1Department of Research and Veterinary Services, Mystic Aquarium, Mystic, CT, USA; 2School of Pharmacy, University of Colorado Health Sciences Center, Denver, CO, USA; 3Department of Chemical Engineering, University of Colorado, Boulder, CO, USA; 4Central Research Division, Pfizer Inc., Eastern Point Road, Groton, CT, USA
A previously beach stranded, juvenile, male, bottlenose dolphin (Tursiops truncatus) was diagnosed with vertebral body osteomyelitis in June of 1999. Initial survey radiographs and later computerized tomography (CT) scans showed a slab fracture involving nearly 1/3 of the caudal vertebra at the leading edge of the flukes with evidence of devascularization. Due to the nature of the lesion and the anticipated difficulties in healing at that site, we decided that surgery was not a viable option and that long-term medical management would be necessary.
Multiple attempts to isolate an etiologic agent were unsuccessful but the animal tested positive for Brucella (card, BAPA, rivanol, and ELISA tests). Initially systemic antibiotics were used to help control the presumed infection, but this form of therapy was not considered a desirable long-term option despite the positive effects of decreasing the neutrophilia and swelling associated with the condition. Intralesional injections were also used but the requirement for frequent injections in a very small area raised concerns that our treatments would eventually result in significant local tissue damage.
Our goal was to maintain long-term efficacious levels of antibiotics at the site of the lesion while avoiding any systemic effects of long-term antibiotic administration. Various sustained release vehicles were investigated including polymethylmethacrylate (PMMA), hydroxyapatite cement (HAC), polymer gels, and microspheres. Polylactide microspheres had the desirable characteristics of being injectable, biodegradable, and providing prolonged sustained release of the antibiotics with which they were impregnated.
The polylactide microspheres were impregnated with doxycycline and injected intralesionally. Serum levels of doxycycline were monitored to assess the elution kinetics in vivo. An in vitro assay examining the elution of drug into phosphate buffered saline was conducted in parallel with the serum assay in an attempt to simulate and monitor the concentration of doxycycline in the region of the lesion.
An initial rise in neutrophils peaked within 2 days post-injection and was presumably due in part to injection site trauma. Following this peak, the WBC profile improved but at 10 days post-injection began to rise again. After several weeks of rising neutrophils, we decided to revert to systemic antibiotics. Oral doxycycline (2 mg/kg BID) was administered for 10 days but failed to check the increasing neutrophilia. Treatment was changed to amoxicillin/clavulanic acid (20 mg/kg BID). A follow-up blood draw 4 days later indicated that total WBC and neutrophils were decreasing and were within normal limits the next week.
Since doxycycline was ineffective at the systemic level it was probably an inappropriate choice for local therapy. The delivery system, however, is promising and we are evaluating other possible antimicrobials to pair with the polylactide microspheres. In the meantime, the dolphin is being maintained on amoxicillin/clavulanic acid and bloodwork is being monitored on a weekly basis.