ANAL furunculosis is a chronic debilitating disease of the perianal, anal and occasionally the rectal tissue.
Although a range of different breeds can be affected, including Irish setters, collies, Old English sheepdogs, Labradors, Bulldogs and spaniels, the German shepherd dog is highly predisposed.
One report suggested that 84% of all cases are seen in the German shepherd. Many of the German shepherd dogs with anal furunculosis (AF) have also been shown to have concurrent colitis suggesting that AF shares many similarities with Crohn’s disease in humans. The fact both diseases respond well to down regulation of the immune response may indicate an immune-mediated mechanism may be involved. Clinical signs can be very variable and are listed in Table 1.
The diagnosis of anal furunculosis can be made on the basis of the typical clinical signs (see Table 1) together with the presence of disease in a predisposed breed such as the German shepherd dog.
The lesions on the perianal area itself vary considerably in severity by both the extent and the depth. The mildest lesion may be small pin-point areas of superficial erosions but can be so severe as to produce extensive circumferential lesions with extensive deep tracts.
Anal sacs involvement is inconsistent but these should always be checked. They may be normal, impacted or ruptured. Initial investigation should include assessment of the perianal area to gauge the extent of the lesions and also to identify if any infection is present.
AF is a painful condition and examination of the perianal area, which should include assessment of the anal glands and exploration of any sinus tracts, can only be undertaken successfully in an anaesthetised animal. Superficial cytology of the area generally reveals pyogranulomatous inflammation with mixed bacterial infection.
Samples for culture taken from the surface of the skin can give misleading results. Cultures should be performed using material gathered by a sterile swab from the sinus tracts or by tissue culture. Other diagnostic tests that may be useful include the institution of a hypoallergenic diet and where marked gastrointestinal signs are seen colonoscopy with biopsy may be useful.
The management of AF will depend on the extent of the lesions and also the temperament of the dog. Many dogs with AF are so painful they will not tolerate topical medication, which can be useful in many of the milder cases that do not have extensive sinus formation.
Prior to the use of immunomodulating drugs, AF was principally managed surgically. The rationale behind this was to remove or destroy the epithelial lining of the tracts and where necessary perform anal sacculectomy. Surgical interventions include surgical excision, cryosurgery, chemical cautery and laser therapy and in some cases in the past tail amputation has been performed.
The rate of resolution of AF after surgery ranges from as high as 97% to as low as 48%. However, side-effects after surgical therapy may include anal stenosis and faecal incontinence which when severe can lead to the euthanasia of patients.
More recent studies looking at medical therapy of AF using immunomodulating and immunosuppressive drugs together with hygiene measures and dietary change has led to a good resolution of AF in many cases without these adverse effects. Current wisdom suggests that where surgical therapy is deemed necessary and is performed after medical therapy, the success rate is much higher.
Dietary therapy has been shown to be useful particularly where the dog has a history of colitis. Diets should be formulated on an individual basis so that a novel protein and carbohydrate source should be fed. The alternative to this is to use one of the many excellent hydrolysed diets that are available commercially. Where the dog has signs of dyschezia, stool softeners may also help make the dog more comfortable.
Hygiene therapy should include antibiotics prescribed on the basis of culture and sensitivity together with topical treatment of the area if the dog will tolerate it. Clipping and cleaning the area is important to prevent faecal soiling.
The author’s choice for topical therapy, once the dog will tolerate it, is a chlorhexidine-based shampoo which is gentle and works well even if there is a purulent haemorrhagic exudate present.
Immunosuppressive/ immunomodulating drugs include systemic therapy with glucocorticoids with or without azathioprine and/or metronidazole; ciclosporine with or without ketoconazole or itraconazole; azathioprine with metronidazole or sulphasalazine. Topical drugs that may be used include glucocorticoids or tacrolimus.
Glucocorticoids have been shown to be useful therapy for AF. Prednisolone given at a dose of 2mg/kg by mouth once daily for two weeks followed by a reduced dose of 1mg/kg by mouth once daily for a further four weeks, then on an alternate-day basis after that has been shown to be beneficial.
One study by Harkin (1996) showed that 33% of dogs’ perianal lesions completely resolved on this regime with a further third showing improvement; the final third remained unchanged.
Prednisolone can also be used combined with azathioprine or metronidazole. Azathioprine has the ability to suppress both humoral and cell-mediated immunity and may be used at a dose of 1.5-2.2mg/kg by mouth for 2-4 weeks to treat AF in combination with prednisolone. After this initial induction period it should be cut to an alternate-day dosage.
The reader is referred to more detailed texts regarding the side-effects and the monitoring that is necessary when azathioprine is used, due to its potential to cause gastrointestinal upsets, bone marrow suppression, hepatotoxicity and pancreatitis.
Metronidazole is known to have immune-modulating, antibacterial and antiprotozoal effects and can also be given in combination with glucocorticoids at a dose of 10- 15mg/ kg by mouth twice daily to treat AF. Side-effects include gastrointestinal upsets, central nervous system toxicity and hepatotoxicity.
Sulphasalazine may be given at a dose of 50mg/kg/day every 8-12 hours for animals with colitis. Anecdotal reports suggest this drug may also be useful in the therapy of AF. However, because of the risk of idiosyncratic drug reactions and keratoconjunctivitis sicca, this drug should be used with care.
Medical therapy should aim primarily to relieve the large bowel signs and secondarily to resolve the cutaneous signs (Table 1). Often, therefore, although drugs such as glucocorticoids, metronidazole or sulphasalazine, which can have side-effects, do not completely resolve perianal lesions, the fact they improve gastrointestinal signs may justify their use.
Ciclosporine is a reversible immunosuppressant/immunomodulator that has been shown to be the most effective current medical therapy for AF. There is now a large weight of evidence-based work showing ciclosporine produces a rapid and complete improvement in perianal lesions in dogs with AF.
Therapy should be started at a dose of 4-8mg/kg/day by mouth, initially for 8-16 weeks. Once clinical signs have substantially resolved, the dose can be cut by 25-50% or can be given at the same dose on an alternate-day basis. The dose should then be tapered to the lowest possible maintenance dose.
For many clients, ciclosporine therapy has been prohibitively expensive for use on a long-term basis on their dogs; however, with the now widespread availability of lower cost generic formulations of ciclosporine, it has become a much more affordable long-term treatment. Previous studies looking at the concurrent use of both ketoconazole and itraconazole to reduce the dose and necessarily the cost of ciclosporine are probably now less useful because of the dramatic reduction in the price of ciclosporine.
Once systemic medication has produced a significant reduction in clinical signs, systemic therapy can be reduced to the lowest possible maintenance rates or topical therapy can be used to replace systemic drugs. Although topical glucocorticoids can be used on lesions of AF, the risk of cutaneous atrophy when used on a long period makes them suitable only for short-term use.
Topical tacrolimus (0.1%) offers a better therapeutic option than glucocorticoids. Tacrolimus has similar pharmacologic actions to ciclosporine but is 10-100 times more potent, it has not shown any significant evidence of cutaneous absorption when given topically and produces none of the cutaneous side-effects associated with glucocorticoids.
Initially, tacrolimus can be applied sparingly to the perianal area with a gloved hand. It can be reduced to the lowest possible maintenance dose rate to keep the dog lesion-free.
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