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What is Cow Parsnip Poisoning?

Cow parsnip has course, saw-tooth edged leaves and grooved, hairy stems. It is between two and five feet tall and has yellow flowers that are arranged in an umbrella shape. Some say that cow parsnip is similar in appearance to Queen Anne’s lace. 

Horses will eat cow parsnip with their normal forage. This is why it is important to practice proper pasture management and remove any plant that could be harmful to your horse. This includes within the pasture, along the fence line and anywhere else on your property where your horse can get to.

Cow parsnip, also known as giant hogweed, is toxic to horses and can cause extreme photosensitization. Cow parsnip is commonly found in fields and along roads throughout the United States. It is toxic both fresh and dried and all parts of the plant can cause illness to your horse.

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Symptoms of Cow Parsnip Poisoning in Horses

Cow parsnip may not look toxic, but it can cause your horse to fall ill quickly. Photosensitization can occur causing your horse to experience severe sunburn. Horses that are white or light skinned will be most affected by cow parsnip poisoning. Horses that have white or light skinned patches on their face or body will also be susceptible to cow parsnip poisoning. Brown or black horses are not affected as long as they do not have any white or lighter skinned patches on their body or face.

  • Blisters
  • Peeling of fur and skin
  • Redness

Sometimes, oozing sores will develop. Care must be taken to prevent infection from setting in.

Causes of Cow Parsnip Poisoning in Horses

Cow parsnip has toxins called furanocoumarins that will interact with the UV rays that are emitted from the sun. The reaction causes photosensitization. Cow parsnip is toxic during all stages of its growth, from seed to mature plant. The highest amount of toxins is found in the seeds. It remains toxic even when dried.  

Horses may be accidentally poisoned by cow parsnip by their owners when they are fed hay that has been contaminated with the plant. It is important to know what is in your horse’s hay. If you cut your own hay, thoroughly check your field prior to cutting for plants that are dangerous to your horses. If you purchase your hay, be sure to purchase from a source that is clean and practices good field maintenance.

Diagnosis of Cow Parsnip Poisoning in Horses

As with any plant poisoning, it can be difficult to definitively diagnose cow parsnip poisoning in horses. If you see your horse eating the plant, take a sample of the plant for your veterinarian to verify species and toxicity. If you do not see your horse eating a suspicious plant, collect samples of your horse’s hay and any plants in their pasture that may be the culprit. Your veterinarian can run analyses on these samples and possibly determine the cause of your horse’s illness.

Your veterinarian will also conduct a physical examination of your horse and do a complete blood count, fecal examination and urinalysis. These will help your veterinarian rule out other diseases and possibly pinpoint the exact toxins that are affecting your horse. 

Once your veterinarian has determined the cause of your horse’s photosensitization, they can begin treatments.

Treatment of Cow Parsnip Poisoning in Horses

It is important that you remove your horse from their pasture and put them in a place with plenty of shade. A barn would be the best option for your horse so they are completely out of the sunlight. The toxins react with sunlight, causing your horse to sunburn, but keeping them out of the sun, the toxins cannot react with the sun. 

Your veterinarian will provide a topical ointment to relieve the pain from the sunburn. Depending on the severity of the sunburn, your horse may need antibiotics to prevent a bacterial infection from starting. 

Horses that are severely sunburned may require supportive care and injectable antibiotics until their sunburn begins to heal. This will keep infection and dehydration from occurring.

Recovery of Cow Parsnip Poisoning in Horses

Your horse’s prognosis is good and they should make a full recovery from cow parsnip poisoning as long as you follow your veterinarian’s instructions. Keep your horse out of the sunlight until they have fully recovered and the toxins are no longer within your horse’s body. 

Remove any potentially poisonous plant from your horse’s pasture and be aware of what is being cut and put into their hay.  Quickly eradicate any cow parsnip that begins to grow in your horse’s pasture. When riding your horse through fields, wooded areas or along roadways stay aware of what they are trying to eat. Do not allow your horse to freely graze on any plants of which you are unsure of their toxicity to your horse.