Stomatitis and excessive salivation in horses

"Doc, you've got to get here quickly. My horse is foaming at the mouth, and I think it might be rabies!" This is a fairly typical call, usually in the early spring after a stretch of wet weather. With further questioning, you discover that the affected horse is acting relatively normal but that a large amount of watery saliva is constantly dripping from its mouth, creating puddles in the barn aisle and anxiety in its owner (Photo 1).

Photo 1: Horse owners and barn managers can't help but notice the often impressive pool of saliva produced by horses affected with various gingival diseases or plant toxicoses. (Photos courtesy of Dr. Marcella)

Stomatitis, or irritation or ulceration of the mouth, is a fairly common event in horses given the aggressive and indiscriminant grazing nature of some horses and their exposure to many plants, shrubs and weeds in their environment. Excessive salivation is a primary sign of stomatitis, along with a reluctance to graze or eat and a decrease in performance related to avoiding the bit.

The causes of equine stomatitis and salivation can vary from mild and self-limiting to severe or even fatal in the case of rabies. As the spring approaches, and we once again enter "slobbers" season, it is a good time to review the causes of excessive salivation in horses so that serious conditions are promptly addressed and not-so-serious ones do not create undue panic. Be sure to first rule out common causes of excessive salivation in horses such as foreign bodies (sticks stuck in the mouth), choke (obstruction of the esophagus) and dental abnormalities.

Rabies virus

Rabies, caused by a rhabdovirus, can make an affected animal salivate and produce often thick, ropey saliva, but these unfortunate individuals are also typically showing more severe or advanced neurologic signs such as anxiety, irritability, sensitivity to being touched, other odd behaviors and incoordination or ataxia. The clinical signs of rabies progress quickly, and by the time drooling is observed, the condition is not likely to be confused with other diseases.

However, horses can manifest the "dumb" form of rabies commonly seen in cattle. These horses are lethargic and depressed, and their salivation may be interpreted as a case of choke. Most cases of rabies exposure in veterinarians or horse owners involve this type of presentation, and caution should always be used when investigating cases of excessive salivation in horses. Practitioners and anyone examining the mouths of these horses are encouraged to always wear gloves, and contamination of wounds on the hands and arms should be avoided.

Vesicular stomatitis

This serious condition affecting horses, cattle and pigs is also caused by a rhabdovirus. Vesicular stomatitis is characterized by fluid-filled vesicles on the tongue, mouth lining, nose and lips that rupture to produce ulcerated lesions. These horses may be unwilling to eat normally and will salivate or drool. Their breath will generally have a slight to more severe necrotic odor because of the nature of the ulcerated tissue in their mouths. They may have mildly elevated temperatures and appear clinically depressed. This disease can become systemic, and ulcerated areas may appear on the udder, sheath or coronary band, making these animals lame as well.

To rule out the possibility that the lesions are caused by other conditions such as sunburn or irritating feeds, blood testing is required. The ulcers usually heal in two weeks to two months. Until that time, the horse is infective and can spread the disease. Vesicular stomatitis is a reportable disease. For all suspected cases, veterinarians must contact state and federal animal health authorities. When a case of vesicular stomatitis is confirmed, the state veterinarian's office will quarantine the affected farm or ranch.

Chemical or mechanical irritants

Other causes of mouth ulcers and irritation in horses may involve chemical or mechanical factors that are most commonly caused by plants in the environment. Occasionally, horses will rub or chew on boards that have been treated or painted with various chemicals and coatings that can be irritating to mouth tissue. Certain leg sweats and blisters contain irritating compounds, and inquisitive horses that chew on or lick at their legs bandaged with these agents may develop oral irritations which can lead to excessive salivation.

Photo 2: All equine oral lesions tend to look similar, and these foxtail-induced ulcers on a horse's lips and gums are not very different from lesions caused by chemical agents in plants or by exposure to paints, cleaning agents, blisters or other environmental toxins.

Burdock, sandbur, raspberry canes and foxtail can all cause a mechanical irritation to a horse's mouth. Buttercups and marsh marigolds contain irritating chemicals that can damage equine gum and tongue tissue. The mechanically irritating plants have sharp, often bristly, seed awns that can come directly from the plant at pasture or when harvested and dried in hay. These fine hairlike awns tend to penetrate the tongue or oral mucosa and cause pustules and ulcers (Photos 2 and 3). Gingival damage may be mild to severe, depending on the amount of irritating material present in the feed or pasture, the stage at harvest (the drier the seed awns, in the case of foxtails, the more intensely irritating) and the appetite of the individual horse.

Photo 3: Foxtail awns, commonly found in hay, tend to be especially noticeable in wet years after droughts, such as the conditions that have been seen in the Southeast over the past few years. These bristly, hairlike filaments easily penetrate the gums and tongue and cause ulcers and irritation, leading to excessive salivation.

Some horses stop eating when they experience some irritation and lessen their exposure, while other horses continue to consume the offending material, causing more lesions. Excessive salivation will be easily observed in these cases and an oral examination will quickly reveal multiple areas of redness, ulceration and occasional hemorrhage. An examination of feed sources and pasture will generally produce a diagnosis and, in most cases, simple avoidance of the offending material will affect a complete cure. Moisture, sunshine and various environmental factors control how much irritating chemical is in plants in a particular pasture or how much weed growth occurs in individual years. It is possible for horses grazing the same pasture to be affected one year and show little to no lesions the following year.

Slobbers

Many cases of excessive salivation in horses are associated with no oral lesions and no evidence of gingivitis or stomatitis other than voluminous amounts of saliva—a condition known as slobbers (Photo 4). Horses typically seem unaffected by this condition but periodically, and seemingly at random, will release a large volume of saliva. These animals continue to eat and drink normally and do not show any changes in behavior or performance.

Photo 4: Horses with slaframine poisoning, or slobbers, constantly drool saliva but show little to no other clinical signs.

Slobber cases commonly occur in the spring and fall and are associated with humid, wet weather and the presence of clover in pastures and fields. Red clover is important because the fungus Rhizoctonia leguminicola preferentially infects this type of clover and, given the optimal conditions of temperature and moisture, produces the mycotoxin slaframine (1-acetoxy-6-amino-octahydroindolizine), also called slobber factor. Slaframine causes excessive salivation, lacrimation and weight loss with long-term exposure and may even cause diarrhea and colic in some individuals. Clover and other plants affected with R. leguminicola exhibit bronze to black patches or rings on their stems and leaves. Slaframine poisoning has consequently also been referred to as black patch disease.

The mycotoxin slaframine can be active in stored hay for up to 10 months, but its biologic activity decreases with time. Fresh hay may contain 50 to 100 ppm of slaframine, and concentrations above 10 ppm have been associated with clinical signs in horses.

Atropine has been used to provide some relief from diarrhea and salivation, and electrolyte supplementation is important to offset the high potassium losses in saliva. Most horses experience no significant clinical signs, however, and this "poisoning" quickly resolves (48 to 72 hours) after withdrawal from the contaminated forage or weather changes that no longer support the growth of R. leguminicola in pasture.

Dr. Marcella is an equine practitioner in Canton, Ga.