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
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.
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.