Aid to digestion

The digestive system of the horse is of crucial importance to the equine practitioner. Health and optimum function of this system are necessary for almost every aspect of a horse's life. Complaints of diarrhea, weight loss, poor weight gain, lack of performance and colic, among others, often accompany equine digestive disease.

The digestive system begins in the horse's mouth. While attention to the care, maintenance and problems of the teeth has greatly increased, another key part of this system has been largely overlooked.

The equine salivary glands are an important component of the total digestive system in the horse. These three paired glands — the parotid, the mandibular and the sublingual — represent a primary step in the digestive process and are responsible for the production of saliva.

Saliva is a viscous fluid that is 99.5 percent water and 0.5 percent salts and organic material — mainly enzymes and mucin.

While saliva is produced almost non-stop in the horse, its flow is greatly increased with the stimulation of chewing or mastication. Saliva production can reach 50ml/minute in adult horses and the composition of this fluid varies with the type of stimulus initiating its secretion. Variations in the sodium, potassium, calcium and bicarbonate concentrations in saliva are noted, depending on the nature of the food being chewed and the amount of saliva flow initiated. Qualities such as degree and type of roughage, Ph, taste and texture all influence saliva production.

Saliva provides lubrication during chewing and helps moisten the mouth and lips. It aids in the process of swallowing and provides a means of rinsing the mouth clean of debris that otherwise might create a culture medium for bacterial growth.

Salivary enzymes also help break down starch into simple sugars (maltose) as the first enzymatic step of the entire digestive process.

The parotid glands are the largest salivary glands, and are found on both sides of the head extending ventrally from the base of the ear and wing of the atlas to the convergence of the maxillary and linguofacial veins and to the caudal border of the mandible.

Each gland is about 20-25cm in length, with an average thickness of 2 cm. The parotid salivary duct begins at the cranioventral angle of the gland and travels rostrally along the medial surface of the mandible. This duct crosses underneath the mandible with the facial vessels and continues along the border of the masseter muscle to empty in the mouth opposite the third upper premolar.

The mandibular salivary glands lie on the inner surface of the lower jaw, extending from the atlantal fossa to the basihyoid bone. The mandibular gland is smaller than the parotid (roughly 19.5 cm by 2.6 cm.) and also has a strong, distinct capsule.

The mandibular duct leaves the gland on the medial surface. It travels over parts of the digastricus and stylo-glossus muscles, entering the mouth at a point referred to as the sublingual caruncle.

The sublingual salivary gland does not have a very distinct capsule, and the sublingual duct is found near the midline below the level of the mucous membranes on the floor of the mouth.

Common diseases, conditions

There are numerous diseases and conditions of these salivary glands and ducts that, although infrequent, should be familiar to equine practitioners.

The most common condition is a foreign-body obstruction of one of the major ducts. Horses present with a large, firm mass palpable over the middle portion of the parotid duct. Because the horse has two glands (right and left), there usually is little swelling or pain or decrease in saliva production following sialothiasis.

Salivary calculi or sialoliths can occur, but F. A. Al-Sobayil and I.M. Ibrahim of the College of Agriculture and Veterinary Medicine at Qassim University in Saudi Arabia feel that these ductal stones (sialolithiaisis) are a fairly rare occurrence, with few documented cases.

The cases of record, however, seem to show that donkeys are more sensitive to stone formation and that stones actually have been found only in the parotid salivary duct.

Analysis of these stones shows them to be multiple layers of calcium carbonate or calcium oxalate that typically develop over months, with the horse's body continually trying to cover the irritating particle much like an oyster creating a pearl.

Treatment of these salivary calculi often requires a surgical approach, with location and removal of the stone. Newer surgical approaches allow access through the buccal mucosa and, while sialothiaisis is almost always a one-sided problem, attention to this condition has brought up the fact that stones are twice as common on the right side of the horse and that adult females have a two-fold higher increase in the incidence of ductal stones.

Trauma to the jaw

Trauma can be a significant cause of problems with salivary glands and their related ducts.

If a small jaw laceration includes tissue along the medial surface of the head, then the parotid duct may be irritated to partially lacerated to completely transected.

These horses will show a constant leaking of fluid that increases when the horse is fed and begins to chew. Because the volume of saliva produced in the horse is quite large, they may show a stream of fluid being released from the site of trauma while the horse is eating. There may be so much saliva draining on the skin that care must be taken to keep the surrounding surface moist and covered with a protectant salve so that the skin does not become scalded or irritated.

Many of these horses are repaired surgically, and more and more papers are being published describing successful resolution of a partial parotid defect using catheters and time. Occasionally these horses may spontaneously resolve their partially torn and leaking ducts, but repair more routinely requires general anesthesia and dye-contrast examimation along with duct recannulization.

The techniques for dye-contrast studies of the various salivary ducts in the horse have been worked out and well described in "Sialography in the horse: technique and normal appearance," by Drs. Seifollah Dehghani, Mina Tadjalli and Alireza Seifali of the School of Veterinary Medicine at Shiraz University in Iran. "Because of the close relationship between the parotid gland, the guttural pouch, retro-pharyngeal and cranial cervical lymph nodes," writes Dr. Dehghani, "and because of the clinical importance of the retromandibular fossa in the horse, diagnosis of disease conditions of the salivary glands is of prime importance." Sialography or a dye study of the salivary ducts is the best way to clearly see which structures are involved in various conditions.

Infections and tumors

Infectious agents can significantly affect the salivary glands. The highest concentration of organisms in a rabies infection are found in saliva and in the lining of the salivary ducts.

Salivary glands can become infected by Streptococcus Equi bacteria, and there are many other infectious disease agents that can infect the salivary glands.

Tumors of the salivary glands do occur, with melanomas and sarcomas the most commonly reported.

In any case of salivary-gland or duct problems, it is important to infuse a dye into the partially or fully torn end segment. The films taken after an infusion of dye allow the practitioner to evaluate the damage/problem and begin formulating a solution.

Routine review and re-evaluation of your horses might help you recognize these various salivary-gland conditions and needs and to be ready to treat them.

Marcella is an equine practitioner in Canton, Ga.