The parotid salivary gland is the largest saliva-producing gland in the horse.
It is located below the ear and extends distally along the back of the ramus to below the curve of the mandible.
There are two other salivary glands: mandibular salivary gland and sublingual salivary gland, both of which are paired, and also many small glands, which are found in the mucous membranes of the mouth.
The parotid salivary gland empties into the common parotid duct which courses along the anterior edge of the masseter muscle and opens into the oral cavity through the cheek at the level of the upper third premolar.
The amount of saliva excreted by the parotid salivary gland is variable, but we estimate the volume to be between 4 and 7 gallons per day for the average horse.
The main function of the saliva is to moisten and lubricate forage during mastication. Saliva is an ionic solution composed mainly of water and also contains some proteins, sodium, chloride, and sodium bicarbonate. This buffered solution is important in controlling gastric pH. Saliva also contains some minimal enzymes that begin breaking down food particles before entering the gastrointestinal tract in the horse. Salivary secretion is controlled solely by the autonomic nervous system. Chewing is the main stimulant for salivation, but salivation can be stimulated by visual or olfactory stimulation or irritation of the mucous membranes and some disease processes. The formation of salivary stones, known as sialoliths, occurs infrequently in the horse, and they are usually found at the distal end of the common parotid duct. These stones can be palpated in the cheek at the level of the first molar.
They are usually oval in shape with a smooth surface and do not cause pain when palpated. The size can vary, and with time they can become greater than two inches.
The stones that we have dissected all contain a grass seed in the center of the stone with deposit rings showing the buildup of calculus around the seed (Figs 62 and 65).
Over time these slowly increase in size. Blockage of saliva may occur at the orifice of the common duct if the stone becomes large enough, however we have not seen any ill effects in the oral cavity if this does occur. Salivary stones are usually unilateral and depend on the entrance of a seed into the orifice of the salivary duct to start the process.
Expressing the stones out through the duct orifice has been described in the literature by using quick forward pressure on the stone from the outside of the cheek. This author has never been successful in eliminating a salivary stone with this method and has always had to remove the stone surgically. There are two surgical approaches possible: incision over the stone through the mucous membrane on the inside of the oral cavity or alternately, an incision made from the outside of the cheek through the skin.(Fig.63)
An outside skin incision has some advantages, such as better exposure, good suture placement, accurate repair of the salivary duct, decreased chance of infection, and increased comfort for the horse during mastication immediately after surgery. Incisions in the mucous membrane in this area are difficult to suture and there is an increased chance of infection. Also, developing fibrous tissue from the incision may cause a blockage of the salivary orifice. Surgical removal of sialoliths is easily performed on a standing horse with sedation and local anesthesia. Reoccurrence of a stone is unlikely because the buildup of calculus seems dependent on a seed of some type to enter the duct and start the process. The seed found in the center of the stone in figure 62 is from cheat grass, which is common in pastures in our area.
A similar article concerning sialoliths from Saudi Arabia stated that the seeds found in the center of the stones were barley seeds in the four cases they described. Two sialoliths from different horses are shown in figure 65. The larger stone had a very foul odor and there was extensive inflammation within the salivary duct. The lining of the duct was thicker than normal, but there were no signs of exudate within the duct. The smaller stone did not show any signs of infection or odor. This may indicate the significance for removing sialoliths as soon as possible after diagnosis is made.
B. W. Fletcher, DVM