THE MERCK VETERINARY MANUAL
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Perianal Fistula

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Perianal fistula is characterized by chronic, purulent, malodorous, ulcerating, sinus tracts in the perianal tissues. It is most common in German Shepherds and is also seen in Setters and Retrievers. Dogs >7 yr old are at higher risk.

The cause is unknown, although many theories have been proposed. Contamination of the hair follicles and glands of the anal area by fecal material and anal sac secretions may result in necrosis, ulceration, and chronic inflammation of the perianal skin and tissues. Affected animals may be predisposed to generalized skin problems. Hypothyroidism, an immunologic defect, or an immune-mediated component may contribute to susceptibility. The likelihood of contamination is greater in dogs with a broad-based tail; deep anal folds may cause feces to be retained within rectal glands and play a major role. The draining tracts are lined with chronic inflammatory tissue and often extend to the lumen of the rectum and anus. Infection may spread to deeper structures involving the external anal sphincter and, therefore, should be treated promptly.

In dogs, signs include attitude change, tenesmus, dyschezia, anorexia, lethargy, diarrhea, and attempts to bite and lick the anal area. Signs in cats are similar to those in dogs but may include matting of fur and sitting in the litter box.

Historically, management of perianal fistulae was frustrating for both veterinarians and pet owners. Surgical therapy traditionally included anal sacculectomy, in addition to destroying the diseased tissues. Surgical techniques included excision, debridement, fulguration, and cryosurgery. Amputation of the tail at its base was once advocated alone or adjunctively with other therapy. Surgery is now only recommended for fistulae resistant to medical therapy. Sequelae of surgery include fecal incontinence, rectal stricture, and recurrence.

Cyclosporine has been an effective treatment at a dosage of 5–10 mg/kg bid for 10–20 wk and then for an additional 4 wk after all fistulae appear to be healed. Concurrent administration of ketoconazole (8 mg/kg/day) allows the dosage (and cost) of cyclosporine therapy to be reduced (1–3.5 mg/kg/day). Prompt treatment is recommended early in the course of the disease to reduce the likelihood of recurrence. However, some dogs are intolerant of ketoconazole. Cyclosporine at a dosage of 5 mg/kg/day can effectively decrease the severity of lesions. In one study, the combination of cyclosporine therapy for 12 wk followed by surgical excision of any remaining draining tracts, along with cryptectomy and anal sacculectormy, successfully resolved disease with minimal recurrence. Topical tacrolimus (0.1% ointment applied sid-bid) in combination with a tapering course of prednisone (2 mg/kg/day for 2 wk, 1 mg/kg/day for 4 wk, and then 1 mg/kg every 2 days for 10 wk) with metronidazole (10 mg/kg, bid for 2 wk) and a novel-protein diet has also been found to be effective in some dogs. Other aspects of medical management include the use of fecal softeners to reduce dyschezia. Perianal cleansing and antibiotics may reduce inflammation.

Last full review/revision December 2013 by Stanley I. Rubin, DVM, MS, DACVIM

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