Clinical Manifestations and Treatment of Discospondylitis in an Adult Captive Harbor Seal (Phoca vitulina)
A 16-year-old male 80 kg captive bred harbor seal (Phoca vitulina) was housed at the Alaska SeaLife Center with 7 other harbor seals for studies of phocid nutrition and physiology. The seals were housed alternately in an outdoor pool with smooth concrete haulout ramp and deck and in a naturalistic rocky shore habitat. Both areas were supplied with natural salt water and had good water quality. All animals were fed a mixed diet of herring and pollock with vitamin supplementation and routinely blood sampled every two weeks with an 18 ga spinal needle via the extradural intervertebral sinus at L3-4 or L4-5. Standard protocol for blood sampling required repeated cleansing of the venipuncture site with alternate swabs soaked in isopropyl alcohol and dilute povidone iodine solution, as well as the use of sterile disposable needles, and sterile technique. The animal was behaviorally conditioned for venipuncture at this site and routinely stationed for voluntary blood sampling.
On February 28, 2000, staff observed acute onset of apparent pain with decreased use of the rear limbs and lethargy, which were first noted after the seal had been moved from the outdoor pool to the habitat the preceding weekend. He was unmotivated for training sessions and would not exit from the pool. There was little change during the following week except that he was eating better but still not coming out reliably. The seal moved on the deck by pulling with his forelimbs while holding the back straight and the rear flippers tucked up, together and off the floor. He swam with little movement of the lumbar area and slept on deck with his left side always down. Exam on 3/6/00 showed no apparent pain on palpation of the back or hips but distinct flinching on superficial touch to the ventral abdominal skin. Thermal mapping using an infrared thermometer (Raytek; Williston, VT) revealed an increase in surface temperature of the skin over the caudal lumbar muscles when compared with other areas of the body. Aspirin (365 mg twice daily) was administered orally in food fish from 3/9/00 through 3/16/00. The apparent sensitivity of the skin to touch decreased but there was no observed change in mobility. Blood cell counts and serum chemistry on 3/7/00 and 3/21/00 were within normal limits. Radiographs taken on those dates showed a collapsed intervertebral disc space at L5-S1 with evidence of bony destruction or lysis of both endplates. The loss of disk space did not seem unusual when compared to the L7-S1 space of a dog or cat, but was striking when compared with the L5-S1 conformation in two other male harbor seals including one of near the same age. Ultrasound scanning of the dorsal lumbar and sacral musculature revealed no apparent abnormalities. Differential diagnosis included lumbosacral disc rupture or degeneration due to unknown trauma or hereditary predisposition and inflammation or infection of the disc due hematogenous infection or needle injury.
Radiographs were referred to a board certified veterinary radiologist who concluded that the disc changes at L5-S1 were most consistent with an active infectious discospondylitis. Repeated CBC and serum chemistries remained within the previously established normal ranges for this animal (WBC 5-9000, 50-70% segmented neutrophils, no band cells). Serial serum samples were submitted for screening for Brucella and were negative on a B. abortus card test. Aerobic and anaerobic blood cultures were also negative. However, a review of this animal's record revealed that he had developed a focal cellulitis in the dorsal lumbar area 8 months previously. Group G Streptococcus had been isolated from a pinpoint purulent drainage site at that time and the condition had apparently resolved uneventfully following treatment with oral cephalexin (3500 mg twice daily for 10 days).
Pending results of the blood culture and the radiologist's review, medical treatment consisted of continued oral non-steroidal anti-inflammatory medication (aspirin 325 mg BID). Ancillary therapy included movement to the outdoor research pool with the gradually sloping, smooth haulout surface and careful handling to avoid struggling or excessive flexion or extension of the back and hips. Massage of the dorsal lumbar and pectoral muscles was employed in an attempt to reduce muscle spasm and pain.1 The seal remained clinically stable with improved appetite and swimming activity, but he continued to avoid moving the rear quarters normally.
On 4/12/00, despite negative blood culture and normal blood cell counts, empirical treatment with oral antibiotic (amoxicillin with clavulanic acid, 1125 mg twice daily) was begun and continued for a total of 38 days. Aspirin was continued for a total of eight weeks. The seal continued to station for voluntary blood sampling and blood cell counts and serum chemistries were evaluated every other week. Gradual improvement in mobility was noted after the third week of antibiotic treatment and continued even after all medication had ceased. Mobility was considered within normal limits 6 months later and no recurrence of symptoms has occurred. Radiographs of the lumbosacral vertebrae have been repeated annually and show continued collapse of the L6-S1 disc space with moderate proliferative osteoarthritic changes in the affected vertebral end plates, but there has been no further reactive change in that or any other disc space.
Potential etiologies for collapsed intervertebral disc space include traumatic or degenerative disc rupture.4 Septic discospondylitis can occur from blood borne bacteria such as Staphylococcus, Brucella or Streptococcus. Alternatively, inadvertent needle puncture of the nucleolus pulposus during blood sampling could cause leakage of cerebrospinal fluid into the disc leading to degeneration and collapse with secondary infection.2,4 Hematogenous spread from other infected tissues is reported to be the most common route of infection in domestic small animal pets.2 In the absence of positive culture results, treatment with beta-lactamase-resistant antibiotics which reach sufficient therapeutic levels in bone and pus is recommended.2 The Streptococcal infection noted 8 months before onset of clinical signs may have been the result of a contaminated venipuncture site and could have resulted in direct or hematogenous spread although the affected disc was caudal to the intervertebral space targeted for blood collection. Amoxicillin with clavulanic acid was chosen in this case because it was effective against the previously reported Streptococcal infection and was well tolerated when fed hidden in fish as part of the seal's regular diet. Moderate increases in serum transaminases (AST and/or ALT) have been reported in humans and other animals receiving ampicillin class antibiotics and non-steroidal antiinflammatory drugs with unknown significance but true hepatic dysfunction is considered infrequent and is usually reversible.3 No significant changes in serum chemistries were noted during or after treatment in this animal.
The authors wish to thank the staff of the Alaska SeaLife Center, Husbandry staff and Research Departments of for their support and care of the marine mammals housed at that facility. We are indebted to Jim McBain, Lawrence Dunn, Frances Gulland and other members of the IAAAM for their comments and advice in this case.
1. Guglielmo A, H Walters, JA Basinger. 1997. Massage Therapy: It's not just for humans anymore...A valuable tool to assist mammals of all kinds to recover from injuries and other problems. Draft manuscript.
2. Luttgen PJ. 1994. Spinal Cord Disorders. In: S.J. Birchard and R.G. Sherding, (eds.). Saunders Manual of Small Animal Practice. W.B. Saunders Company, New York. Pp. 1161
3. Mosby. 1998. Amoxicillin; Clavulanate Potassium. In: Mosby's GenRx. Pp. II-117.
4. Park RD. 2000, Personal communication