ABSTRACT: Disease of the anal sacs is a common reason for presentation of dogs to a veterinary surgeon. It is less common in the cat. Little is known of the aetiology or pathogenesis of anal sac disease. Management of anal sac disease is, therefore, often symptomatic, frequently ending with surgical removal of the sacs as a response rather than a planned strategy.

This paper will discuss a diagnostic approach based upon current knowledge (Table 1).

Anatomy and physiology

The anal sacs of the dog are paired, cutaneous pouches situated either side of the anus. They lie between the external anal sphincter muscle and the rectum. They are connected to the outside by a short duct that opens laterally to the anus. Anal sacs form a reservoir for the secretions of the apocrine and sebaceous glands lining the sacs.

Anal sacs empty during defecation, squeezed by the external anal sphincter against the faeces in the rectum. There is considerable variation in the colour, consistency and odour of secretions from normal anal sacs, from black to green, from watery to viscid (Figure 1 and Table 2).

Figure 1: Normal anal sac secretion is of varied appearance. (Image courtesy of Carl Gorman.)

Microbiology

In the normal dog, the most common bacteria found in anal sac secretions are micrococci, Escherichia coli, Streptococcus faecalis and Staphylococcus spp.

Malassezia pachydermatis yeasts are found in normal and diseased sacs. In the diseased anal sac, Gram-negative bacteria and anaerobes are also found.

Clinical approach

History taking, thorough clinical examination and diagnostic tests are all important for successful diagnosis and management (Tables 3, 4 and 5).

History

History taking must focus on the presenting signs, together with gathering information about possible concurrent disease. Does the dog scoot (rub the anus on the ground)? If it does not, then has it ever done this? Scooting is highly suggestive of anal disease, but some dogs with significant anal sac disease never do. Some dogs experience sudden pain in the hindquarters. If there is caudal pruritus and associated tail-base alopecia, owners must be questioned about flea control; concurrent facio-pedal pruritus or ear disease may suggest underlying atopy (or food allergy).

Examination

Examination of the dog or cat requires the following to be assessed:

•   systemic examination (concurrent disease, suitability for future surgery)

•   dermatological examination (allergic, endocrine or parasitic disease) including perineum (sinus or fistula, intertrigo) and tail and vulval folds, if present

•   palpation of anal sacs (present? thickened? mass present?)

•   carefully express anal sacs and collect contents for cytology.

Gross and microscopic examination of the anal sac contents is important in all cases.

Gross and microscopic examination

Whilst the nature of the anal sac contents may be suggestive of a type of disease, only the presence of blood is always abnormal. Examination of a sample stained with Diff- Quik can reveal bacteria of three types : cocci (staphylococci or streptococci), small rods (likely Gram-negative coliforms) or large rods (anaerobes).

If a perianal mass is detected, then biopsy material may be taken. If malignancy is suggested, then radiographic assessment of the local and regional lymph nodes must be performed and assessment for para-neoplastic syndrome should be made.

Management of anal sac disease

Obviously the therapeutic approach to anal sac disease is determined by the specific disease present. Therapeutic options include topical, systemic and surgical options. If simple anal sac impaction is present, then increasing the dietary fibre may help in management.

Topical therapy

Since the anal sac is a pouch of skin, any infection in the lumen of the sac is essentially on the skin surface. Topical therapy here means the instillation of antibacterial and/or anti-inflammatory products into the anal sac, by means of a nasolachrimal cannula or cat catheter (Figure 2).

Figure 2: Irrigation of an anal sac using a nasolachrimal cannula. (Image courtesy of Carl Gorman.)

The choice of therapy depends upon the organisms demonstrated. Products suggested include sterile saline, antibiotic preparations (metronidazole or clindamycin), antiseptics (povidone iodine or chlorhexidine solutions) or antibiotic/steroid preparations. If anal sac impaction is a problem, then cerumenolytic agents may be used. Treatments may have to be repeated, as recurrence of disease is common.

Systemic therapy

In cases of acute infection or abscess, the use of systemic antibacterial or antifungal treatment may be necessary. Identification of possible causative organisms is important, as anaerobes or yeasts require specific treatment. Suitable antibiotics include clindamycin, metronidazole and amoxycillin/clavulanate.

Treatment should continue beyond clinical cure and microscopical examination of anal sac contents to assess efficacy is important. Recurrence is commonplace, and may indicate the need for surgical removal of anal sacs.

Surgical therapy

Surgical removal of anal sacs is indicated if medical therapy fails, or if recurrence is frequent in the absence of manageable underlying disease. There are many techniques described for the surgical removal of anal sacs; however, filling of the sacs with caustics, such as phenol or silver nitrate, is never indicated.

Two techniques now commonly employed are closed and open removal.

Closed anal sac removal

The patient is anaesthetised and the perineum clipped. The anal sacs are emptied, and then filled with a heated gel. This hardens, defining the extent of the sacs. A purse string suture may be placed to avoid faecal contamination of the surgical sit
e. The perineum is then prepared for surgery, and a curved incision is made over the palpated distal portion of the sac.

Blunt dissection is then used to isolate the sac and duct, taking care not to damage the nervous supply to the external sphincter. A ligature is then placed on the duct, and the sac removed. The wound is then repaired.

Advantages of this technique are the lack of damage to the external sphincter muscle. Disadvantages include the leaving of some anal sac duct and possible damage to local nerves. Should a hole be made in the sac, the filler can escape and make the surgery more difficult.

Open anal sac removal

The author favours the open technique for anal sac removal. This technique results in complete removal of all anal sac and ductal tissue.

The patient is anaesthetised and the perineum prepared for surgery. It is important to allow the dog to empty its rectum prior to surgery. A gauze swab may be placed in the rectum to prevent leakage of faecal material.

An incision is made along the duct and into the sac either by cutting onto a probe, or by inserting one blade of a straight pair of Metzenbaum scissors into the duct. The next incision is medial to the duct, lifting the whole duct away from the anal canal. The distal open wall is grasped with a pair of tissue forceps, and the sac is freed from underlying tissue by blunt and sharp dissection and removed. The wound is then repaired, taking care to appose the cut ends of the external sphincter muscle.

The advantages of this technique are speed, and that having entered the sac, it is easy to identify whether it has been removed in its entirety. This is especially important in chronic infection, as infected ductal remnants can cause as much pruritus as the whole sac.

Disadvantages are that the external sphincter muscle is cut, and takes some time to regain normal function. This does not lead to incontinence, but occasional faeces may be dropped in the first days after surgery.

Pain relief is important following surgery. The application of a viscid ointment (Vaseline or Sudocrem) to the suture line to prevent contamination with faecal matter. Sutures can usually be removed after seven days.

Author

David H Scarff

BVetMed CertSAD MRCVS

David Scarff has been a practice- based dermatologist for nearly 20 years following time spent teaching at the Royal Veterinary College. He has an interest in the conditions affecting areas of specialised skin in the dog and cat.

• VOL 25 • No4 • April 2010 • Veterinary Nursing Journal