Perianal fistula is a specific disease of the canine characterized by ulcerating fistulous tracts, often with a malodorous
purulent discharge around the anal orifice. The tracts are usually infected and filed with chronic inflammatory tissne. The
extremely severe and chronic cases the tracts may extend to the lumen of the rectoanal canal, becoming true fistulas.
This disease is most commonly seen in the German shepherd with a few cases reported in Setters and Retrievers. Dogs of either
sex can be affected and there seems to be some age correlation with older dogs, over seven years, being most often affected.
Although the etiology of this disease is not clear, many possibilities have been advanced. The conformation of the German
shepherd may be a predisposing factor in that the broad-based tail is held close to the anal region and thus maintains a film
of fecal material and anal sac secretion over the perianal region. This conformation and the poor ventilation afforded by
such tail carriage may provide a suitable environment for establishment of infection of circumanal glands, hair follicles,
and other glands in the perianal region. This conformation and. the poor ventilation afforded by such tail carriage may provide
a suitable environment for establishment of infection of circumanal glands, hair follicles, and other glands in the perianal
region, and thus abscessation fistulization. This author feels the deep folds just inside the anus also play a major role
in this disease and lead to collection of feces in rectal glands and resulting fistulous tracts.
Many of the dogs affected may also have a generalized skin problem and be hypothyroid; some may also have poor T-cell function.
Thus some suspect that perianal fistulas are an expression of generalized skin and systemic problems.
The anal sacs have been shown to be only secondarily involved in the disease process.Statistically, anal disease is seen less
frequently in German Shepherds. However, the infection can spread to deeper structures which can eventually canse severe problems
for the dog. One of these structures is the external anal sphincter, which plays a vital role in fecal continence; rarely
do the tracts extend to the bowel lumen.
Clinical Presentation and Diagnosis
The primary clinical signs associated with perianal fistulas are tenesmaus, constipation and dyschezia with licking and biting
of the anal area. Weight loss, anorexia, lethargy and diarrhea may occur as the disease progresses. In severe cases there
will be a copious, foul-smelling mucopurulent discharge in the perianal area and mild fecal incontinence. Occasionally, rectoanal
hemorrhage will occur,
Diagnosis is made by direct visual examination of the perianal area, revealing the fistulous lesions, which vary according
to the chronicity of the disease. It is important to differentiate this disease from primary anal sac disease, which has lead
to abscessation and fistula formation. Anal sac disease is rare in German Shepherds and other large breed dogs. The lesions
associated with anal sac disease are found generally over the anal sac area, while perianal fistula lesions can be found 360
degrees around the anus. Perianal adenocarcinomas will also have ulceration and fistulization in this area; however, on palpation
the perianal gland tissue around the anus will be considerably thickened. A biopsy may be necessary to differentiate the two.
The severity of the disease is correlated with the extensiveness of the lesion, the amount of scar tissue that has been laid
down and the length of time the disease has been allowed to progress. Ulceration and necrosis of the skin in the perianal
region is often seen; the lesions may extend two to five centimeters from the anus and further in severe cases. The fistulous
tracts may be seen extending deep into the tissue and may contain hair and fecal material. Microscopically, these tracts are
lined by chronic granulation tissue or stratified squamous epithelium growing in from the skin. In severe cases the formation
of fibrous tissue around the anus may prevent easy dilation of the anal orifice and result in stricture and fecal impaction.
Rectal examination is essential and may have to be performed under anesthesia. An assessment should be made of the depth of
the lesions, extent of circumferential involvement and any degree of anal stricture that may be present due to the chronicity
of the disease, These factors affect the extensiveness of the surgical intervention and prognosis for the case.