How I approach… Perianal Fistula Disease in Dogs

Perianal fistula disease is a common and difficult problem that is seen all too often in clinical practice. Its chronic, progressive nature can make it a real challenge for the veterinarian, but Lindsay McKay offers some pointers to optimize therapy and control the risk of recurrence.

Introduction

Perianal fistula disease (PFD), also known as canine anal furunculosis, is a condition where draining tracts form in the skin and deeper tissues of the areas surrounding the anal region of dogs. For most affected animals it is a painful and debilitating condition, with clinical signs that vary from licking of the affected area to hemopurulent discharge and odor, to difficulty defecating or even obstipation. Although a wide variety of breeds can be affected by this condition, German Shepherd dogs are over-represented, suggesting a genetic predisposition. Early recognition and treatment is important so that affected animals maintain a good quality of life, and thorough client communication is key, as most dogs will require extended maintenance therapy to keep the disease in remission.

Etiology

Our knowledge of PFD has changed dramatically from when this disease was first described in the 1960’s. Originally it was thought to be the result of anatomic factors such as (i) a broad-based tail (ii) low tail carriage and (iii) increased density of apocrine sweat glands in the region surrounding the anal canal (1). For decades the disease was treated with surgical intervention, ranging from tail amputation, debridement and debulking of the fistulous tracts, to anal sac removal. While surgical intervention may be necessary in some cases, most veterinarians now usually employ medical management to treat PFD. This new approach comes from our more recent understanding that this disease is – at least in part – due to an immune dysfunction. Canine PFD shares many characteristics of certain variants of Crohn’s disease in humans, including clinical signs, histopathology, and response to cyclosporine therapy (2-6). Crohn’s disease is thought to develop from an autoimmune attack against cells of the gastrointestinal tract or associated microbial antigens (7). A specific causative antigen has not been identified for canine PFD, but it has been proposed that the inflammation is due to inappropriate immune responses to normal flora in the feces or skin of the perineal area (5). Additionally, as with people, a genetic predisposition to the development of the disease has been identified (8,9); over 80% of dogs diagnosed with PFD are German Shepherd dogs (GSD) (10). Research revealed that GSDs with a specific MHC class II allele and haplotype are five times more likely to develop perianal fistulae (9). Lastly, it is also thought that there is a strong correlation between PFD, colitis, and food allergy in dogs (11). Thus, PFD has a complex multi-factorial pathogenesis that likely varies between dogs, especially breeds other than GSD. [...]