Salivary Gland
D. Dunning
College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA.
Diseases of the salivary glands and ducts are uncommon in the dog and cat, with a reported
overall incidence of 0.17 to 0.3% . Reported conditions involving the salivary glands include
rupture, inflammation, dilation, necrosis, fistula, a foreign body, autoimmune disease, calculi,
and neoplasia. The onset of many of these conditions is frequently insidious, with vague
findings on physical examination. Definitive diagnosis often necessitates fine-needle
aspiration and cytology, radiology, ancillary imaging, biopsy, and exploratory surgery.
Complete knowledge of the anatomy of the head and neck, accurate dissection, removal of the
affected glands and duct systems, and drainage may be necessary to successfully treat many
of the conditions outlined in this chapter.
Anatomy and Function
The four major salivary glands in the dog and cat are the paired parotid, mandibular,
sublingual, and zygomatic glands.The parotid gland is a triangular bilobar gland located at the
base of the horizontal ear canal. The parotid duct runs rostrally along the lateral surface of the
masseter muscle between the dorsal and ventral buccal branches of the facial nerve and opens
into the oral vestibule at the level of the 3rd-4th and 2nd cheek tooth in the dog and cat,
respectively. Secretion from the gland is primarily thin and serous, with a mucous component
also present in the dog. Sympathetic innervation is provided by the external carotid plexus of
nerves, which travel in tandem with the parotid artery. Fibers of the auriculotemporal branch
of the trigeminal nerve give parasympathetic input. The parotid artery is the main blood
supply to this gland. Venous drainage is via the superficial temporal and great auricular veins.
Often mistaken as an enlarged lymph node on cervical palpation, the mandibular gland is
located directly beneath the bifurcation of the external jugular vein, lying caudoventrally to
the parotid gland, between the linguofacial and maxillary veins. It is an ovoid, capsulated
gland that is fused to the monostomatic part of the sublingual gland. The mandibular duct
travels rostrally, medial to the digastricus muscle to empty at the sublingual caruncle on the
floor of the oral cavity. Secretion from this gland is mixed, both serous and mucous.
Sympathetic never fibers reach the gland by means of a perivascular plexus around the
glandular artery. Parasymphathetic innervation is provided by the chorda tympani of the facial
nerve. The main artery supplying blood is the glandular branch of the facial artery, which
enters the gland medially and in close association with the mandibular duct. Entering the
dorsal part of the deep surface are one or two branches of the caudal auricular artery. The
chief vein draining the gland terminates into the lingual vein. A second smaller vein drains the
caudal portion of the gland and terminates in the facial, maxillary, or lingual veins.
The sublingual salivary gland is composed of both a monostomatic and a polystomatic
component. The monostomatic portion is compact with a fibrous capsule that is contiguous
with the rostral border of the mandibular gland. Its duct opens into the oral vestibule near the
sublingual caruncle. The diffuse polystomatic part of the sublingual salivary gland spreads
into tissues ventral to the floor of the oral cavity and drains via several ducts opening into the
oral cavity on either side of the frenulum of the tongue. Similar to that of the mandibular
gland, innervation is provided by perivascular plexus around the glandular artery and the
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corda tympani of the facial nerve. Blood supply to the monostomatic portion is provided by
the facial artery; the sublingual artery supplies the small polystomatic part of the sublingual
gland. Small satellite veins accompany the facial and sublingual arteries to drain the gland.
The zygomatic gland is seated deep to the zygomatic arch, on the dorsolateral surface of the
medial pterygoid muscle, forming most of the orbital floor. It has both major and minor ducts
that open into the vestibule opposite to the maxillary molar. The zygomatic gland is a mixed
salivary gland, ovoid in shape, and is innervated by the glossopharyngeal nerve. Several ducts
drain this small gland, but the major zygomatic duct opening is opposite the last maxillary
molar. The minor duct openings are difficult to visualize with the naked eye, opening just
caudal to the major duct. The first branch of the infraorbital artery provides blood supply to
the gland. Venous drainage terminates into the deep facial vein on the lateral surface of the
gland.
In addition to these primary glands, smaller, variable glands are present in the soft palate, lips,
tongue, and cheeks and are collectively referred to as buccal glands. These glands drain into
the oral cavity via numerous small ducts; the secretion is mixed serous and mucous in
character.
Secretion of saliva is under control of the autonomic nervous system, which controls both the
volume and type of saliva secreted. Salivary secretions serve many functions in the dog and
cat, some of which are to lubricate and bind masticated food into a consumable bolus, to
solubilize dry food, to flush the oral cavity from debris, to prevent overgrowth of oral
microbial population, to initiate starch digestion, and to provide evaporative cooling and
maintain core body temperature homeostasis.
Specific Disease Conditions
Salivary Mucocele (Sialocele)
Mucoceles are the most common salivary gland disorder of dogs, but are less frequent in the
cat. A mucocele, or sialocele, is defined as the accumulation of saliva in the subcutaneous
tissue adjacent to the gland or duct. Unlike a cyst, a mucocele does not have an epithelial
lining. True brachial and zygomatic cysts have been reported in both the dog and cat, but
these are rare. The exact pathogenesis underlying mucocele formation is usually unknown,
but trauma, foreign bodies, and infrequently, sialoliths have been implicated. A case report of
a mucocele associated with dirofilariasis has been reported in the dog. Poodles, dachshunds,
Australian silky terriers, and Siamese cats may be predisposed.
Although a mucocele may arise from any one the salivary glands or their associated ducts, the
sublingual and mandibular glands are most frequently implicated. Clinical signs are
dependent on the site of saliva accumulation. The most commonly reported clinical sign
associated with mucocele formation is the appearance of a soft, fluctuant, painless mass that
must be differentiated from abscesses, tumors, and other retention cysts of the neck. Saliva
will generally accumulate in a gravity-dependent site, within the cranial cervical or
intermandibular region, but it can also appear within the oral cavity under the base of the
tongue. The accumulations are referred to as ranulas. A less common site is in the pharyngeal
wall, which can obstruct airways and cause significant life-threatening dyspnea and heat
stroke.
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In the initial phases of accumulation, the soft tissue surrounding saliva is inflamed and may be
painful on palpation. This stage is short-lived and usually not detected. Once established, the
slowly enlarging saliva-filled mass is non-painful; however, secondary infection should be
suspected if the mass is persistently painful or a fever is noted. Additional differentials for the
swelling in this area include abscesses, neoplasia, and other retention cysts of the neck. A
complete oral exam should be performed in the event of a ranula, particularly if oral bleeding
is noted. Diagnosis of a mucocele is confirmed via paracentesis of a golden-brown or bloodtinged viscous fluid that typically does not need further analysis. If any question exists
regarding the identity of the fluid, a mucus-specific stain, such as a periodic acid-Schiff will
confirm the diagnosis.
Definitive treatment requires surgical removal of the affected gland and duct. Positioning the
animal in dorsal recumbency usually delineates the affected side, as origin of the saliva is
typically unilateral and most often originates from the mandibular sublingual gland complex.
If the mucocele appears to be bilateral, exploration of the site of swelling will demarcate the
side that is affected. The salivary glands may be removed bilaterally without any detrimental
effects to saliva production, if any uncertainty remains as to the origin of the saliva
accumulation. Treatment limited to lancing and draining or periodic aspiration risks infection,
does not address the primary problem, and is contraindicated. A retrospective review of
mucoceles in the dog revealed a 42% recurrence rate when treatment was limited to surgical
drainage. Aspiration and drainage are indicated with pharyngeal mucoceles, but only as an
emergency palliative procedure to relieve dyspnea until the animal can be anesthetized and
the offending glands and the redundant tissue blocking the airway removed. If a ranula is
present, marsupialization may be necessary in addition to glandular removal and drainage.
Treated appropriately, mucocele recurrence is uncommon (less than 5%), unless the glandular
tissue is not fully removed.
Sialoadentitis
Inflammation of the salivary gland, or sialoadentitis, is an incidental finding frequently noted
at necropsy in the dog that rarely manifests as a clinical problem. Blunt trauma, mucoceles,
penetrating bite wounds, foreign body migration, invasive tumor infiltration, and systemic
viral infection have been reported to cause inflammation of the salivary gland. Sialodentitis
has been reported with rabies, distemper, and paramyxovirus. Clinical signs are characterized
by pyrexia, lethargy, and painful, swollen salivary glands. Severe inflammation can result in
abscessation and rupture of the gland into the oral cavity or through the skin, with fistula
formation. Sialodentitis of the zygomatic gland frequently results in exopthalmos, retrobulbar
swelling, divergent strabismus, and trismus. Mild sialodentitis requires no treatment, and
recovery is usually rapid and complete. A salivary abscess necessitates surgical drainage or
removal of the gland and therapeutic antibiotics.
Fistula
Salivary fistulas are typically the result of penetrating injury or abscessation of the salivary
gland. The transcutaneous flow of saliva prevents second intention healing. Definitive
treatment entails dissection of the fistulous tract and salivary gland removal. In people,
botulinum toxin A, injected into the involved gland with ultrasound guidance, has been
successfully employed to abate the flow of saliva and allow wound healing via second
intention, while preserving the gland in situ.
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Sialolithiasis
Salivary calculi or sialolithiasis is a rare condition; only two cases have been reported in the
dog. Clinically, these dogs present with a painful swelling and rupture of the affected parotid
gland owing to obstruction of saliva outflow. Diagnosis is made by palpation of the sialolith
within the duct, skull radiographs, ultrasound and/or sialography. Treatment entails removal
of the calculi, with cannulation and lavage of the duct to remove any residual debris. Healing
is via second intention and closure of the duct in not necessary. Sialolith composition is
usually calcium phosphate or calcium carbonate.
Immune-Mediated Disease
Immune-mediated disease localized to the salivary gland is rarely recognized in the dog and
cat. Keratoconjunctivitis sicca and xerostoma (Sögren syndrome) have been reported in the
dog and are seen in association with other autoimmune diseases, such as rheumatoid arthritis,
systemic lupus erythematosus, and polymyositis.
Necrotizing Sialometaplasia
In people, necrotizing sialometaplasia is a benign, mildly painful, self-limiting disease
characterized by ischemic necrosis of the palatine gland with secondary proliferation
(metaplasia) of the salivary duct. Histologic changes in humans are difficult to differentiate
from neoplasia and include: lobular necrosis of salivary tissue, squamous metaplasia
conforming to duct and/or acinar outlines, preservation of salivary lobular morphology,
variable inflammation, and granulation tissue. In contrast, clinical signs in the dog and cat are
characterized by severe acute retropharyngeal pain accompanied by enlarged, hard
mandibular salivary glands, anorexia, gagging, and vomiting. The underlying pathobiology is
unclear; however, traumatic ischemia is suspected to be the cause of the vasculitis and
thrombosis. It has also been hypothesized that this syndrome may be an unusual form of
limbic epilepsy. Treatment consists of surgical excision of the affected gland and multimodal
pain management or the short-term administration of anticonvulsants for their antiemetic
properties. The milder disease in humans is self-limiting; in some cases, facial or pharyngeal
pain is more extensive. The prognosis is guarded in the dog, as some animals continue to
experience severe pain and vomiting, despite surgical excision and supportive care. The
prognosis in cats is more favorable for complete recovery.
Neoplasia
Salivary gland neoplasia, like all other salivary gland disease, is rare, with an overall
incidence of 0.17%. Within the realm of salivary gland disease, however, neoplasia represents
a relatively frequent condition, with 30% of all salivary gland biopsies being classified as
neoplastic on histopathology. Siamese cats are at higher risk than other breeds of cat, but there
does not appear to be a breed predilection in the dog, as previously reported.The glands most
commonly affected are the parotid and mandibular salivary glands, accounting for
approximately 80% of all the neoplastic cases reported. Clinical signs of salivary gland
neoplasia include a mass affect in the region of the gland, dysphagia, weight loss,
exophthalmos, and halitosis. The most common histopathologic type of tumor was simple
adenocarcinoma in both the dog and cat. Other reported histopathologic tumor types include
squamous cell carcinoma, mucoepidermoid carcinoma, anaplastic carcinoma, and complex
carcinoma. Adenomas are rarely reported and comprise only 5% of all salivary tumors.
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Fibrosarcomas, lipomas, mast cell tumors, and lymphomas may also incorporate salivary
gland tissue by direct extension and invasion.
Table 24-1. Distribution of Salivary Gland Tumors in the Dog and Cat.
Gland
Dog
Mandibular
30%
Parotid
50%
Sublingual and minor glands
12%
Zygomatic
4%
Indeterminate
4%
Cat
59%
19%
6%
3%
13%
In general, cats are diagnosed at a later stage of disease than dogs. A retrospective study
reviewing salivary gland neoplasia in the dog and cat revealed that early diagnosis
significantly improved survival times in dogs but not in cats. In this study, cats seemed to
have a more aggressive disease, with over half the feline patients having nodal involvement,
distant metastases, or both at the time of diagnosis.2 Furthermore, clinical staging was
prognostic in dogs, but not in cats. The median survival times for dogs and cats reported in
this multi-institutional study was 550 days and 516 days, respectively. Local infiltration and
metastasis to regional lymph nodes and lungs was common, as was local recurrence after
surgical excision [2]. Radiotherapy, with or without surgery, offered the best prognosis.
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