Perianal Fistulas, Anal Furunculosis

Canine anal furunculosis, also called perianal fistula, is a formation of an abnormal channel between the anal canal and the skin surrounding the anus. The continual discharge of watery pus from the fistula can irritate the skin and result in itching, discomfort and pain. Most anal fistulas are caused by abscesses that spread from inside of the anus to the outer surface of the skin. The disease is most common in German Shepherd and Irish Setter breeds, but has been seen in other breeds. Males outnumber females by 2 to 1.

What causes this condition is unknown. Researches have explored many possible causes, including overproduction by local secretory glands, poor ventilation associated with low tail carriage, anal sac disease, or hip dysplasia. Dogs older 7 years are at higher risk. Contamination of the hair follicles and glands of the anal area by fecal material may result in tissue damage and long-term inflammation of the skin and tissue surrounding the anus. Low thyroid hormone levels and an immune system defect may also 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 significant role.

The current theory involves a multifactorial immune-mediated disease process. An immune-mediated process is suspected because both canine anal furunculosis and Crohn's disease respond to therapy targeted at the immune system. Accumulating evidence shows that Crohn's disease is the result of an unbalanced host immune response to intestinal triggers in genetically susceptible humans. Because German Shepherds with canine anal furunculosis also have clinical and histological evidence of inflammatory bowel disease, it is possible that dietary antigens, bacterial antigens, and superantigens are initiators of canine anal furunculosis as well.

Signs of Perianal Fistula

Clinical signs are characterized by multiple draining tracts and ulcers immediately surrounding the rectum. Animals may present pain and spasm when attempting to evacuate the bowels (tenesmus), difficulty in defecation and constipation. Affected area is usually very painful. An association between perianal fistulas and inflammatory bowel disease is suspected.

Clinical Signs

• Pain or spasm when attempting to pass urine or evacuate the bowels (tenesmus)
• Passage of red blood through the rectum
• Constipation
• Diarrhea
• Ribbon like stool
• Increased frequency of defecation
• Perianal pus-filled discharge and/or bleeding
• Perianal licking
• Self mutilation
• Perianal pain
• Scooting
• Offensive odor
• Low tail carriage
• Weight loss

Diagnosis

Diagnosis is based on history, physical findings and skin biopsy. Culture and sensitivity tests are performed if antibiotic therapy is being considered as part of the treatment. The most common organisms recovered from culture include: E. coli, Staphylococcus aureus, beta-hemolytic Streptococcus, and Proteus mirabilis. Keep in mind that antibiotic therapy has very little impact on the clinical outcome.

Treatment

Management of these cases is often disappointing because they do not respond consistently to antibiotics, corticosteroids and surgery. Keep the affected area clean by clipping the hair and flushing the lesions with chlorhexidine or povidone iodine. Post-operative complications include recurrence of fistulas, fecal incontinence, tenesmus, and difficulty in defecation. The carbon dioxide laser has been an effective adjunctive tool in treating canine anal furunculosis in some dermatology practices in the United States.

Chronic Disease

It is important for owners to understand that canine anal furunculosis is a chronic relapsing and remitting disease that can be managed but not necessarily cured. Lifelong therapy may be required as with other immune-mediated diseases. If one drug combination does not achieve the defined goal, another drug protocol is warranted.

The first goal of therapy should be to alleviate large bowel clinical signs such as tenesmus, blood discharge from the rectum, constipation or obstipation, diarrhea, ribbon-like stool, increased frequency of defecation, and perianal pain. The second goal of therapy should be to reduce the diameter, depth, extent, and recurrence of sinus tracts.

It is important to keep the perianal region clean and dry. Clipping and cleaning the perianal region under sedation can help. Baby powder may reduce regional relative humidity. At home, antimicrobial shampoo therapy may be helpful once the patient will tolerate it.

References

1. Nesbitt G.E. & Ackerman L.J. Miscellaneous Canine Skin Diseases. In: Canine and Feline Dermatology: Diagnosis and Treatment. Veterinary Learning Systems, Trenton, New Jersey, 1998
2. Mathews KA et al. Cyclosporine treatment of perianal fistulas in dogs. Canadian Vet Journal.
3. Dermclub 1, 2006: Managing Anal Furunculosis in Dogs.

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