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Toxic Plants

By: Dr Andrew Hoffman

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Colic and Diarrhea

Several plants fall into this category. One common culprit is Oak (Quercus spp), which contains tannins, the toxic principle. Tannic acid and its toxic metabolites (digallic acid, gallic acid, and pyrogallol) are contained in the leaves, especially when green, as well as the bark, blossoms, buds, stems, or acorns. Oak ingestion causes hard, dark, feces, and colic, later turning to bloody diarrhea, oral ulcers and signs of choke. Liver and kidney damage, and increased blood calcium to phosphorous ratio are additional signs.

Horses that wander into shrubs or woodlands, restricted from good quality pasture or hay, may ingest oak. Horses with bad teeth, given only complete pelleted feed, and young curious horses may taste the leaves. Horses out on the trail may experiment with leaf ingestion if tied near a source. The summertime, when leaves are plentiful, is the most likely time to see oak toxicity. In ruminants, oak causes kidney damage, with signs of urinary dysfunction and edema – these signs are generally not seen in horses.

Other plants that cause colic include the leaves and nuts from Horse Chestnuts and Buckeye (containing aesculin), Morning Glory (containing pseudotopine, an atropine-like toxin), and Solanacae plants (Jimson weed, Potato, and Tomato) which similarly contain atropine-like alkaloids. These alkaloids cause paralysis of the gut and colic, diarrhea, slowing of the heart and breathing, and pupil dilation.

Diarrhea is caused by numerous plants, but fortunately good management and knowledge of these plants can easily keep horses from harms way. Examples include Foxglove and Oleander that contain cardiac glycosides. These are ornamental plants that are readily identified – in addition to diarrhea, they cause signs of choke or regurgitation, and pronounced cardiac arrhythmias, leading to death.

Other plants that cause diarrhea, include Pokeweed (Phytolacca americans), Coffee or Senna weeds (Cassia spp.), the yellow field plant Buttercup (Ranunculus spp), Nightshade and Potato, Tomato, and Avocado (also causes sudden death, but not from flesh of fruit). Mountain Laurels (Kalmia spp), Azaleas (Rhododendron spp), Mountain Pieris (Pieres spp) and Maleberry contain toxins (grayanotoxin and arbutin). These are rarely eaten by horses, but can cause excessive greenish salivation, colic, frequent defecations, diarrhea, weakness and ataxia.

There is a tendency for goats, rather than horses or donkeys, to get into Rhododendron-like plants, where severe hypocalcemia and rapid death ensue from eating a few clippings or leaves. Castor bean (Ricinus communis) is a highly distasteful bean that can get inadvertently mixed into horse feeds, causing colic, diarrhea, depression, severe sweating, leading to severe muscle cramping, convulsions, and seizures or anaphylaxis at higher levels of ingestion. Fortunately, no recent reports (after 1945) could be found. Black Locust (Robinia pseducacia), a very common tree in the USA, contains a similar toxin in the leaves (called 'lectin'), that in addition to diarrhea, cause hypersensitivity reactions.

Slobbering (Hypersalivation)

Slobbering, frothing, or drooling are signs, that must be distinguished from diseases that interfere with swallowing. When caused by toxic plants, the salivation arises from the mouth – when the swallowing function is impaired, there is often saliva and food material coming from the nose as well. Salivation can stem from the irritation of a foreign body in the mouth or tongue, or from direct toxic stimulation of the salivary glands, that drain in the mouth.

Some plants are infested with fungi that cause salivation directly, e.g. alfalfa or clover infested with Rhizoctonia leguminacola; this fungus contains slaframine, a toxin that stimulates the salivary gland through histamine-like actions). A common location for a thorn or foreign body to lodge is the base of the tongue. The tongue can also get infected secondary to the plant material (thorn, awn) migrating into the tissue of the tongue.

Common plants that cause mechanical injury and slobbering include burdock, awn grasses, sand burrs, foxtail barley awns, prickly pear cactus, horse nettle, buffalo burr, porcupine grass, wheat awns, stinging nettle, and cockle burrs. The injury can be very superficial or can result in deep abscessation requiring surgery. The eye can alternatively get injured by these plants, or by fungal agents that enter the cornea after injury from one of these.

Plants that interfere with prehension and swallowing or food, in addition to a whole host of neurologic signs, include Russian Knapweed and Yellow-Star Thistle (these causing severe stupor).

Botulism, Equine protozoal myelitis, West Nile Virus, and Botulism must be distinguished from plant toxicities that interfere with swallowing or induce slobbering.

Liver Disease

The cause of liver disease is often mysterious in horses, but plant-derived toxins must be considered. By the time you see the disease, it may be well after the fact, because the liver has a great reserve, i.e.major damage is required before liver function is compromised. A toxic exposure may be long gone before liver disease appears.

Liver disease may be the result of multiple or long-standing exposure(s) rather than pin-point exposure. However, if a plant was responsible for liver disease, it is probably still in the pasture, and worth identifying. More than one horse effected with liver disease should definitely trigger a suspicion of plant toxicity. The plant toxin also leaves a mark for the pathologist that distinguishes plant-induced from other causes of liver disease. This is another good reason for an autopsy in horses with liver disease.

Photosensitization (sunburn)

Some plants contain "photodynamic" substances that accumulate in the skin. At this location, they are easily excited by the sun, and release "radiant energy" (heat) that simply burns the skin. White hair affords little protection to sunlight, so burns occur in white regions only.

Examples of plants that contain these photodynamic toxins include St John's wort (Hypericum) and Buckwheat. Don't go feeding your horse St John's wort no matter how sad he looks.

Liver disease (jaundice signs) with secondary photosensitization

Other plants cause liver disease directly, and the accumulation of photodynamic chemicals and photosensitization (sunburn) as a secondary effect of liver failure. Examples of plant that contain toxins (ca'lled "pyrrolizidine alkaloids") include Senecio spp (Tansy Ragwort, Lamb'stongue, Groundsels, and Butterweed -- ironically, these contain yellow, i.e. jaundice colored flowers). Others include Fiddleneck (Amsinckia spp), Hound's tongue (Cynoglossum spp), Rattlepod or Rattlebox (Crotolaria spp), Heliotrope, Alsike clover (Trifolium hybridum), and Kleingrass pasture (Panicum coloratum). Probably the most important of these are Alsike clover and Kleingrass, both of which cause there liver damage through a fungus (releasing "mycotoxins").

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