Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066



 (also called “perineal fistulae,” “anorectal abscesses,”
“perianal fissures,” “perianal sinuses,” and “pararectal fistulae.”)

Most people are not in the habit of inspecting the area under their dog’s tail unless the dog seems to be doing something that indicates a problem. A dog will lick under the tail, scoot his or her rear end on the ground, or seem painful sitting or raising the tail. When the tail is lifted and the anus inspected, a dog with perianal fistulae will show deep open crevices, some oozing pus, all around the anal sphincter. Odor may be noted as may be straining to defecate. This condition waxes and wanes but ultimately over time is progressive, ulcerating the surface of the anus and its surroundings.



At the present time, no one knows but recent information suggests an immune-mediated basis. There seems to be some genetic basis as the German shepherd dog seems predisposed to development of this condition, though this phenomenon may be more about conformation than about a specific genetic factor for the disease. The German shepherd dog also has an increased number of apocrine sweat glands, the type of sweat glands that produce stinky oily sweat (as opposed to watery sweat), in the anal area relative to other breeds. The average age of onset is 5 years and approximately twice as many males are affected as females. The anal glands may or may not be involved in the fistulation.

Approximately 80% of affected dogs are German shepherd dogs.



A biopsy is necessary to confirm the diagnosis as the following conditions can appear similar to perianal fistulae:

  • Squamous Cell Carcinoma of the anus (cancer)
  • Hyperplastic Anus (common in older unneutered male dogs)
  • Perianal Adenoma (benign tumor also common in older unneutered male dogs)
  • Anal Sac rupture



After the diagnosis is confirmed, there are several aspects to therapy. It should be realized from the beginning that it will take 2-5 months to get the lesions under control and that maintenance therapy will likely be needed for the remainder of the animal’s life.

    Since these lesions are commonly infected at least at the beginning of treatment, antibiotics are typically prescribed. A topical antibiotic may be helpful for long term infection control. It is important to realize that antibiotics alone will not control this deeply rooted problem; they are merely adjunctive to control complicating infections.
    Because of the ulcerations, defecation may be painful. To minimize the straining, stool softeners may be prescribed.
    Food allergy seems to be a possible etiology in this condition so using foods that the patient could not possibly be allergic to has been a recommended adjunctive therapy. Such diets are typically made from unusual protein sources such as rabbit, duck, kangaroo, fish, or venison. Most veterinary hospitals stock an appropriate food. It is important to note that there is nothing especially hypoallergenic about these unusual proteins; the idea is that the patient has likely never eaten them before. If the patient has never been exposed to these proteins, there should be no possibility of allergy (developing allergy requires multiple exposures to a protein).
    In older times, an assortment of surgical procedures were used to trim the diseased tissues of the perineal fistulae but immunomodulating drugs have largely supplanted surgery. In particular two medications have emerged: cyclosporine (an oral drug) and tacrolimus (a topical drug). Many dogs will need both to control their disease at least at first but often eventually the topical product can be used alone.

Cyclosporine was originally developed for organ transplant patients to prevent organ rejection by modulating the immune response without necessarily suppressing it. Medication is given twice daily and improvement should be seen within the first two weeks of use. Cyclosporine is an expensive medication and there is tremendous controversy over whether or not generics are bioequivalent. Please see our pharmacy page on cyclosporine for more details.

Cyclosporine tissue levels can be boosted with the concurrent use of ketoconazole, an antifungal drug. This "trick" can be used to cut the dose of cyclosporine and save money. There is controversy regarding whether blood levels of cyclosporine area helpful in determining dose and what the relationship actually is between tissue levels and blood levels. Because of the long term use needed to control perianal fistulae, it is best to monitor liver enzymes or liver function tests in the patient; cyclosporine blood tests may or may not be recommended by your doctor.

Tacrolimus is a much stronger immunomodulator than cyclosporine and because it can be applied directly to the fistulae, high tissue levels can be achieved right in the area they are needed. The potential for side effects and expense is also much more favorable than that of cyclosporine but only 50% of dogs experienced resolution with tacrolimus alone after 16 weeks versus 85% with cyclosporine.

Another immunosuppressive protocol involves prednisolone, azathioprine, and metronidazole and is particularly helpful to patients with concomitant inflammatory bowel disease. This protocol is substantially less costly than cyclosporine but 30% of dogs can be expected to fail to respond.

Whatever the immunosuppressive protocol, it is best to re-evaluate the patient every 3-5 weeks to see if modifications are necessary. Most of the medications listed above are reviewed in our pharmacy library if more details are desired.


Before the advent of cyclosporine, perianal fistulae were treated surgically with mixed results. Presently, surgery is only recommended for patients for whom immunosuppression has failed or where the anal glands are involved. The goal of surgery is to remove the proliferative or dead tissue, prevent or treat any anal or rectal strictures (narrowed areas caused by scarring), and change the “environment” of the perineal region. Tail amputation may be required; in fact, in one study, tail amputation alone was 80% successful in preventing recurrence of the fistulae.

If the anal glands are involved in the fistulae, they will have to be removed. In milder cases, chemical cauterization of fistulae (which destroys abnormal tissue and allows normal tissue to heal in) may be helpful. Cryotherapy, where a freezing agent is used instead of a chemical one, has been less effective (more scarring, less control over the area treated etc.) Laser therapy, on the other hand, has been 95% successful in preventing recurrence and controlling pain (20% of patients developed fecal incontinence but most of these cases were controlled with diet).

The more extensive the surgery, the more the potential for complications. Stool softeners are typically needed for a month after surgery and the owner should be comfortable cleaning the anal area. Fecal incontinence, narrowed anus, and inability to control the fistulation are the chief complications with surgery.

Despite innovations such as cyclosporine, perianal fistulae can be extremely frustrating.

For more information, visit the American College of Veterinary Surgeons page at:

Page posted: 5/13/09
Page last updated: 7/28/2013