Management of perianal fistulae in five dogs using azathioprine and metronidazole prior to surgery

Department of Veterinary Clinical Sciences, The University of Sydney, New South Wales 2006

Objective To evaluate combination therapy with azathioprine and metronidazole in German Shepherd Dogs with perianal fistulae.

Design Prospective study

Procedure Five dogs (31.5 to 36.0 kg) with perianal fistulae were treated with azathioprine (50 mg per dog orally every 24 h) and metronidazole (400 mg per dog orally every 24 h). Patients were re-evaluated at 2 week intervals by inspection, palpation, photographs of the perianal region and assessment of white blood cell counts where possible. Treatment was continued until improvement in lesions reached a plateau. Surgical excision of residual fistulae and anal sac remnants was then performed, with medical therapy continued for an additional 3 to 6 weeks.

Result Signs attributable to anal irritation were reduced or eliminated in all dogs within 2 weeks, although visible healing of lesions progressed more slowly. Ulcerated lesions reduced in surface area and depth, and some fistulae healed completely. Non-healing area were usually associated with anal sac rupture or chronic fibrosis. Visible improvement typically reached a plateau 4 to 6 weeks after commencing treatment. Immunosuppressive therapy continued for 5 to 24 weeks before surgical intervention to remove anal sacs (four dogs) and/or residual fistulae (five dogs). All dogs remain disease free 7 to 10 months postoperatively. No important complications of treatment were encountered.

Conclusion Azathioprine with metronidazole effectively reduced perianal irritation, and the severity and extent of lesions prior to surgery. Treatment was economical even in large dogs and associated with few untoward sequelae. The combined use of immunosuppressive and antimicrobial therapy followed by surgery minimized potential morbidity associated with aggressive use of either medical or surgical treatment alone.

Perianal fistulae (anal furunculosis, hidradenitis) refers to a chronic inflammatory condition involving the anus, perianal skin, anal sacs and adjacent tissues of dogs. Typical lesions comprise multiple ulcerated, arborising sinuses (1). Over the years, many medical and surgical treatments have been tried, (2) but none have been uniformly successful, and complications including persistence, recurrence, fecal incontinence and anal stenosis are reported in up to 52% of cases (1).

The pathogenesis of perianal fistulae remains poorly understood although there is increasing evidence suggesting an immune-mediated etiology. Histopathological and bacteriological studies indicate the inflammatory process is initially sterile (3) and characterized by infiltration of lymphoid cells, plasma cells and eosinophils, consistent with immunological activation (4,5). Similar lesions occur in human patients with Chron's disease, and the fistulae in these patients are responsive to various immunosuppressive drugs including cyclosporine (6). In a series of dogs with perianal fistula and concurrent colitis, anal lesions improved with immunosuppressive doses of prednisolone in a proportion of cases (7). In a more recent study, cyclosporine monotherapy produced complete resolution of perianal lesions in 85% of dogs treated, although disease recurred in a substantial proportion of cases when treatment ceased (8). Based on these preliminary data, an immune-mediated etiology seems likely, although the underlying antigenic trigger has not been identified.

Cyclosporine has recently been recommended as the treatment of choice for perianal fistulae in dogs (9). In Australia, however, the cost of the drug and associated therapeutic monitoring required to avoid toxic plasma concentrations is prohibitive, running into thousands of dollars for a German Shepherd Dog. The aim of the present study therefore was to identify a treatment regimen that was simple, safe, economical and readily accessible for practitioners. While glucocorticoids are the best known and most widely used immunosuppressive agents, side effects commonly seen at effective dosages (polyuria, polydipsia, polyphagia, panting and muscle wasting), and untoward sequelae (pancreatitis, pulmonary thromboembolism, sepsis) can be unacceptable. Azathioprine was selected for its ready availability and familiarity among practitioners (10), economy and simplicity of a 24 h interval dosing. Furthermore, therapeutic monitoring is readily achieved using routine hematological analysis to detect neutropenia or thrombocytopenia associated with bone marrow suppression. Azathioprine is metabolized to the active moiety 6-mercaptopurine, an antimetabolite that competes with endogenous purines in nucleic acid synthesis. It produces more generalized immunosuppression than cyclosporine, with suppression of both antibody-mediated and cell-mediated immunity, but sparing the nonspecific components of the hosts' defenses (11-13). Metronidazole is also widely available, inexpensive and safe at moderate doses. It was selected both for its immunomodulatory effects and because it was thought likely to be effective at reducing growth of fecal anaerobic bacteria colonizing the perianal region (14,15).

A prospective clinical study was conducted to determine whether a combination of azathioprine and metronidazole was useful for managing perianal fistulae in dogs. Although the combination was found to markedly improve the appearance of the perianal lesions, minor surgical intervention was required to effect a cure in all cases. Results in five cases comprise the basis of this report.

Perianal fistulae were diagnosed in five German Shepherd Dogs (31.5 to 36.0 kg) based on typical history and physical findings. The diagnosis was subsequently confirmed by characteristic histopathological findings in the three cases in which tissue was submitted. Informed consent of the owners was attained prior to dogs entering the study. Three were spayed females, while the male dog and the remaining bitch were entire.

The severity of initial lesions were assessed and documented by inspection, probing to determined depth, and rectal palpation. The perineum was clipped and cleansed with dilute chlorhexidine (0.05%). Owners were asked to keep the area clean using water as needed. Dogs received azathioprine, 50 mg orally every 24 h and metronidazole, 400 mg orally every 24 h. Owners were asked to monitor changes in demeanor and appetite as indicators of possible complications. Lesions were reassessed fortnightly by inspection, palpation and photography. Total and differential blood leukocyte counts were performed every 2 weeks for 4 to 8 weeks until the clinician was confident neutrophil counts were stable. Generally, therapy was continued until no further improvement in perianal lesions was noted. In the first two dogs drug treatment was continued for substantially longer, but because of lack of further improvement in these cases, surgical excision of remaining fistulae and/or diseased anal sacs was recommended in subsequent cases as soon as improvement reached a plateau. Open anal sacculectomy and sharp excision of all affected tissue with primary surgical closure was performed in all cases. Dogs were hospitalized for 3 to 7 days after the operation and observed for tenesmus, dyschezia and fecal incontinence. They received amoxycillin-clavulanate (750 mg subcutaneously or orally every 12 h) and were fed a soft, highly digestible diet supplemented with psyllium (Metamucil, Proctor and Gamble; 1 tablespoon orally every 12 h), and docusate sodium fecal softener (Coloxyl, Sigma) administered orally as needed to achieve regular passage of soft, formed stools, until the perianal wounds had healed. Therapy with azathioprine and metronidazole was usually continued for 2 to 6 weeks after suture removal.

Owners were contacted by telephone 7 to 10 months after surgery and asked whether clinical signs or lesions had recurred.

Details of the five dogs are presented in Table 1.

Table 1. Characteristics of perianal fistulae in five German Shepherd Dogs.

F entire female, FN spayed female, M entire male

L left, R right

Two typical cases are described in detail to demonstrate the evolution in case management.

A 9-year-old desexed female German Shepherd (31.5 kg) was referred with a 3 month history of perianal fistulae that had not responded to topical treatment with iodine and improved only slightly with systemic antimicrobial therapy. The bitch was reported to have difficulty defecating, licked her anus frequently and had a malodorous mucopurulent anal discharge. Multiple coalescing sinuses were present adjacent to the anus, affecting an area 1.5 cm wide, up to 2 cm deep and extending 270^o circumferentially around the anus, sparing dorsal and ventral areas. The left anal sac ruptured and neither sac could be palpated distinctly. Extensive thickening and fibrosis were noted during digital palpation, especially ventrally, and the bitch resented rectal examination.

Three weeks after commencing therapy with azathioprine and metronidazole, the affected area had reduced by approximately 50%, and clinical signs had resolved. On the right side, fistulae had almost healed, apart from a small area adjacent to the anal sac opening. On the left, fistulae had reduced in area but were still up to 2 cm deep, and a discharging sinus communicating with the left anal sac persisted. After 9, 11 and 16 weeks there was little or no further improvement in the lesions, although clinical signs had not returned. During this time the dog required treatment for acute moist otitis externa, which responded to cleansing, topical treatment with a preparation of corticosteroid and antibiotic, and a course of amoxycillin-clavulanate (750 mg every 12 h orally). As residual fistulae appeared to be associated predominantly with the ruptured left anal sac, bilateral open anal sacculectomy and sharp excision of remaining fistulae were performed. The excised left anal sac was at the center of a mass of inflammatory tissue infiltrated by mixed mononuclear cells and neutrophils, indicative of chronic active inflammation. A striking feature was the presence of large lymphoid follicles, which have been reported previously in these cases (4). Recovery from surgery was uncomplicated and the dog defecated normally the following day. Surgical wounds healed well, and azathioprine/metronidazole was continued for 2 weeks after suture removal and then stopped. There has been no recurrence of lesions 9 months after surgery.

A 7-year-old entire male German Shepherd (32.5 kg) was presented with a 4 month history of tenesmus, anal licking, reduced appetite and bloody anal discharge. Rectal examination was resented. The entire anus and rectal mucosa were swollen and there were multiple punctate-draining sinuses in a 1 x 2 cm area to the right of the anus around the duct of the right anal sac and involving approximately 90^o of the anal circumference. A single 4 mm fistula communicating with the right anal sac was situated ventrally, whereas the left anal sac was expressible.

The owners reported resolution of clinical signs within a few days of commencing treatment: appetite and demeanor improved, and anal licking and tenesmus were reduced. After 2 weeks the affected area was reduced (1 x 1 cm), and was better demarcated from surrounding tissues as associated swelling and inflammation had resolved. After 4 weeks the dog was clinically normal, although the size of lesions was similar. Mild neutropenia (3.1 x 10⁹/L, reference 4.1 to 9.4) was present without consequent signs. Rectal examination revealed a well-demarcated firm fibrous area in the region of the right anal sac, and the associated fistula remained open. Since evolving experience suggested resolution of anal sac disease was unlikely without surgery, we elected for early surgical management. Treatment with azathioprine was discontinued for 1 week until neutropenia resolved and recommended at a reduced dose (50 mg orally every other day). Right anal sacculectomy, fistulectomy and castration were performed without complications. Three weeks after surgery the wounds had healed and treatment was discontinued. Seven months after surgery the dog remained free of lesions, defecated normally, and only licked its anus occasionally.

Once daily therapy with azathioprine and metronidazole was effective in controlling signs in dogs with perianal fistulae (Table 2).

Table 2. Responses to azathioprine/metronidazole therapy in the five dogs characterized in Table 1.

L left, R right

Anal irritation, licking, dyschezia and tenesmus in all dogs resolved within the first 2 weeks of treatment and several owners noticed an almost immediate response. Since it is generally accepted that azathioprine has a delayed onset of action (11), the possibility that the metronidazole may have contributed to early improvement cannot be discounted. In each case, lesions became smaller and/or shallower, and associated inflammation and discharge were substantially reduced. Improvement occurred most rapidly in the first 2 to 4 weeks, and generally there was little further improvement after 4 to 6 weeks.

The drug treatment was well tolerated and serious side effects or adverse sequelae were not encountered. Therapy was not considered onerous by the owners, therefore compliance was likely to be satisfactory. While side affects of azathioprine (myclosuppression, gastrointestinal upsets, pancreatitis, hepatotoxicosis) and metronidazole (neurological signs) have been reported (10,16), they are uncommon at the doses used in the present trial, and generally reversible in dogs after discontinuation of medication. Mild neutropenia in one dog was not associated with clinical signs and resolved with dose reduction. Minor skin cut infections and otitis externa that developed in some patients during treatment may have been associated with immunosuppression but were probably unrelated.

Unlike previous reports using cyclosporine treatment, where complete resolution occurred in most cases (8,9), disease was not completely eliminated using azathioprine and metronidazole in any dog. Despite a good early response to treatment, all dos required surgical excision of residual inflammatory foci. This may have reflected lesser efficacy than that achieved with cyclosporine. Alternatively, these cases may have been more chronic or had more extensive anal sac involvement than the dogs given cyclosporine, as persistent fistulae were invariably associated with either dense fibrous tissue or diseased anal sacs. It is possible that a longer duration of therapy or more potent immunosuppression may have produced complete healing without surgery. It could be argued, however, that removing the anal sac and/or residual abnormal tissue would reduce the likelihood of subsequent disease recurrence, as is the case in dogs treated by surgical excision and 'prophylactic' anal sacculectomy (17). Although anal sacs are involved only secondarily (1, 4), damage may be irreversible if fibrosis or duct occlusion occurs. In one study using cyclosporine alone, lesions recurred in 8 or 17 dogs within 2 to 24 weeks of discontinuing treatment (8). In contrast, there was no recurrence in any of our patients, although a longer period of follow-up is required to establish whether the dogs were 'cured'.

The severity of lesions has been shown to be a prognostic indicator for successful surgical outcome, with major surgery most likely to result in unacceptable outcomes such as fecal incontinence and anal stenosis (1, 17, 18). The reduction in area and depth of fistulae by medical therapy here greatly facilitated surgery as all residual fistulae and affected anal sacs were readily amenable to excision with primary closure. Apart from temporary fecal incontinence in one dog, few complications were associated with surgery.

We believe that azathioprine was probably the major active component of therapy in these dogs. While often used in combination with glucocorticoids for initial management of immune-mediated disease (11), its efficacy as a sole immunosuppresive agent has been demonstrated, for example in human and canine patients with acquired myasthenia gravis (19). Although the metronidazole's immunomodulatory effects (inhibiting cell mediated immunity and neutrophil chemotaxsis) and antimicrobial activity against fecal anaerobes may have contributed to the improvement, this drug is not effective against coliforms, staphylococci or streptococci which are commonly isolated from affected tissues (3). Metronidazole is also considered useful in management of perianal fistulae in active Chron's disease in human patients (6) when combined with other therapies. Additional antimicrobial therapy was considered unnecessary in these dogs because bacteria were considered likely to be surface contaminants an/or normal flora, and deeper tissues usually contain few organisms (3). Furthermore, unlike glucocorticoids, azathioprine has little or no effect on neutrophil and macrophage function, the nonspecific arm of cellular immunity which represent the first line of defense against pyogenic bacteria. Amoxycillin-clavulanate was administered before and after the operation because the surgical site was contaminated. No wound infections occurred despite concurrent immunosuppresive therapy.

A similar conceptual approach to management of these cases was described recently (K. Wyatt personal communication). Dogs with perianal fistula were treated with immunosuppressive doses of prednisolone as well as azathioprine. Although the physical appearance of the lesions was reported to improve markedly in all cases, several dogs died to major complications including pancreatitis, pulmonary thromboembolism and sepsis. This emphasizes the dangers of combination immunosuppressive therapy using high doses of glucocorticoids. On the other hand, the postoperative morbidity and potential complications of aggressive surgical interventions are well documented (18, 20). We propose that a balanced approach, using drugs and surgery as in the present series, may produce best results while minimizing the likelihood of disease recurrence or surgical complications.

While therapy with azathioprine and metronidazole appears unlikely to resolve perianal fistulae without adjunctive surgery, particularly in cases associated with anal sac rupture, it remains a useful medical treatment for this condition. Treatment is economical, safe and produces a rapid clinical improvement. Daily or alternate day therapy with this drug combination may also have a place in managing perianal fistulae in dogs whose owners are unprepared to pursue curative surgery, for example in geriatric animals or when anesthesia and surgery are contraindicated. Further studies are warranted using similar drug combinations in a larger number of dogs to determine the optimal approach for individual patients and whether complete and permanent resolution of lesions id feasible without surgery or serious drug side effects.

The authors wish to thank the various veterinarians that referred cases for treatment. Richard Malik is supported in part by the Post Graduate Foundation in Veterinary Science, The University of Sydney.

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