Diseases of the Anal Sac

Russell Muse, Tustin, California

Diseases and abnormalities of the anal sac are common concerns faced by pet owners and the general practitioner. Unfortunately, the volume of research, literature, and information available about anal sac disease does not correspond to the frequency with which these problems occur. Fortunately, over the past few years, a number of papers have begun to establish normal and abnormal parameters associated with anal sac contents and disease. Knowledge of normal anal sac structure and function, and an appreciation of the various disorders that can involve these tissues, are critical to successful recognition and management of anal sac diseases.

Anal sacs are paired invaginations of the skin located between the internal and external sphincters of the anus. Each sac is connected to the surface by a duct that opens at the mucocutaneous junction of the anus in the dog. The anal duct of cats opens onto a prominence just lateral to the anus. The anal sacs are lined with stratified squamous epithelial cells and contain large apocrine glands with smaller numbers of sebaceous glands. In addition, the walls of the anal sac are lined with elastic and smooth muscle fibers. The duct is lined with both apocrine glands and large sebaceous glands.

The anal sacs provide a reservoir for the secretions of these glands admixed with desquamated epithelial cells. This forms the brown, oily to waxy secretion that normally is evacuated as a result of pressure from fecal excretions. However, change in character of the secretion or alteration in muscle tone or fecal form may cause overfilling and plugging of the sacs and resultant fermentation, inflammation, and infection. Because of the thinness of the anal sac and the ease with which it is normally evacuated, it is not distended in normal dogs. However, if enlarged, it can be palpated easily at the 4 and 8 o’clock positions between the smooth muscle of the anal canal and the striated muscle of the external anal sphincter.

Physical Characteristics

The normal characteristics of anal sac secretion and histologic evaluation long have been used to attempt to discern disease of the anal sac; however, these data also have been evaluated and reported only occasionally. Several studies have provided sources of information regarding the gross, cytologic, and bacteriologic characteristics of anal sac secretions in the normal dog (Lake et al, 2004; Robson et al, 2003) and in dogs with various dermatologic diseases (Pappalardo et al, 2002). Another recent study assessed the value of cytologic analysis in differentiating between normal dogs and dogs with anal sac disease and explored the relationship between cytologic findings and the frequency of behaviors associated with anal sac disease (James et al, 2010).

In the studies by Robson and colleagues (2003) and Lake and colleagues (2004), the anal sac secretions in normal dogs ranged from thin liquid or watery to thick liquid or pasty. Color of the discharge was quite variable and was noted to be light or dark brown, creamy whitish-yellow, grayish-tan, orange-yellow, or reddish-brown. Color and consistency were similar in the right and left anal sacs in most dogs in the study by Robson and associates, whereas Lake and associates reported similar consistency in fewer than half of the dogs they examined.

In a study that compared anal sac secretions in normal dogs and in dogs with various dermatologic disorders (atopic dogs, dogs with pyoderma, and atopic dogs with Malassezia dermatitis) (Pappalardo et al, 2002), no significant differences were noted among the groups in anal sac size, color, or consistency, or in the presence of granules in the content. However, an unpleasant odor was found significantly more frequently in dogs with Malassezia infection and in those with atopy.

Cellular Characteristics

Cellularity of anal sac secretions in normal dogs was found to be somewhat similar in two studies, which identified keratinocytes (Robson et al, 2003) and parabasal epithelial cells (Lake et al, 2004). Neutrophils were noted in both studies, although degenerate neutrophils were predominant in the Robson study and nondegenerate cells in the report by Lake and colleagues. In another study (Pappalardo et al, 2002) neutrophils were noted in anal sac secretions of 12.5% of normal dogs but 30% of dogs with Malassezia and atopic dermatitis, 70% of dogs with pyoderma, and 80% of dogs with uncomplicated atopic dermatitis. Erythrocytes and eosinophils were not noted with any regularity in any study. In an evaluation of cellularity of secretions in normal dogs versus dogs that exhibited behaviors typical of anal sac disease (scooting and anal pruritus), although a larger mean count of neutrophils was noted (25% of normal dogs versus 31% of dogs with behavior typical of anal sac disease), the difference was not statistically significant (James et al, 2010).

Bacterial Characteristics

Bacterial counts also were detailed in these studies. One report (Robson et al, 2003) indicated that coccoid bacterial counts in anal sac secretions were low, with 77.9% of microscopic examination fields containing few coccoid organisms and only 2.4% demonstrating more than 319 organisms per oil immersion field. These findings are in contrast to a study by Lake and colleagues (2004), which reported that 86% of anal sac secretions contained mostly gram-positive cocci. Pappalardo and associates (2002) reported that anal sac secretions from 48.75% of normal dogs had “abundant” numbers of bacteria present. However, all dogs with skin diseases had higher bacterial counts, ranging from 60% in dogs with atopic dermatitis without complications to 90% in dogs with pyoderma. Finally, James and colleagues (2010) showed that although intracellular bacteria were present more frequently in secretions from normal dogs than in those from dogs with behaviors associated with anal sac disease (14% versus 4%, respectively) the difference was not statistically significant. Each of these studies did find that intracellular bacteria and large numbers of Malassezia organisms are a relatively uncommon finding. Previous studies revealed various gram-positive coccoid and rod-shaped bacteria to be a normal part of the flora of the anal sac; these include Streptococcus faecalis, Streptococcus faecium, Escherichia coli, Clostridium perfringens, Staphylococcus intermedius, Proteus spp., and coagulase-negative staphylococci. The most recent study to evaluate cultures of anal sac secretions (Pappalardo et al, 2002) found that the most common organisms were Proteus mirabilis, E. coli, Staphylococcus intermedius, β-hemolytic Streptococcus spp., S. faecalis, Bacillus spp., and Pseudomonas aeruginosa.