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The spring parsley plant is a short perennial that grows to about 6 to 10 inches high. The oval leaves grow to about eight inches long. The tiny yellow flowers grow in an umbrella shape with five petals each. They tend to grow on dry or sandy places around rocks, as does sagebrush. Although the toxins (xanthotoxin and bergapten) cause photosensitization throughout the bloodstream, hair and skin pigment protects your horse from most of the sun’s UV rays, so the amount of skin affected depends on skin and hair color. The areas around the muzzle, face, ears, and eyes are affected more often than the other areas because of the lack of hair.
Spring parsley (Cymopterus spp.) poisoning is a condition your horse gets after eating any spring parsley plant and then being exposed to sunlight UV (ultraviolet) rays. The toxins (furanocoumarins) cause photosensitization, which triggers a serious skin reaction that resembles a bad sunburn on areas of light skin that is exposed to the sunlight. The condition is so extreme that it can produce blisters, weeping lesions, and even damage your horse’s eyes. The furanocoumarins in the plant are spread from the stomach throughout your horse’s body via the bloodstream and when exposed to sunlight, these toxins release a photodynamic compound that reacts with the sun. This effect is also possible through plant contact with the skin and may occur right away or days later.
The symptoms are dependent on the amount of spring parsley that was consumed and how much sunlight or UV rays your horse was exposed to. The worst of the symptoms will be on white or light colored skin that is exposed.
There are almost 50 types of spring parsley plants. Some of the most common include:
The cause of spring parsley poisoning in horses is the consumption of any part of a Cymopterus plant, even if it is cut and dried. It is the furanocoumarins (xanthotoxin and bergapten) in the plant that trigger a photosensitivity in the skin and eyes after being exposed to UV sunlight. Although most horses will not eat spring parsley, they have been known to do so in some cases, which include:
If plant ingestion is suspected, the veterinarian may choose to walk the pasture and paddocks in an effort to locate the spring parsley plant as well as other noxious weeds that may be present on your property. If you did not see your horse eat the offending plant, the diagnosis may be more difficult because there are several other conditions that can present the same symptoms as spring parsley poisoning. Diagnosing any condition includes a complete physical assessment, vital signs, lameness examination, and body condition score.
The veterinarian will examine your horse’s entire body thoroughly, looking for areas of sun damage and blistering. Skin scrapings may be taken for microscopic analysis. A blood chemistry panel will reveal an increase in bilirubin, alkaline phosphatase, gamma glutamyltransferase, and sorbitol dehydrogenase. The veterinarian will likely check liver enzymes due to the possibility of hepatic photosensitization as well. Other laboratory tests can confirm the presence of high levels of porphyrin in urine, feces, and blood.
Treating your horse for spring parsley poisoning is mostly supportive and preventive because there is no cure for photosensitivity. Keeping your horse out of the sunlight and allowing grazing in the evening when the sun has gone down is the only way to prevent more damage to the skin.
Pain medication and corticosteroids may be helpful in relieving the symptoms of photosensitization. Topical steroid cream or ointment may also be used to help soothe the skin. Anti-inflammatory medications such as flunixin meglumine can also be given to decrease the severity of the disorder.
Horses usually recovery quickly after treatment and will continue to thrive once the poison is out of their system. It may be months before your horse is able to be out in the sunlight again, so it is best to continue night grazing until the veterinarian tells you it is okay to allow your horse out in the sun. Continue any medications prescribed and call your veterinarian with any questions you may have.
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