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Perianal Fistula
BASIC INFORMATION
DEFINITION
A chronic, inflammatory disease of the tissues surrounding the anus of dogs. The lesions are painful, ulcerative, and sometimes deep and have draining tracts adjacent to the anus. The anus itself is not usually involved.
SYNONYM(S)
Anal furunculosis
EPIDEMIOLOGY
SPECIES, AGE, SEX: Dogs, usually >5 years old; males may be overrepresented.
GENETICS AND BREED PREDISPOSITION: German shepherds and possibly Irish setters are predisposed.
RISK FACTORS: Possibly broad-based tail, low tail carriage, and increased density of apocrine sweat glands in the perianal region.
ASSOCIATED CONDITIONS AND DISORDERS: Possibly caused by a food allergy; manifested as pruritus and/or signs of colitis.
CLINICAL PRESENTATION
HISTORY, CHIEF COMPLAINT: Initially, owners may notice a foul odor or observe that the dog licks its perianal region excessively. As the lesions progress, the dog may have dyschezia, hematochezia, tenesmus, and fecal incontinence; the dog may engage in self-mutilation. In severe cases, inappetence, lethargy, and weight loss are possible.
PHYSICAL EXAM FINDINGS: In mild to moderate cases, physical examination abnormalities are confined to the perianal region. In more severe cases, the dog may also be in poor body condition. Examination of the perianal region may be very difficult in these animals due to pain, and sedation is often necessary. Visual and rectal examinations are indicated:
* Visually, the lesions appear as multiple, ulcerated, draining tracts that may be superficial or extend deeply into the perianal tissues. The lesions may extend 360 degrees around the anus as well as involve the ventrum of the tail base.
* Upon rectal palpation, rectal strictures, loss of anal tone, anal sac rupture or abscessation, and/or roughened rectal mucosa may also be found.
ETIOLOGY AND PATHOPHYSIOLOGY
* An immunologic basis is suspected based on clinical improvement with immunosuppressive drugs as well as a few pathologic studies identifying sterile, chronic inflammatory changes.
* Secondary bacterial infection is common, often due to fecal microflora or skin contaminants.
DIAGNOSIS
DIFFERENTIAL DIAGNOSIS
* Perianal neoplasia (anal sac adenocarcinoma)
* Anal sac abscessation/rupture
* Trauma (bite wounds)
* Perianal hernia (early phase of fistula, prior to ulcerated lesions)
INITIAL DATABASE
* Complete blood count (CBC), serum chemistry panel, and urinalysis: often within normal limits
* Abdominal radiographs: may show evidence of constipation
ADVANCED OR CONFIRMATORY TESTING
* Contrast radiography may be needed to better delineate the lesion in cases with rectal stricture because a colonoscopy may not be possible if the stricture is profound
* Colonoscopy/proctoscopy: may reveal inflammatory colitis (lymphoplasmacytic), rectal stricture
TREATMENT
THERAPEUTIC GOAL(S)
* Reduction in number and size of fistulae
* Control of infection
* Treatment of colitis, rectal stricture if present
ACUTE GENERAL TREATMENT
* Under sedation or general anesthesia: clipping of hair, removal of feces and debris, and cleansing of surrounding tissues.
* Enemas and stool softeners may be needed if constipation is present.
CHRONIC TREATMENT
* Immunosuppression is the initial therapy of choice:
o Cyclosporine: 80-90% success rate; expensive in larger dogs. Starting dose, using emulsion form (Atopica, Neoral): 3-5 mg/kg PO q 12h, adjusted to maintain whole blood trough cyclosporine levels 400-600 ng/ml. Adding ketoconazole (10 mg/kg PO q 24h) allows a reduction in cyclosporine dose to 1 mg/kg PO q 12h, with similar serum levels.
o Azathioprine (2 mg/kg PO q 24h, for 2 to 4 weeks, then tapered) may be an effective alternative.
o Prednisone alone (effective in ≤33% of animals): 2 mg/kg PO q 24h, for 2 to 4 weeks as needed for lesion improvement, then gradual taper to 0.5 mg/kg q 48h.
o Tacrolimus 0.1% topical, applied q 12-24h: resolution of lesions in 50% of dogs.
* Antimicrobials (e.g., cephalexin 22 mg/kg PO q 8-12h) are used as adjunct therapy for perianal dermatitis.
* Dietary therapy has been used in conjunction with immunosuppression. Novel antigen diets have been recommended becuase there is a suspected association with food hypersensitivity. Most important, animals should avoid high-fiber (bulking) diets, and owners should feed highly digestible diets that will result in a softer, smaller stool.
* Stool softeners (e.g., lactulose 0.25-0.5 ml/kg PO q 12h, titrated to stool consistency) as necessary to allow for easier defection.
* Surgery: if medical management does not resolve lesions to an acceptable degree. Use of an Nd:YAG laser and cryotherapy have been reported. Surgical procedures that may be needed include anal sacculectomy, removal of skin overlying the tracts, debridement of diseased tissue, and rectal pull-through for rectal strictures.
* Balloon dilation of rectal strictures is recommended as first line of therapy because surgical removal of strictures may result in recurrence of strictures or fecal incontence.
DRUG INTERACTIONS
Ketoconazole decreases the catabolism of cyclosporine and is given for this purpose
POSSIBLE COMPLICATIONS
* Recurrence of fistulae with discontinuation of immunosuppressive drugs
* Rectal stricture and constipation secondary to chronic inflammation
* Fecal incontinence secondary to chronicity or surgery
RECOMMENDED MONITORING
* Follow-up examinations of the perianal region every 2 weeks
* Whole blood trough cyclosporine levels
* CBC (risk of cytopenias if azathioprine is being used)
PROGNOSIS AND OUTCOME
* Fair to good prognosis with early treatment
* Long-term prognosis may be guarded with more severe lesions and the need for indefinite medical therapy
PEARLS & CONSIDERATIONS
COMMENTS
Most dogs will respond to medical management but may also require long-term therapy to maintain remission.
CLIENT EDUCATION
Advise clients on the clinical signs and lesions that warrant early intervention if lesions recur
SUGGESTED READING
Ellison GW, Bellah JR, Stubbs WP, van Gilder J: Treatment of perianal fistulas with Nd:YAG laser-Results in twenty cases. Vet Surg 24:140-147, 1995.
Harkin KR, Walshaw R, Mullaney TP: Association of perianal fistula and colitis in the German shepherd dog: Response to high-dose prednisone and dietary therapy. J Am Anim Hosp Assoc 32:515-520, 1996.
Mathews KA, Sukhiani HR: Randomized controlled trial of cyclosporine for treatment of perianal fistulas in dogs. J Am Vet Med Assoc 211:1249-1253, 1997.
Misseghers BS, Binnington AG, Mathews KA: Clinical observations of the treatment of canine perianal fistulas with topical tacrolimus in 10 dogs. Can Vet J 41:623-627, 2000.
Patricelli AJ, Hardie RJ, McAnulty JF: Cyclosporine and ketoconazole for the treatment of perianal fistulas in dogs. J Am Vet Med Assoc 220:1009-1016, 2002.
Tisdall PLC, Hunt GB, Beck JA, Malik R: Management of perianal fistulae in five dogs using azathioprine and metronidazole prior to surgery. Aust Vet J 77:374-378, 1999.
AUTHOR: LISA E. MOORE
EDITOR: DEBRA L. ZORAN