The cow parsnip is a large perennial related to the carrot, with an umbrella-type group of small, white flowers which can grow up to eight feet tall. It is a native wildflower throughout North America and Canada, but many consider it a weed. Cow parsnip may be found growing on roadsides, in forests, fields and along river edges.

The furanocoumarins in the cow parsnip are a natural insect repellant and some Native American tribes used to rub the flowers on their skin to keep bugs away. In fact, some tribes used cow parsnip as food, but that has not been done for years due to the toxicity. Experts now know that the toxins in the cow parsnip can interact with DNA to cause mutagenic and carcinogenic changes. In addition, once your dog has been affected by phytophotodermatitis, a condition called hyperpigmentation can occur if you do not keep the skin covered. The inflammation caused by the furanocoumarins can be bad enough to cause necrosis, killing the tissues and may lead to amputation.

Cow parsnip poisoning is caused by furanocoumarins in the foliage of Heracleum maximum, which can produce phytophotodermatitis or photosensitivity if your dog eats the plant or if the sap is absorbed through the skin. In addition, exposure to cow parsnip can also cause ocular damage leading to permanent blindness. The furanocoumarins are to blame for the rash and blisters of dermatitis that can present as just a mild redness or severe enough to cause necrotic dermatitis with extreme inflammation. In rare cases, the swelling can get so bad that it may lead to amputation of a limb with necrosis. The burning and blistering may not show up until days later if your dog has not been out in the sun since eating the cow parsnip because it takes sunlight to trigger the reaction that causes it. Cow parsnip can also cause intestinal irritation with nausea and vomiting.

The cow parsnip has the ability to cause topical, intestinal, and optical toxicity, though the symptoms depend on the method of exposure and amount of toxins that were absorbed or ingested.

Cow parsnip (Heracleum maximum) is from the Apiaceae family and is known by several other names:

The cause of cow parsnip poisoning is the furanocoumarins. Your dog can be exposed by:

Diagnosing cow parsnip poisoning depends on the symptoms your dog has. For example, if your dog has phytophotodermatitis, the veterinarian will strongly suspect a plant is to blame and she will want to know what kinds of plants your dog may have been exposed to. If you did not see your dog actually consume or come into contact with cow parsnip, tell the veterinarian what kinds of plants your dog may have access to.

Your dog’s vital signs, weight, blood pressure, coat and skin condition will be assessed. A skin scraping will be taken and examined under the microscope to rule out bacterial or fungal infections. Blood chemistry and complete blood counts will be performed to determine the levels of bilirubin, phosphatase, gamma glutamyltransferase, and liver enzymes. Using a special light with UV rays less than 300 nm, the veterinarian will be able to get a look at your dog’s eyes to check for irritation or cloudiness.

Your dog’s treatment plan depends on the method and area of exposure as well as the severity of the symptoms.

The veterinarian will probably give your dog a corticosteroid injection to relieve some of the pain and swelling as well as a topical ointment or cream. An antibiotic will be prescribed to prevent secondary infection from scratching.

If your dog is dehydrated, the veterinarian will start intravenous (IV) fluids or inject subcutaneous fluids just under the skin that will slowly rehydrate your dog. Antiemetic medication will probably be given as well.

The veterinarian will apply an ophthalmic ointment to relieve the pain and prescribe oral antibiotics to prevent infection.

Keep your dog indoors during the day as much as possible until the pigmentation is back to normal. This may take a few weeks or even up to a year, depending on the severity. If you need to take your pet outside, use sunblock specifically made for dogs. If you have concerns about the healing process of your pet's skin, contact the veterinary clinic for a follow-up appointment.