Canine Perianal Fistula or Anal Furunculosis

Perianal fistulas or anal furunculosis is a chronic, progressive disease characterized by ulceration of the perianal tissue (tissue surrounding the anus) in the dog. The clinical signs of perianal fistulas may be present for years, gradually worsening over time. A fistula is defined as an abnormal passage or communication between an internal organ and the surface of the body or between two organs, and it is typically seen as multiple draining tracts, in this case surrounding the rectum. With perianal fistulas there is communication between the rectum and the perianal area. Clinical signs may include tenesmus (straining), dyschezia (painful elimination of stool), constipation, licking of the anal area, and mucopurulent anal drainage with an odor. The end result may be anal stricture or fecal incontinence.

The cause of perianal fistulas has yet to be definitively determined, but evidence currently suggests that the condition is actually immune mediated. The German shepherd dog is the most common breed affected, but it may be seen in other large breed dogs such as the Irish setter. The typical age of onset is from 5 to 7 years. In the German shepherd the condition is believed to have a definite immunologic basis with inflammation of the entire large intestine (colitis) occurring. Many of the breeds which suffer from perianal fistulas tend to have a broad sloping tail head which may actually predispose them to the formation of fistulas.

In the past, treatment for perianal fistulas included surgical ablation (removal of the fistula) or deroofing (removal of the skin covering each tract). In general, surgery has been replaced by the use of immunosuppressive drugs. The response to cyclosporine, prednisone, and tacrolimus has been encouraging. Cyclosporine is generally considered to be the drug of choice and is typically used on a 16-week course of treatment. Surgery, once widely used, is presently reserved for cases that involve the anal sacs or for selective cases that do not respond to medical management.

The only form of cyclosporine approved for the use in animals is Atopica®, manufactured by Novartis. The dose range for Atopica is from 1.75 to 5 mg/Kg. The initial response to higher doses of cyclosporine has been shown to be faster than that seen with the lower doses. The dose of Atopica may be expensive, especially at the higher end of the dose range. Concurrent use of ketoconazole at a dose of 2.5 to 10 mg/Kg will decrease the clearance (removal) of cyclosporine by the liver through its competitive binding action, thereby extending the useful life of the Atopica. This combination of ketoconazole with cyclosporine may result in a cost saving of 30 to 80% on the normal dose of cyclosporine. Cyclosporin is best absorbed on an empty stomach, although some dogs cannot tolerate cyclosporine without food due to gastrointestinal upset.

In studies, 41% of dogs with perianal fistulas had recurrences when cyclosporine treatment was discontinued and required management with additional cyclosporine or surgical excision.

Tacrolimus, available as Prograf® from Astellas Pharma, is a topically-applied immunosuppressive ointment that has a similar mechanism of action to cyclosporine and may be used with cyclosporine or alone if small fistulas recur. Although tacrolimus is 10 to 100 times more potent than cyclosporine, it is not used systemically due to the severity of the side effects. When used topically it has potent anti-inflammatory effects with minimal systemic absorption. Tacrolimus suppresses antigen-presenting T cells; it inhibits production of interleukins, interferon, cytokine expression, as well as factors that stimulate various white blood cells. For humans, the FDA has issued a public health advisory warning of a potential cancer risk when tacrolimus is used. Currently there are no studies that have addressed the potential cancer risk in dogs. Individuals should be advised to wear gloves during the topical application of tacrolimus. For the control of recurrent fistulas a 0.1% solution or ointment is used once to twice daily on the affected area.

An alternative topical ointment, pimecrolimus, is available under the trade name Elidel®, from Novartis. This ointment acts similarly to tacrolimus. Veterinary dermatologists have reported that the efficacy of Elidel is similar to or less effective than tacrolimus.

In cases of financial hardship, prednisone has been used with some success. Ancillary use of antibiotics may also be of benefit, especially when faced with secondary bacterial infection.

To prevent reoccurrence, a novel protein diet (a diet whose protein source the pet has not been previously exposed to) is suggested, good anal hygiene should be implemented, and the anal area should be monitored for fistula recurrence.

References:

Elkins, A.D. “Canine Perianal Fistula – Medical Approach. NAVC Clinician’s Brief. January 2008. Pp. 14 - 16.

Ettinger, Stephen and Edward Feldman. Textbook of Veterinary Internal Medicine. 5^th Edition. Vol. II. W.B. Saunders 2000. Pp. 1264-1266.

Guitian, A.K. and J. Gregory et al. “Evaluation of the Effect of two dose rates of Cyclosporine on the Severity of Perianal Fistulae Lesions and associated clinical signs in Dogs.” Journal of Veterinary Surgery. Vol. 35. 2006. Pp. 543-549.

Newton, Heide and Craig Griffin. “What’s New in Dermatologic Therapy?”. Veterinary Medicine. June 2007. Pp. 384-391.