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Canine Perianal Fistula — Medical Approach

A. D. Elkins, DVM, MS, Diplomate ACVS

Internal Medicine

|January 2008|Peer Reviewed

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Shadow, a 7-year-old neutered German shepherd dog, was presented with signs of tenesmus, dyschezia, licking of the anal area, and mucopurulent anal drainage with an odor.

History. These signs had been present for more than a year but had progressively worsened.

Physical Examination. Shadow was reluctant to have his tail lifted. Following sedation, a rectal examination revealed mild thickening (2 to 3 cm) inside the anus. There were multiple draining tracts within the entire circumference of the anus (Figure 1, above).

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Ask yourself ...
For this advanced case of perianal fistula, what is the approach to management that is most appropriate and gives the best chance of a successful outcome?

A. Surgical management with a laser to ablate each fistula
B. Surgical management with deroofing of each fistula, followed by electrocoagulation or fulguration
C. Administration of prednisone and cephalexin for several weeks
D. Treatment with the immunosuppressive drug cyclosporine because perianal fistulas in German shepherd dogs are due to an immunologic defect that affects the colon as well as the perianal area

  • Correct Answer: D -- Treatment with cyclosporine

The cause of perianal fistulas remains unclear. German shepherd dogs are the most common breed affected, but fistulas may be seen in other breeds, such as Irish setter dogs. Many of the breeds in which perianal fistulas occur have broad sloping tail heads, which may predispose them to fistulas. In addition, there is a reported link between colitis in the German shepherd dog and perianal fistulas.1

Clinician's Brief
Treatment Options. One treatment for perianal fistulas advocated in the past is tail amputation. Surgical treatment (surgical ablation or fistula deroofing) has also been incorporated into treatment regimens, but has generally been replaced by the use of immunosuppressive drugs.

The response to immunosuppressive drugs, such as cyclosporine or prednisone, is encouraging (Figure 2). Azathioprine, another potent immunosuppressive agent, may be tried if prednisone or cyclosporine is unsuccessful. This author has not had to resort to its use.

Surgery is now usually reserved for cases that involve the anal sacs or for selective fistulas that do not respond to medical management.

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Cyclosporine. In 1997, Matthews and colleagues performed a study in which 20 German shepherd dogs with perianal fistulas were treated with cyclosporine or a placebo.2 At 4 weeks, perianal fistulas in all dogs in the cyclosporine group had improved compared with 70% of those in the placebo group. At 16 weeks, 85% of the perianal fistulas in the dogs treated with cyclosporine had completely healed. However, lesions recurred in 41% of dogs after discontinuation of the drug; these dogs required additional cyclosporine treatment or were considered for surgical excision.

A downside to cyclosporine therapy is its expense. Also, while the drug is best absorbed on an empty stomach,3 some dogs cannot tolerate cyclosporine without food due to gastrointestinal upset. One possible way to decrease costs is to combine cyclosporine with ketoconazole (2.5 to 10 mg/kg), a drug that decreases clearance of cyclosporine by the liver as well as acting as a competitive binder. This combination could lead to savings of 30% to 80%.

The starting dosage of cyclosporine is 1.75 to 3 mg/kg Q 12 H. The microemulsified formulation is more readily absorbed. Many generic forms of cyclosporine are available for human use, but cyclosporine A (Atopica; Novartis, www.novartis.com) is the only form approved for use in animals. Cyclosporine is often combined with an antibiotic or a novel protein diet (ie, fish/potato, rabbit/potato) (see Table).

Additional Options. When applied locally, the immunosuppressive ointment tacrolimus has a mechanism of action similar to that of cyclosporine.4Although expensive, tacrolimus can be used with cyclosporine therapy or if small fistulas remain or recur.

In the case of financial constraints, prednisone in place of cyclosporine has shown some benefit. Since many owners object to the side effects of prednisone, another option is to try azathioprine (1 to 2 mg/kg orally Q 24 H). This may be combined with prednisone (1 mg/kg orally Q 12 H for 2 weeks followed by 0.5 mg/kg Q 12 H). Ancillary antimicrobial therapy may be a beneficial option (see Table).

TAKE-HOME MESSAGES

  • In German shepherd dogs perianal fistulas are an immunologically based, inflammatory disease of the entire large bowel; they are not cases of sole fistulas in the perineal area.5,6
  • Perianal fistula is a diagnosis of exclusion; it typically occurs in dogs 5 to 7 years of age. The major differential diagnoses are anal sacculitis with rupture and squamous cell carcinoma.
  • Fistulas are best treated medically with immunosuppressive drugs such as cyclosporine. Novel protein diets, good hygiene of the anal area (including clipping the hair and wiping or cleaning the anal area with a disinfectant after bowel movements), and tacrolimus are beneficial.
  • This condition is expensive to treat.
  • Owners need to be aware that although most dogs improve with medical management, a significant number will relapse. It may take 4 to 6 weeks of therapy to see significant improvement. For those dogs that relapse, intermittent therapy with cyclosporine or prednisone maybe advised. Surgery is reserved for cases with anal sac involvement or fistulas that do not respond to medical management.

CANINE PERIANAL FISTULA-MEDICAL APPROACH • A.D. Elkins

References


1. Association of perianal fistulas and colitis in the German shepherd dog: Response to high dose prednisone and dietary therapy. Harkin KR, Walshaw R, Mullaney TP. JAVMA 32:515-520, 1996.
2. Randomized controlled trial of cyclosporine for treatment of perianal fistulas in dogs. Matthews KA, Sukhiani HR. JAVMA 211:1249-1253, 1997.
3. Cyclosporine and ketoconazole for the treatment of perianal fistulas in dogs. Patricelli AJ, Hardie RJ, McAnulty J. JAVMA 220:1009-1015, 2002.
4. Tacrolimus and cyclosporine-New treatments for perianal fistulas. Davidson EB. NAVC PROC, 2003, p 1045.
5. Perianal fistulas in dogs. Matushek KJ, Rosin E. Compend Contin Educ Pract Vet 13:621-627, 1991.
6. Treatment of perianal fistulas in dogs. Ellison GW. JAVMA 203:1680-1682, 1995.

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