Abstract

Diagnosis of diseases affecting deep skeletal structures is often difficult in pinnipeds due to vague clinical signs, poor ability to palpate structures as a result of thick blubber layers and limitations of traditional diagnostic imaging techniques. In recent years, The Marine Mammal Center (TMMC) has been fortunate to have access to an Imatron C-150 (Imatron Ultrafast CT) electron beam computerized tomography (CT) scanner. The advantages of this technology will be discussed with the use of 4 case examples. Gross post mortem findings, CT images and computer reconstructions will be presented.

Case 1

Aria (CSL 3083), a yearling female California sea lion (Zalophus californianus) was presented to TMMC with a swollen left hind flipper that was non-weight bearing. There was a tract draining moderate mucopurulent discharge from the left tarsal region. Despite aggressive antibacterial treatment based on culture and sensitivity results, repeated anesthesia and surgical debridement, and intermittent improvement of clinical signs, the discharge was not resolved after 9 weeks of treatment. Radiographs were taken throughout the course of treatment and revealed moderate lytic changes on the distal articular surface of the left tibia and periosteal reaction of the distal and proximal thirds of the left tibia, indicative of arthritis and osteomyelitis. However, the extent of bone involvement was not made clear until a CT scan was performed. A CT study was made of the hind end of the animal with 3.0 mm thick sections using 130 kV, 645 mA and an exposure of 0.30 seconds. The scan revealed severe periosteal reaction involving the entire length of the tibia, marked bone remodeling and severe lytic changes affecting articular surfaces. Because of the chronic nature of the infection, poor response to aggressive therapy and the severity of the lesions as indicated by the CT scan, it was thought that the animal could not regain sufficient use of the limb to ensure survival upon release. The sea lion was euthanized before recovering from anesthetic.

Case 2

Brody (ES 1212), a male weanling northern elephant seal (Mirounga angustirostris) was admitted to TMMC and treated topically and systemically for bilateral corneal ulcers and keratitis for approximately 5 weeks when a draining tract appeared over the ventral mandibular symphysis with a small amount of mucopurulent discharge. The lesion appeared to resolve within 10 days after antibiotic treatment based on culture and sensitivity results but returned approximately 4 weeks later. Antibiotic therapy was resumed based on new culture and sensitivity findings. However, within 12 days, mucopurulent discharge began to freely flow from the left lower canine. The animal was anesthetized and oral examination revealed mucopurulent debris surrounding the base of both lower canines. Surgical removal of the canines was undertaken after antibiotic therapy failed to resolve the lesion. Some osteomyelitis was evident after removal of the left lower canine. All antibiotic treatment was stopped 10 days after the surgery. Ten weeks after the surgery, Brody became lethargic, anorexic and developed bilateral nasal discharge. Mucopurulent discharge from around the area of the lower canines returned 2 weeks later. A CT study was conducted using 6.0 mm thick sections, 130 kV, 615 mA and an exposure of 1.00 seconds to reveal a severe ascending osteomyelitis of the mandible characterized by severe bilateral mandibular osteolysis with pathologic multifragmented fracture of the medial aspect of the left mandible. Routine radiographs had failed to reveal the extent of the infection. The animal was euthanized due its poor prognosis.

Case 3

Moonface (ES 1445), a weanling female northern elephant seal was admitted to TMMC underweight and with a wound on the right side of the head midway between the eye and ear draining mucopurulent discharge. The wound was flushed and the animal was treated systemically with antibiotics. Swellings on the lower jaw lateral to each lower canine developed 3 weeks after the animal's arrival. The animal was anesthetized for oral examination that revealed no indication of tooth involvement. Probing the head wound revealed a draining tract at least 5 cm deep. Regular flushes and antibiotic treatment continued for another 2 weeks with no resolution of the discharge. The animal was re-anesthetized and a tract was discovered that led to the right lower second premolar. The first premolar was found to be discolored and loose and was easily removed. Radiographs were non-diagnostic. A CT scan was performed 2 days later to evaluate the extent of any associated osteomyelitis. The study included 3.0 mm thick sections, 130 kV, 613 mA and an exposure of O. 10 seconds. The scan indicated thickening and periosteal reaction of the right mandible. Treatment was continued in an effort to resolve the osteomyelitis, however, the animal died 5 weeks later. Postmortem findings confirmed the right mandibular osteomyelitis and histology was consistent with sepsis that may have been due to dissemination of the osteomyelitis.

Case 4

Zuma (ES 1553), a weanling male northern elephant seal was presented to TMMC underweight and with moderate, bilateral mucopurulent discharge and bilateral facial abscesses. The facial abscesses had resolved within 10 days and the bilateral discharge had turned to thick, purulent discharge from the left nares only. A CT study was performed 6 days later with 6.0 mm sections using 130 kV, 640 mA and an exposure of 0.60 seconds. The scan revealed a calcified opacification in the left nasal cavity with associated lysis and fracture of the left ventral nasal bone and maxilla. A nasal flush and culture and sensitivity were performed and the animal was placed on antibiotic treatment. The animal was re-anesthetized for rhinoscopy that revealed exposed cartilage in the left nasal cavity. Unfortunately, the animal died during the procedure. Gross postmortem and histological examination confirmed the necrotizing rhinitis.

The CT scan has repeatedly been of value when diagnosing bone lesions in marine mammals at TMMC especially in chronic diseases with clinical signs, blood values and radiographs that did not indicate the severity of the underlying condition. The advantages of the CT scan include better detail than routine radiographs, visualization of complex structures without superimposition, and the ability to generate a three-dimensional image to enhance assessment of the degree of severity. Disadvantages may include cost, accessibility and the need for chemical immobilization to allow increased exposure time and completion of the procedure.