PRIMARY PHOTODERMATITIS INDUCING PLANTS
The common cause of plant-induced dermatitis or skin inflammation is photosensitization, which can be induced by the consumption of plants. These plants induce photodermatitis as a result of the accumulation of photodynamic compounds in the skin, which when exposed to ultraviolet rays from sunlight fluoresce, releasing radiant energy that causes cellular damage. The less pigmented the skin, the more ultraviolet rays are able to reach photosensitive compounds in the skin, and the more susceptible that skin consequently is to photosensitization. Horses with completely pigmented skin are often fully protected even though the photodynamic pigments are present in the skin. In such animals, excess tearing or lacrimation and an abnormal intolerance to light may be the only manifestations of photosensitivity.
Photodermatitis may be primary or secondary. Primary photosensitization develops when horses eat plants containing photosensitive pigments, which are absorbed and accumulate in the skin. Two plants associated with primary photosensitization in horses are buckwheat (Fagopyrum esculentum) and St. John’s wort (Hypericum perforatum). Horses are also potentially at risk from plants such as spring parsley (Cymopterus watsonii) and bishop’s weed (Ammi majus), which contain photoreactive substances that induce primary photosensitization in other livestock and poultry and, therefore, may in horses.
Secondary, or hepatogenous, photosensitization occurs more commonly in animals than primary photosensitization. Unlike primary photosensitization, liver disease is the underlying cause for secondary photosensitivity. The plant toxins themselves are not photoreactive, but they cause liver damage. Once 80% or more of the liver is affected, it is unable to eliminate phylloerythrin, a normal breakdown by-product of plant chlorophyll, which then accumulates in the blood. Phylloerythrin fluoresces when exposed to ultraviolet light, causing the cellular damage resulting in photosensitization. The prognosis for animals with secondary photosensitization is always far poorer than that for primary photosensitization because the underlying liver disease is frequently irreversible and eventually fatal in most affected animals. The plants that are most frequently associated with secondary photosensitization in horses in North America are discussed in the article on ‘‘Liver Disease-Inducing Plants.’’ Those most commonly responsible for causing primary photosensitization in horses, buckwheat and St. John’s wort, are described here.
Treatment of the dermatitis due to photosensitization, whether it is primary or secondary, is to keep the animal completely out of the sun, preferably stalled. Sunlight through a glass window isn’t harmful, as ultraviolet rays are filtered out by the glass. Gentle daily cleaning of the skin with a mild organic iodide solution will aid recovery. Antibiotic therapy may be necessary if there is secondary bacterial infection of the skin.
St. John's Wort or Klamath Weed
Buckwheat
|
