Chapter 36: Diseases of the Anal Sac

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Diseases of the Anal Sac

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.

Gross Characteristics

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.

Thus, although these recent studies seem to indicate variability in the physical characteristics of anal sac secretions, they do add new information to this field to help the clinician assess the presence or absence of disease of the anal sac. Despite these advances, evaluation and observation of clinical behavior still is the most important component in establishing the existence of an anal sac disorder.

Disorders of the Anal Sac


Diseases of the anal sac vary from impactions to infections (sacculitis) with or without abscess to neoplasia. Anal sac impactions have been reported to occur in 2% to 12% of dogs (Scott et al, 2001). Clinically these usually result in pruritus or the classic scooting behavior that is noted by most owners. Although anal sac disease is a common cause of anal pruritus, the clinician should be alert to the fact that many other causes of pruritus of the anal or perianal area have been documented and include any number of dermatologic, psychologic, metabolic, nutritional, and gastrointestinal diseases. Skin diseases such as atopy, cutaneous adverse food reaction, insect hypersensitivity, Malassezia or bacterial dermatitis, parasitic skin disease, and keratinization defects all may result in persistent pruritus of the anal or perianal area. Diagnostic procedures such as cutaneous cytologic evaluations, examination of skin scrapings, culture for dermatophytes, food trials, and allergy testing may be necessary to uncover the causes of any residual pruritus. However, if the pruritus completely resolves after expression of the anal sac, a true anal sac disorder is likely. In addition, I strongly recommend that the routine expression of anal sacs in dogs during grooming, bathing, and boarding by veterinarians, technicians, groomers, and others be discouraged. Repeated manipulation and inadvertent trauma may predispose normal dogs to chronic recurrent anal sac disease.

The incidence of impactions is increased in smaller breeds (<15 kg), especially miniature and toy poodles, Chihuahuas, American cocker spaniels, and English springer spaniels (Scott et al, 2001). The presence of dermatologic problems such as primary or secondary keratinization defects in the spaniels also may be an exacerbating factor. Impaction of the anal sacs is less common in cats. Secretions of the anal sacs in cats are somewhat different from those in dogs because the anal sacs of cats contain larger numbers of sebaceous glands, which may allow for more lipid production and subsequently easier expression. In addition, since the anal duct opens laterally to the anus in cats, it may be less susceptible to occlusion by fecal matter.

Impaction may be a result of alterations of glandular secretion. Loss of apocrine secretions may lead to the accumulation of hard granular material that cannot be expressed by the normal bowel movement of the dog. Glandular hypersecretions associated with generalized seborrhea also may cause plugging of the duct. Chronic loose stools or bouts of diarrhea may result in pasty secretions and subsequent impaction. Dietary manipulation with feeding of higher-fiber diets or endoscopic investigation for the possibility of a gastrointestinal disease that is causing chronic soft or loose stools may be of benefit. In addition, tapeworm segments have been identified in anal sacs and may cause impaction.

Impactions should be gently but thoroughly expressed manually. Complete expression of the anal sac is best accomplished via digital compression through the rectal canal. In addition, a complete palpation of the anal sac can be achieved. Expression should relieve the pruritus associated with a simple impaction. If pruritus is not resolved, additional causes of anal pruritus should be investigated as discussed previously. Sedation and irrigation of the anal sac with sterile saline may be required to soften firmer secretions or manage more recurrent impactions. Every attempt should be made to identify and correct underlying causes of chronically recurrent impactions. If the pruritus is not relieved by manual expression, infection or inflammation of the anal sac also may be present.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 36: Diseases of the Anal Sac
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