Anal sacs are often referred to as anal glands, but this is a misnomer because structurally anal sacs are invaginations of the skin. They are located at the positions of four and eight o’clock between the internal and external anal sphincter muscles (Figure 1). The sacs are round to oval and variable in size. Although there was a suggestion that larger breeds of dogs have larger sacs, a study failed to support this hypothesis (Pappalardo, 2002).
Each anal sac is connected to the surface via a single duct that can be up to a centimetre in length. The surface lining of the sac is squamous keratinising epithelial tissue which is supported by underlying connective tissue, rich in sebaceous and apocrine glands.
The contents of the sacs are made up of keratinised epithelial cells, secretions from the sebaceous and apocrine glands and microorganisms. The material varies in consistency, colour and odour and is normally fatty and serous in nature. The consistency of the exudate ranges from watery to waxy, its colour from cream to dark brown and its odour generally depends on the presence or absence of infection, with the latter making it more pungent.
Various studies have examined the exudate cytologically (Pappalardo, 2002; Lake et al., 2004; Robson et al., 2008; James et al., 2011). Generally, on cytology pigment granules, extracellular bacteria, Malassezia and neutrophils are reported; however, the presence of neutrophils within the exudate does not always indicate an infection. Streptococcus faecalis, Staphylococcus pseudintermedius, Escherichia coli, Proteus mirabilis, beta-haemolytic streptococci, Pseudomonas aeruginosa and Bacillus have all been isolated from the anal sacs of normal dogs and dogs with pyoderma (Pappalardo, 2002). In the same study, Malassezia was isolated but the distribution suggested that it was less prevalent in the anal sacs of normal dogs.
Dogs with anal sac disorders are often brought in by their owners because they have been seen “scooting” or biting and licking their tail base, anus and/or lateral thighs. Some present with an acute moist dermatitis, commonly known as “hotspot”, on their rump. Differential diagnoses for “scooting” include atopic dermatitis, adverse food reaction and intestinal parasites. Before embarking on lengthy investigations into allergic disease, the anal sacs should be checked as they may be impacted or infected or there may be neoplastic changes.
Anal sac impaction
Impaction is due to the failure of material to extrude from the gland through the duct during defecation. It is probably the most common condition affecting the anal sac and it is more common in Poodles, Pugs, Cavalier King Charles Spaniels, Bichons Frises and other small breeds. Exactly why this happens is not known; however, obesity and soft faeces have been suggested as contributory factors.
Distended sacs can be felt externally as soft protrusions around the four and eight o’clock positions around the anus.
Generally, they are best emptied by inserting a gloved finger, using a lubricant such as K-Y Jelly, into the anus and then placing the finger on the medial wall. The thumb is then used to fix the external wall and the material gently squeezed out onto cotton wool or a swab.
Owners often ask the question, “why do they get impacted and how often do they need to be emptied?” Unfortunately, there is no straight answer to this query, as there is so much variation between individuals. Some dogs respond to increased fibre in the diet and others require regular emptying. The frequency needed varies between individuals and could be necessary as frequently as once a week. In such cases, surgical excision may be a more desirable longer-term solution.
Anal sac infections
Precisely why anal sacs get infected is unknown. It may be associated with frequent episodes of impaction, allergic skin disease, obesity, incomplete emptying during defecation, chronic bowel disease, hormonal disorders and the tight position of the tail against the anus.
Cytologically it is difficult to distinguish between infection and impaction because neutrophils and bacteria are present in both disorders.
An infected anal sac can develop an abscess, which is recognised by the presence of pain, erythema and often draining sinus or perianal swelling. In advanced cases, if the abscess is ruptured an ulcerated area at the site of the anal gland can be seen.
Treatment options for infected sacs include flushing the sac with either saline or an antiseptic solution through a 4G cat catheter or nasolacrimal cannula inserted through the duct opening. Topical drugs containing antibiotics and antifungals can also be infused into the sac. Off-label ear preparations or off-licence bovine intramammary preparations can be used. In a few cases, systemic antibiotics may be indicated, and it is important to warn the owner that even with appropriate treatment and management the condition can recur in some predisposed individuals.
In some cases, surgical ablation of the anal sac is indicated.
Anal sac neoplasia
Anal sac adenocarcinoma and squamous cell carcinoma have been reported. In addition to the thickening of the sac wall or palpation of a nodule, clinical signs of neoplasia include tenesmus, constipation and polyuria/polydipsia. Cocker Spaniels, Springer Spaniels and King Charles Spaniels are at-risk breeds. Further investigations into the causes of polyuria/polydipsia may reveal hypercalcaemia, which may be suggestive of a paraneoplastic syndrome associated with adenocarcinomas. If so, further investigations to stage the disease, treatment options and prognosis should be discussed with an oncologist.
Anal pruritus and scooting are common in dogs of all breeds but more so in some. The cause of this behaviour needs to be ascertained, because some of the differentials are pernicious or require long-term management (eg atopic dermatitis). It is best to warn owners that long-term management, or excision, may need to be considered.