Common Ailments Caused by Plant Poisoning

The major and most common clinical signs of plant poisoning in equines are most easily grouped by the body area, organ, or system affected.  Plant toxins can have varied, complex, and dangerous effects on equines. 

These effects are best categorized by the type of illness or disorder; included are some examples of plants that may cause each type.

• Physical Damage

Physical damage from plants may result in ulceration, cuts or lesions to the mouth, tongue, gums, cheeks, nose, eyes, throat, and esophagus, and may possibly progress into the digestive tract. 

Examples of plants include puncture vine (Tribulis terrestris), sandbur (Cenchrus spp.), cheatgrass (Bromus tectorum), and medusahead rye (Taeniatherum caput-medusae).

• Colic

There are several types of colic, depending upon the action of the plant and toxins. Colic may be caused by a direct irritant, spasms caused by the nervous system, impaction or obstruction, excessive fermentation, and displacement in which portions of the intestine become displaced or twisted. 

Plants include leafy spurge (Euphorbia esula), Rocky Mountain iris (Iris missouriensis), horsetail (Equisetum spp.), bitter rubberweed (Hymenoxys odorata), mustards (Sinapis and Brassica spp.), nightshades (Solanum spp.), field bindweed (Convolvulus arvensis ), and buttercups (Ranunculus spp.).

• Pregnancy & Foals

Possible teratogenic (meaning resulting in deformation of foals) and gestational (during pregnancy) effects of toxins are spontaneous abortion, defects in the fetus or foal, decreased milk production, temporary or permanent infertility, prolonged pregnancy, and the need for cessation of the pregnancy.  These are much more likely and problematic if the plant toxin is consumed within the first trimester of pregnancy. 

Teratogenic plants include tall fescue (Festuca arundinacea), milkvetches (Astragalus spp.), locoweeds (Astragalus & Oxytropis spp.), lupine (Lupinus spp.), Sudangrass (Sorghum bicolor ), and California false hellebore (Veratrum californicum).

• Photosensitization

Equines with lighter colored hair, including light or white patches, will have higher susceptibility to photosensitization in these areas caused by plant toxins. However, the animal’s physiological system itself is not any more sensitive to these toxins than that of dark-haired animals. There are two types of photosensitization, primary and secondary.  In primary photosensitization, the plants are eaten and the toxin is absorbed, and the effects may be cumulative over time.  Dermatitis is due to photosensitization, and may be severe enough to cause loss of the entire skin, extreme pain, and to prohibit the animal from eating and/or drinking, potentially resulting in death.  In secondary photosensitization, liver disease is the underlying cause of the cellular damage.  A severely damaged liver is unable to eliminate by-products of chlorophyll, which then accumulate in the blood and cause cellular damage that presents as photosensitization. 

Primary photosensitizing plants include St. Johnswort (Hypericum perforatum), buckwheat (Fagopyrum esculentum), some ryegrasses (Lolium spp.), and burr trefoil or burclover (Medicago polymorpha).  

Secondary photosynthesizing plants include alsike clover (Trifolium hybridum), fiddleneck (Amsinckia menziesii ), houndstongue (Cynoglossum officinale), stickseed (Hackelia spp.), Kleingrass or klinegrass (Panicum coloratum), indigos (Indigofera spp.), and rattlepod or rattlebox (Crotalaria spp.).

• Liver Disease

There are relatively few toxins that directly cause liver disease.  Symptoms may occur only after the majority of the liver is destroyed. Acute liver disease is irreversible, and often results in death in a short time. 

Along with the species listed above for secondary photosensitization, species causing hepatotoxicosis include several species of ragwort, butterweed, and groundsel (Senecio and Packera spp.).

• Sudden Death

Equines do not normally select for plants containing the compounds causing sudden death, if other forage is available. There are three basic types of compounds causing sudden death: cyanogenic glycosides, cardiac glycosides such as digitoxin, and alkaloids.  Very small amounts of these toxins may be enough to cause sudden death, which often is the first clinical sign or symptom. 

Examples of plants include serviceberry (Prunus spp.), death camas (Zigadenus spp.), larkspur (Delphinium spp.), yew (Taxus cuspidata), hemlocks (Cicuta spp.), lily of the valley (Convallaria majalis), oleander (Nerium oleander), milkweeds (Asclepias spp.), dogbane (Apocynum spp.), and Sudangrasses (Sorghum spp.).

• Anemia

Anemia resulting from ingestion of plant toxins is due to changes in blood chemistry or physiology, with ensuing hemolysis or hemorrhaging. 

There are three common plants known to cause anemia: onions, both garden variety domestic and wild (Allium spp.), red maple (Acer rubrum), and moldy white or yellow sweetclovers (Melilotus spp.).

• Neurological

Neurological disorders are normally fairly evident, as the clinical signs usually include significant changes in behavior. Chemicals causing neurotoxicosis include terpenes, alkaloids, nitroglycosides, thiaminase, tremetol, and cyanogenic glycosides. 

Plants include yellow starthistle (Centaurea solstitialis), Russian knapweed or hardheads (Acroptilon repens), locoweeds, milkvetches, western brackenfern (Pteridium aquilinum), horsetails (Equisetum spp.), white snakeroot (Ageratina altissima) and Sudangrasses.

• Muscle weakness or lameness

Two of the causes of muscle weakness or lameness may be a deficiency in calcium or an excess of selenium.  Selenium toxicosis may be either chronic or acute. The chronic form is the result of the ingestion of Se accumulators for several months and is often called staggers, blind staggers, or Alkali disease. Acute selenium poisoning usually results in death before clinical signs are evident. Another cause of lameness is direct contact with the shavings, sawdust, or pollen of the black walnut tree. Laminitis may also be due to a severe episode of colic. 

Plants that may cause muscle weakness or lameness include hoary alyssum (Berteroa  incana) and day-blooming jessamine (Cestrum diurnum). 

Plants causing selenium toxicosis are milkvetches, woody asters (Xyloriza glabriscula), saltbush (Atriplex spp.), curlycup gumweed (Grindelia squarrosa), bastard toadflax (Comandra spp.), broom snakeweed (Gutierrezia sarothrae), and goldenweeds (Haplopappus spp.).

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