibiotic was discontinued. There was severe pain on opening the jaw. No laboratory abnormalities were detected.A second MR imaging was performed. There was reduction of the extent of diffuse hyperintense and contrast enhancing regions within the left temporalis and pterygoid musculature. The previously identified fluid collections were absent. Instead, there was a focal, oblong, 1.0 Â 1.0 Â 0.7 cm lesion within the ventromedial left temporalis muscle just caudal to the left pterygopalatine fossa. This lesion was hyperintense on all pulse sequences with a hyperintense, presumably contrast enhancing, 2-3-mm-thick wall that contained a small internal, linear, hypointense focus on both T2-and T1-weighted images (Figs. 3 and 4). The musculature immediately surrounding this focus was hyperintense on proton density and T2-weighted imaging sequences, and was moderately contrast enhancing. Diagnosis and outcomeAt surgery, a fibrous encapsulated abscess filled with gray, purulent material and a grass awn foreign body were found. The grass awn was removed and the area was flushed and closed. Actinomyces canis and Prevotella bivia were isolated. The patient was treated with an analgesic and appropriate antibiotics. One year postoperatively there is complete resol...

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