Metatarsal Draining Tract in a German Shepherd Dog

Remi, a 4-year-old spayed German shepherd dog, was presented to a dermatology referral clinic for evaluation of a unilateral draining tract proximal to the right metatarsal pad of 4 months’ duration. The draining tract was not pruritic and only noted after blood-tinged spots were occasionally seen on the floor of the home. The discharge was reported to originate from the draining lesion on the right pelvic limb, and the owner did not believe the lesion had changed over time. 

Remi lived primarily indoors with access to a fenced yard. She was housemates with another dog and a cat; both were unaffected. Remi was otherwise apparently healthy, and no lameness or discomfort of the area had been observed. Vaccinations and flea, tick, and heartworm preventives were current. 

The referring clinician performed cytology of the right metatarsal exudate that revealed scattered extracellular cocci bacteria with pyogranulomatous inflammation, and cefpodoxime (5 mg/kg PO every 24 hours) was administered for 21 days with no improvement. Routine CBC and serum chemistry profile were unremarkable. Orthopedic examination was within normal limits; radiography was thus performed, and results were normal. Subsequent surgical exploration by the referring clinician revealed no foreign body inside the wound, but a subcutaneous sinus extending ≈1.5 cm proximally was found. Enrofloxacin (5 mg/kg PO every 24 hours) and carprofen (2.2 mg/kg PO every 12 hours) were administered for 14 days with no improvement. Bacterial cultures were not collected.

Remi was referred to the dermatology clinic for evaluation. 

On presentation at the dermatology referral clinic, Remi was bright, alert, and responsive. Vital signs were within normal limits. BCS was 4/9, pain score was 0/4, and muscle condition score was adequate. 

Dermatologic examination revealed matted fur, indurated and inflamed skin, and several small draining tracts with serous discharge on the right ventral metatarsus extending ≈6 cm proximal to the metatarsal pad (Figure 1). The left metatarsal region was also mildly affected, and there was a focal area of fistulation, hypotrichosis, hyperemia, and edema of the skin proximal to the left metatarsal pad (Figure 2). Scattered areas of pinpoint erosions were also present.

Cytologic examination of right and left limb lesions revealed pyogranulomatous inflammation characterized by neutrophils, macrophages that were often multinucleate with foamy cytoplasm, and smaller numbers of plasma cells and lymphocytes; no infectious organisms were seen. A deep skin scraping was negative for mites.

A full-thickness punch biopsy of affected tissue under local anesthetic was performed; results revealed a deep pyogranulomatous, nodular-to-diffuse dermatitis and panniculitis with fibrosis and fistulous tracts. The infiltrate contained neutrophils, macrophages, plasma cells, and lymphocytes. Periodic acid-Schiff, acid-fast, and Gram stains were negative for fungal and bacterial organisms, and tissue cultures for aerobic and anaerobic bacteria and fungi were negative. No demodectic mites or other parasites were identified on biopsy.

Bilateral metatarsal fistulae were suspected, and treatment was initiated according to current recommendations, pending biopsy results.¹ There was a 30-day washout period between administration of carprofen and prednisone. An anti-inflammatory dose of prednisone (1 mg/kg PO every 24 hours for 7 days) was administered due to the high index of suspicion for bilateral metatarsal fistulae.¹ Rapid improvement of lesions resulted. Prednisone was reduced to 1 mg/kg every 48 hours beginning on day 8, but lesions worsened by day 14. 

On day 14, topical tacrolimus 0.1% ointment was initiated every 12 hours for 14 days; marked reduction in edema occurred after topical tacrolimus was added to the management plan, and tacrolimus allowed for furthering tapering of prednisone. On day 28, prednisone was reduced to 1 mg/kg PO every 72 hours for 3 doses, then stopped. Topical tacrolimus was simultaneously reduced to once every 24 hours and administered for an additional 21 days. 

Metatarsal fistulae were resolved at the 8-week recheck, and topical tacrolimus was tapered to every 48 hours for 30 days, then twice weekly for 30 days, and discontinued on day 118.

Lesions were resolved 8 weeks after treatment was started. Remi was still in complete remission 2 months later, and bilateral metatarsal fistulae had not recurred after 6 months. 

Prognosis for metatarsal fistulae is excellent with appropriate medical management. Immunomodulatory or immunosuppressive therapy often leads to long-term remission but may be required chronically. Lesions typically persist when left untreated and do not respond to surgical removal alone. Spontaneous remission is possible but rare.²