Salivary tissue is abundant in the oral and pharyngeal cavities of both dogs and cats. The major salivary glands in both these species are the paired parotid, mandibular, sublingual and zygomatic glands. Cats in addition have paired mandibular molar salivary glands. In dogs and cats the sublingual gland comprises two portions: a caudally situated monostomatic part and a rostrally located polystomatic part. The monostomatic part lies within the same capsule as the mandibular gland.

The duct of the parotid salivary gland opens into the buccal cavity opposite the caudal aspect of the upper carnassial tooth at a small papilla. Behind this, opposite the first molar tooth the major zygomatic duct opens on the same ridge of mucosa, with the minor ducts from this gland (of which there are two to four) opening individually caudal to the major duct. In two-thirds of dogs the mandibular and sublingual ducts open into the buccal cavity together ventrolateral to the sublingual papilla or caruncle, which lies just lateral to the rostral end of the lingual frenulum. In those dogs where the ducts enter separately, the duct of the sublingual gland enters caudal to that of the mandibular gland.

A number of different disease processes may be encountered that originate from or involve the salivary glands: sialoadenitis, necrotising sialometaplasia and sialoadenitis; sialolithiasis; ductal foreign bodies; benign and malignant neoplasia and salivary mucoceles.

Clinical disease of the salivary gland is uncommon in cats, with neoplasia the most frequently diagnosed problem. Similarly, in dogs clinical disease of these glands is uncommon (<0.5% of cases), with salivary mucocele the most commonly recognised problem.

Salivary Mucocele (Sialocele)

Salivary mucoceles are the most frequently recognised clinical condition of the canine salivary gland, but occur infrequently in cats. A salivary mucocele is a cavity in the subcutaneous or submucosal tissues, created by and containing saliva from a disruption of a salivary gland or duct. The lining of the sialocele is usually formed from inflammatory connective tissue and therefore sialoceles differ from true cysts, because they do not have secretory epithelial linings.

In dogs the sublingual gland/duct is most often affected. Attempts to induce sialoceles experimentally in dogs by traumatising or ligating the mandibular gland or duct have been unsuccessful, resulting in either atrophy of the gland or rapid healing of the salivary duct. Consequently, the aetiopathogenesis of the clinical condition remains unresolved. However, a number of mechanisms have been proposed:

 Trauma, e.g., blunt and penetrating injuries in addition to iatrogenic trauma.

 Sialolithiasis. This condition is rare in dogs and mainly affects the parotid gland. The sialocele forms following rupture of the obstructed duct.

 Foreign bodies. Grass awns have been reported to cause obstruction of the parotid duct. A sialocele may form if the obstructed duct ruptures.

 Inflammation. Histopathological assessment of affected salivary glands often reports the presence of inflammation: whether this is a consequence of ductal injury or cause of the sialocele is difficult to establish definitively.

 Neoplasia. Sialoceles may occur in association with glandular neoplasia.

An inherent predisposition in certain breeds has been suggested with poodles, Greyhounds, German Shepherd Dogs, Dachshunds and Australian Silky Terriers over-represented. Poodles are also over-represented in studies of other salivary gland disease such as infarction.

General Clinical Presentation and Signs

Salivary mucoceles have no age or sex predilection, but as noted above certain breeds have been reported as predisposed. Frequently there is a delayed presentation, but some patients may present acutely.

The sublingual salivary duct is most frequently damaged in dogs but sialoceles have been reported secondary to damage to all the salivary glands. Submucosal sialoceles in the oral and oropharyngeal cavities are often referred to as ranulas. Sublingual ranulas are commonest and may communicate directly with cervical sialoceles. These also result from damage to the sublingual duct or glands, although the mandibular salivary duct may be affected. Submucosal sialoceles are occasionally found in the pharynx (pharyngeal sialoceles) and may result from damage to the palatine salivary tissue, to the mandibular or sublingual salivary duct as it courses close to the pharyngeal mucosa, or to the parotid salivary gland.

Affected animals can have a number of different clinical presentations. Frequently, they present with swellings in the submandibular or cervical region. The swellings are generally subcutaneous, fluid filled, soft, mobile and non-painful. Initially they are located at the point of saliva leakage but they tend to sink over time making lateralisation to establish the side of injury difficult. In these cases the mandibular and/or sublingual glands are usually affected and the condition may be unilateral or bilateral.

Other presentations include dysphagia, oral bleeding and drooling (ranula or pharyngeal sialocele), upper respiratory tract obstruction (pharyngeal sialocele), exophthalmos or pain on opening the mouth (zygomatic sialocele) and swellings in the masseteric region (parotid duct). Only rarely are affected animals systemically unwell.

Diagnosis

It is important to obtain a complete history from the owner and perform a thorough physical examination before undertaking further investigations. In cases of cervical sialoceles it is important that the owner be questioned carefully to establish whether the swelling was initially lateralised to the left or right hand side. In many cases, the initial investigation of the disease can be performed in the conscious animal, but deep sedation or general anaesthesia will be required to allow complete evaluation of the oropharyngeal region and/or diagnostic imaging. Fine-needle aspiration of the fluid swelling should be performed at an early stage. The gross and cytological appearance of the fluid contained within sialoceles is very distinctive in uncomplicated cases. The fluid is generally honey coloured, although it may be blood stained and consequently darker in appearance. It is mucinous and will form a long string when dripped from the needle following aspiration, even if there is contamination of the fluid with blood. Cytologically the fluid will have a very low cellularity and mucin is detected by using special stains such as periodic acid Schiff.

In cases of cervical sialoceles no further investigation may be required if the disease can be lateralised either historically, clinically or by positioning the patient on its back and allowing the fluid swelling to drop to the affected side. However, it may be difficult to lateralise the disease process or establish the affected glands. In these cases positive-contrast sialography may be of benefit. The ducts of the zygomatic and parotid salivary gland are generally easy to identify and cannulate. However, the opening of the mandibular duct can be difficult to identify as it sits on the ventral aspect of the mucosal fold on the sublingual caruncle. As noted above, in one-third of dogs the mandibular and sublingual ducts enter the oral cavity separately with the duct of the sublingual gland entering caudal to that of the mandibular gland. Small intravenous catheters (22-24 gauge) or lacrimal cannulae can be used to cannulate the salivary ducts. 2-3 ml of contrast agent is injected into each salivary duct in turn and radiographs are obtained. The normal ductal structure of each gland should be highlighted unless the contrast leaks through a defect into the sialocele. Topical lemon juice has been recommended to increase saliva flow to assist in duct identification, but the author has not found this to be a useful technique.

Treatment

 Drainage. Conservative management of salivary mucoceles by aspiration of the fluid is purely palliative. Sialoceles typically recur within a short period of time and infection is common following repeated aspiration: at worst repeated aspiration can lead to sinus formation.

 Sialoadenectomy. Excision of the affected salivary gland(s) is the treatment of choice. Most often the mandibular/sublingual complex is affected, but occasionally excision of the zygomatic or parotid salivary gland may be required. Drainage of the associated mucocele is usually achieved at surgery but in some situations a surgical drain may be required to aid management of dead space. Recurrence following extirpation of the mandibular/sublingual salivary gland complex is low (<5%). Potential surgical complications include haemorrhage, seroma, infection and hypoglossal nerve paralysis or neuropraxia.

 Marsupialisation. Sublingual (ranulas) and pharyngeal mucoceles may resolve following marsupialisation, but recurrence of the problem is common (10-33%).

 Ductal repair. Primary repair of ductal defects has been reported.

 Ductal ligation. Parotid duct ligation leads to progressive and almost complete atrophy of the parotid gland in normal animals. This has been used successfully to manage some parotid sialoceles. The duct must be ligated proximal to any lesion and not all cases respond well, but this is worth considering before parotid sialoadenectomy.

Salivary Gland Neoplasia

Salivary gland tumours are rare and typically affect older dogs and cats. However, in some studies up to 30% of salivary gland masses were diagnosed as neoplastic. A number of different tumours has been reported, but primary and malignant epithelial neoplasms are the most important. The parotid and mandibular salivary glands are most often affected and spaniels and poodles may be predisposed to the problem. Care should be taken to avoid confusion between lymphadenopathy of the parotid and/or buccal lymph nodes and disease of the parotid salivary gland.

Salivary gland tumours are often locally invasive extending through the capsule of the gland and frequently have spread to regional lymph nodes or distant sites by the time of diagnosis. Sialoadenectomy is unlikely to be curative but can be performed palliatively.

Sialolithiasis

Sialolithiasis is a rare clinical condition in dogs although one histopathological study of salivary gland disease in 245 dogs and cats reported an incidence of 0.4%. It has most commonly been reported to affect the parotid salivary duct in dogs, and has been seen as a complication following parotid duct transposition. It has recently been reported in the mandibular/sublingual duct complex. Removal and primary repair has been described with success in the case of a parotid duct sialolith in a dog but excision of the affected salivary gland(s) is also a reasonable option.

Sialadenitis; Necrotising Sialometaplasia; Sialadenosis

Sialadenitis, necrotising sialometaplasia and sialadenosis are three separate diseases that have similar clinical signs and treatments, but are very different conditions histopathologically. Clinical signs include dysphagia, nausea, gagging, ptyalism, pain on opening the mouth and local swelling and pain of the affected salivary glands. Necrotising sialometaplasia may be an extension of sialadenitis and a number of aetiological factors have been proposed: trauma; infection; immune-mediated disease; and oesophageal disease. Terriers have been reported as predisposed to these conditions. A number of therapies have been proposed including surgical excision of the affected gland(s), antibiotics, glucocorticoids and phenobarbitone.

Sialadenosis causes enlargement of the affected salivary glands (frequently the mandibular gland but occasionally the zygomatic) in addition to ptyalism, retching or gagging, gulping and inappetence. If the zygomatic gland is involved exophthalmos, epiphora and divergent strabismus may be seen. The diagnosis is one of exclusion with a positive response to phenobarbital therapy.