Perianal Fistula in Dogs

By Tammy Hunter, DVM; Ernest Ward, DVM

What is perianal fistula?

Perianal fistula, also known as anal furunculosis is a serious medical condition that most commonly affects German Shepherd dogs, but may also occur in other purebred or mixed breed dogs. Perianal refers to the area immediately surrounding the anus or termination of the digestive tract. A fistula is an abnormal connection or tunnel that forms between two tissues, organs, or vessels that normally do not connect. In affected dogs, the condition is usually associated with an infection in the perianal region, and there are usually one or more draining tracts present.


What are the clinical signs of anal furunculosis?

In its early stages of development, there may be few clinical signs and the condition may go unnoticed. Some cases are discovered during a routine physical examination or when the dog is being bathed or groomed.

"In its early stages of development, there may be few clinical signs and the condition may go unnoticed."

As the disease progresses, the affected dog will usually strain during defecation, and there will be blood in the feces. A decreased appetite (anorexia) is common in dogs with perianal fistulas. Usually, the dog will lick excessively at its tail and rectal regions. Some dogs will be reluctant to sit, some may not wag their tail normally, and some may become aggressive if the tail or hindquarters are touched. Behavior changes are common. Some dogs will become withdrawn or act depressed, and others may appear agitated or even aggressive. If your dog demonstrates any of these signs, contact your veterinarian.

What tissues are affected by perianal fistulas?

Perianal fistulas are characterized by chronic, purulent (pus producing), smelly, ulcerating, sinus tracts (tunnels) in the anal region and surrounding skin. The condition could be compared with a carbuncle - a many headed abscess. This is why some veterinarians prefer use of the term furunculosis - a skin condition characterized by the development of recurring boils.

What causes perianal fistulas to form?  

The cause is not fully understood although impaction or infection of the anal sacs (anal glands) and adjacent sinuses and crypts has been suggested. Poor air circulation around the anal region has long been proposed as a major contributory factor.

"More recent studies indicate that the condition is more likely caused by an autoimmune disease."

More recent studies indicate that the condition is more likely caused by an autoimmune disease. There also appears to be a genetic component to the condition, since some families of German Shepherd dogs appear particularly prone. One study indicates that approximately 80% of cases occur in German Shepherds.

Is there any sex or age predisposition?

Yes. Perianal fistulas are more common in intact males and occur primarily in middle-aged dogs.

How is the condition treated?

Surgical treatment of perianal fistulas was previously the treatment of choice. However, medical management with immune-modulating drugs is now the preferred therapy.  Improvement can often be seen quickly; however, it can take several months of treatment to get the disease under control. Medical management is divided into an induction phase and a maintenance phase using drugs such as tacrolimus (brand name Protopic®), a topical medication and cyclosporine (brand name Atopica®), an oral medication. Antimicrobials are also often prescribed to treat secondary bacterial infections. A hypoallergenic food trial may be recommended since many cases have been linked to an underlying allergy. Increasing ventilation of the perianal region by clipping the area, particularly in long-coated dogs, combined with careful systematic bathing and cleaning is a useful palliative measure.

In severe cases, surgery to debride or remove as much infected tissue as possible, with or without simultaneous cryotherapy (or freezing) may be beneficial. This will reduce the amount of infection and may stimulate improved healing. Surgery has a high risk of potential complications and is used only in cases where medical treatment fails.

Is treatment curative?

Many cases will resolve after lengthy treatment. Regardless of whether medical or surgical treatment is pursued, these cases are often frustrating for the pet owners and uncomfortable for the pet.

Up to 20% of cases are unresponsive and require intermittent treatment for life. In these cases, treatment, although not curative, is certainly palliative and reduces the pain and discomfort that the dog experiences.

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