Anal sac disease in dogs is a frequent reason for dogs to be brought to a veterinary surgeon, although it is rare in cats. Due to limited understanding of its aetiology and pathogenesis, treatment is often symptomatic, frequently culminating in the surgical removal of the sacs. This chapter will outline a diagnostic approach grounded in current knowledge and will also briefly discuss anal furunculosis (perianal fistula) in dogs.
What is Anal Sac in Dogs?
(A) Anatomy and physiology
The anal sacs of the dog are paired cutaneous diverticula situated either side of the anus. The external anal sphincter muscle and the rectum enclose them. They are connected to the outside by a short duct which opens lateral to the anus. These structures form a reservoir for the malodorous secretions of the apocrine and sebaceous gland lining the sacs. The anal sacs are not to be confused with the perianal (hepatoid) glands or the circum-anal glands.
The material which collects in the anal sacs in the dog is a mixture of sebaceous and apocrine secretions, together with desquamated cells from the stratum corneum of the skin lining them. The apocrine glands are situated around the fundus of the sac, whereas the sebaceous glands surround the duct.
The anal sacs release their contents during defecation, when the sac is squeezed by the external anal sphincter against the faecal mass in the rectum. It is likely that the sac secretions play a role in social behaviour in the dog and cat.
Secretions from normal anal sacs can vary significantly in color, consistency, and odor, ranging from black to green and from watery to thick.
(B) Microbiology
The normal flora of the anal sacs has been examined. In the healthy dog, the most common isolates were micrococci, Escherichia coli, Streptococcus faecalis and Staphylococcus spp. Fungal cultures were not performed. In another study, 46% of normal anal sacs were demonstrated to contain Malassezia pachydermatis.
What are the Differential Diagnosis for Anal Sac Disease?
The differential diagnosis for the anal sac disease in dog and cat must include all conditions that can cause pruritus of the caudal trunk, tail base or perineum, together with those conditions that result in fistulae or subcutaneous swellings in the perianal region.
Differential diagnosis of anal sac disease
| Presentation | Differential Diagnosis |
| Caudal Pruritus | Flea bite hypersensitivity Adverse food reaction Atopic dermatitis Cheyletielibsis Perineal intertrigo (skin fold disease) Tail fold intertrigo Vulval fold intertrigo Anal sac infection Anal sac impaction |
| Perineal Swelling | Anal sac abscess Anal sac neoplasia Perineal hernia Perianal (hepatoid) neoplasia |
| Perineal Sinus/Fistula | Ruptured anal sac abscess Anal furunculosis |
What are the Clinical Approach for the Anal Sac Disease in Dogs?
The clinical approach to anal sac disease does not differ from that for most dermatoses. Accurate diagnosis and effective management rely on a comprehensive history, detailed clinical examination, and appropriate diagnostic tests.
(A) History
The history-taking process should emphasize the presenting symptoms while also gathering details about any potential concurrent diseases.
If the presenting sign is caudal pruritus, then owners must be questioned about:
- Flea control
- Owner lesions – fleas and Cheyletiella
- Pattern of disease – is there concurrent pruritus of the face or feet, suggesting atopic dermatitis?
- Dietary history – important if there is any likelihood of concurrent allergic disease
- Breed of dog – important if tail fold intertrigo is suspected
- Whether the dog ‘scoots’ (rubs its anus on the ground); if the animal does not, then has it ever? Scooting is highly suggestive of anal disease, but some dogs with significant anal sac disease never scoot
- Licking the anal region. Cats are able to lick the anal region , and do so regularly. Owners may be unable to distinguish excessive licking from what is normal
With the presence of perianal fistulation or sinus formation , the breed of dog is important. The overwhelming majority of dogs with anal furunculosis (perianal fistula) are German Shepherd Dogs or their crosses.
Previous anal sac disease is important; in a dog with a chronic history of anal sac impaction, anal sac abscess is more likely.
Anal sac neoplasia is often associated with paraneoplastic signs: polyuria and polydipsia may occur due to pseudo hyperparathyroidism. In the presence of such a history, blood samples should be examined for haematology and serum biochemistry to include urea and calcium levels.
(B) Physical Examination for Anal Sac Disease in Dog
The examination of the dog or cat involves the following steps:
- A systemic examination is necessary to assess for any concurrent diseases and to evaluate the animal’s suitability for potential future surgery.
- Dermatological examination (allergic, endocrine or parasitic disease)
- Assessment of perineum (sinus or fistula, intertrigo)
- Assessment of tail and vulval folds if present
- Careful palpation of anal sacs (present? thickened? mass present?)
- Careful expression of anal sacs and collection of contents (difficult to express?)
Following the history and clinical examination a differential diagnosis list may be narrowed, and a diagnostic plan drawn up. Should there be signs compatible with allergic, endocrine or parasitic skin disease, then these should be pursued.
Diagnostic Tests for Anal Sac Disease
(A) Gross and microscopic examination
The first diagnostic test in all cases of anal sac disease is the gross and microscopic examination of the anal sac contents. Whilst the nature of the anal sac contents may be suggestive of a type of disease, only the presence of haemorrhagic discharge is always abnormal.
From the microscopic examination it is possible to assess relative numbers of bacterial types. Gram positive cocci, Gram-negative rods and large Gram positive rods (anaerobes) may be seen, together with yeasts. Malassezia yeasts may be present in large numbers in diseased sacs, especially in the presence of Malassezia dermatitis elsewhere.
(B) Bacterial culture
Bacterial culture of the contents of diseased glands has revealed a number of organisms: essentially those found in normal glands together with diphtheroid, Pseudomonas spp., Proetus spp. and Clostridium welchii. Should bacterial culture be required, it is important to attempt to limit contamination of samples by faecal flora by careful sample collection and rigorous antisepsis.
Interpretation of gross and microscopic appearance of anal sac contents
| Gross appearance of contents | Microscopic appearance | Interpretation |
| Watery brown material | Desquamated cells, few bacteria or yeasts; no inflammatory cells | Normal |
| Thick, waxy material ± clumps | Aggregates of keratinized material, few bacteria or yeasts | Anal sac impaction |
| Purulent, malodorous exudate ± haemorrhage | Toxic neutrophils, many bacteria | Acute infection, abscess |
| Watery to creamy brown-green | Bacteria ± yeasts; some inflammatory cells | Chronic infection |
(C) Biopsy
If a perianal mass is detected, then biopsy material may be taken using either fine needle aspiration or a Trucut needle.
(D) Radiography
If a malignancy is suspected, then radiographic assessment of the local and regional lymph nodes (especially in the sub lumbar region) must be performed.
Anal Sac Infection
| Causes of Anal Gland Infection | Underlying causes probably include allergy and endocrine disease; also iatrogenic damage from expression. |
| Clinical Signs | Clinical signs include scooting and perineal/caudal trunk pruritus. Anal sacs are usually easy to empty but may have thickened walls. Infection also occurs in the cat. Examination of contents reveals bacteria or yeasts ± inflammatory cells. |
| Treatment | Treatment approaches may involve flushing, applying topical antibiotics, administering systemic antibiotics or antifungal medication, or opting for surgical removal. If sacs are removed, an open technique is preferable, as ductal remnants may be the cause of continued pruritus. |
Anal furunculosis (Perianal fistula)
| Pathogenesis | Immune mediated disease |
| Clinical Signs | Dyschezia Tenesmus Blood in stool Frequent licking of perineum On physical examination, perianal sinus formation is noted |
| Treatment | – Treatment includes either medical therapy or surgical ablation of affected tissue. – Medical therapies include ciclosporin, topical tacrolimus, antibiotics, and steroids if there is concurrent colitis. – Management of concurrent illness including hypothyroidism, colitis or dietary sensitivities is important. – Surgical treatment entails removal of all affected tissue, and anal sacs if involved. Complete anal resection with rectal pull-through may be necessary in severe cases. |
Anal Sac Impaction
| Clinical Signs | Clinical signs include scooting, and self-trauma to perineal and tail base region. Anal sacs may be difficult to express. Expressed material is thick and flocculant |
| Treatment | Treatment includes regular emptying of the anal sacs. Surgical removal is indicated if condition is recurrent |
Anal Sac Abscess
| Clinical Signs | Clinical signs include pain in the perineal region and a discharging sinus or perianal mass. Examination of the discharge reveals inflammatory cells and bacteria. The condition may follow anal sac impaction. |
| Treatment | – Usually responds to a short course of appropriate systemic antibiotics. In the German Shepherd Dog, anal sac abscess may progress to perianal sinus. – If the problem is recurrent, then anal sac removal is indicated, once the abscess has healed. |
What is the Treatment of Anal Sac Disease in Dogs?
The therapeutic approach to anal sac disease is determined by the specific disease entity present. Options include topical and systemic drugs, and surgery.
(A) Topical therapy for Anal Sac Disease
Since the anal sac is composed of a skin pouch, any infection within the sac’s lumen is essentially an infection on the skin’s surface. For this reason topical therapy is sometimes more effective than systemic treatment.
In this context, topical therapy involves instilling antibacterial and/or anti-inflammatory agents into the anal sac using a nasolacrimal cannula or a cat catheter. The choice of therapy depends upon the nature of the problem, and the organisms demonstrated. Recommended products include sterile saline, antibiotic preparations such as metronidazole or clindamycin, antiseptics like povidone-iodine or chlorhexidine solutions, and antibiotic/steroid combinations. In the USA, ointments combining neomycin and thiostrepton are frequently used. If anal sac impaction occurs, cerumenolytic agents can be utilized. Treatments may have to be repeated, and recurrence of disease is still a possibility.
(B) Systemic therapy
The use of systemic antibacterial or antifungal agents may be necessary in cases of acute infection or abscess. Identification of possible causative organisms is important, as anaerobes or yeasts require specific treatment. Drugs commonly used include clindamycin, metronidazole and amoxicillin/clavulanate. Treatment should be continued beyond clinical cure, and microscopic examination of anal sac contents to assess efficacy of treatment is important. Again , recurrence is commonplace, and may indicate the need for surgical removal of the anal sacs.
(C) Surgery
Surgical removal of anal sacs is indicated if medical therapy fails, or if recurrence is frequent in the absence of manageable underlying disease. If perianal masses are present, their removal will often necessitate removal of the ipsilateral anal sac, as damage to the sac may lead to recurrent abscessation or sinus formation.
There have been many techniques described for the surgical removal of anal sacs. It must be emphasized, however, that the filling of the sacs with caustics such as phenol or silver nitrate is never indicated.
The two techniques now commonly employed are closed and open removal.
(i) Closed anal sac removal
A closed technique for the removal of anal sacs has been used for many years now, and some surgeons will use no other procedure.
- Anaesthetize the patient. Clip and prepare the perineum for surgery.
- Empty the anal sacs and refill with a heated gel. The gel hardens to define the extent of the sacs.
- A purse-string suture can be placed to prevent fecal contamination of the surgical site.
- Make a curved incision over the palpated distal portion of the sac.
- Use blunt dissection to isolate the sac and duct, taking care not to damage the nervous supply to the external sphincter.
- Place a ligature on the duct and remove the sac.
- Repair the wound.
The advantages of this technique are the lack of damage to the external sphincter muscle and the avoidance of damage to the anal canal. The drawbacks include the potential for leaving part of the anal sac duct intact and the risk of damaging nearby nerves. Should a hole be made in the sac, the filler gel can escape and make the surgery more difficult.
(ii) Open anal sac removal
A ‘lay-open’ technique for anal sac removal has been described more recently, and has gained favour with many surgeons. This technique is quicker than the one described above, and results in complete removal of all anal sac and ductal tissue. It is important to allow the dog to empty its rectum prior to surgery, as it is not possible to place a purse string suture.
A ‘lay-open’ technique for anal sac removal has been described more recently, and has gained favour with many surgeons. This technique is quicker than the one described above, and results in complete removal of all anal sac and ductal tissue. It is important to allow the dog to empty its rectum prior to surgery, as it is not possible to place a purse string suture.
- Anaesthetize the patient. Clip and prepare the perineum for surgery. A swab placed in the rectum may prevent leakage of faecal material during surgery.
- Make an incision along the duct and into the sac: either place a probe into the duct and cut on to it; or insert one blade of straight Metzenbaum scissors into the duct.
- Make an incision medial to the duct and lift the whole duct away from the anal canal.
- Grasp the distal open wall with a pair of tissue forceps and free the sac from underlying tissue by blunt and sharp dissection.
- Repair the wound, taking care to appose the cut ends of the external sphincter muscle.
The advantages of this technique are speed and the ease of identifying whether the whole sac has been removed. A disadvantage is that the external sphincter muscle is cut and takes some time to regain normal function. This typically does not result in incontinence, though some fecal leakage may occur in the initial days following surgery.
This technique is also suitable for anal sac removal in the cat.
Neoplasia
If an anal sac mass is present, then the extent of the removal depends upon the biopsy results. If there is distant spread, then removal is inappropriate.
