Perianal Fistulas (Anal Furunculosis)

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Thank you to Tina Oswald and Ruth Jones for putting together this article for SCARF.

Perianal fistulas are tunnel-like lesions in the skin surrounding the anal area of dogs, usually in dogs between the ages of 5-7 years of age. The lesions can become more severe ultimately surrounding the entire anus, and secondary infections may be present. German Shepherds are most commonly affected along with Irish Setters, however this condition can occur in any breed of dog. Perianal fistulas may cause severe pain and discomfort and, if not controlled, may be responsible for a dramatic reduction in quality of life. Even when treated, they may recur -- it is typically a chronic disease which is managed, not cured.

Signs and Symptoms

Tenesmus -- feeling of incomplete defecation (i.e. straining)
Dyschezia -- painful or difficult defecation
Licking of the anal area
Mucopurulent anal discharge with an odor (Mucus & Pus)
Visible ulcerating lesions around the anus

Causes

The exact cause remains unclear. Many breeds affected with Perianal Fistulas have broad sloping tail heads, which may lead to a predisposition. There has also been a reported link between colitis and Perianal Fistulas in German Shepherds.

Risk Factors

Breeds whose tail is often carried low between the hip bones, covering the anus, is thought to be a predisposing factor.
There may be a genetic component.

Diagnostic Tests

A complete physical examination which includes a digital rectal examination is usually performed if one suspects their dog has Perianal Fistulas.
Cell or tissue samples from the anal sacs or fistula may be collected for microscopic examination (aspiration cytology or histopathology) or for bacterial culture and sensitivity.

Treatment Guidelines

Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.

In the past, the treatment used was to amputate the dog's tail. However, the course of treatment most commonly used now is immunosuppressive drugs such as cyclosporine or prednisone. Azathioprine is another immunosuppressant that may be prescribed if the prednisone or cyclosporine is unsuccessful.
The most common drug used, which gives the greatest improvement, is cyclosporine. The downside of this drug is the cost. However, there is one possible way to decrease the cost by 35%-70% and that is by combining ketoconazole with the cyclosporine (ketoconazole, slows the metabolism of cyclosporine allowing the dose to be reduced). -- The downside of the combination is that it adds the potential side effects of ketoconazole.
For small fistulas or preventative maintenance, can be treated with topical immunosuppressive ointments.
If medication is not effective, surgery may be performed to excise the lesions. If the anal glands are involved, anal sacculectomy may be recommended.
Switching the affected dog to a novel protein diet is also part of the treatment.

References

Northern American Veterinary Conference (NAVC) Clinicians Brief -- January 2008 Vol 6 Number1
"Perianal Fistula" in the Merck Veterinary Manual
House AK et al. 2006. Evaluation of the Effect of Two Dose Rates of Cyclosporine on the Severity of Perianal Fistulae Lesions and Associated Clinical Signs in Dogs. Veterinary Surgery 35:543-549.
Patricelli AJ et al. 2002. Cyclosporine and ketoconazole for the treatment of perianal fistulas in dogs. Journal of the American Veterinary Medical Association 220(7): 1009-1016.