All parts of the cow parsnip plant are toxic to cats, especially the seeds. When ingested or touched/contacted followed by exposure to UV light, the toxins cause a photosensitive reaction which can be quite severe. If your cat is in contact with cow parsnip or accidently ingests it, you should remove the cat from exposure to UV light and seek veterinary care and advice.

Cow parsnip (Heracleum maximum) is a tall, invasive, weedy herb, native to North America, that grows 6 feet tall with 2 to 2.5-foot leaves and one-foot wide, white flowers. It is often confused with giant hogweed, which is a taller plant and wider, with flowers up to 1 meter across.

Cow parsnip toxicity results in photosensitivity in cats that have been exposed to its toxic compounds. Photosensitization symptoms manifest on the skin, ears, muzzle and vulva of infected cats as:

Cow parsnip contains the active toxins xanthotoxin, bergapten, and psoralen, which are furanocoumarins and nitrates. The toxins can be absorbed through the skin by physical contact with the plant, or by ingestion. In cats, absorption through the skin is the most common cause of exposure to cow parsnip. The toxins are photodynamic/reactive agents that your cat's body is unable to metabolize. These compounds become deposited in your cat's tissues and when those tissues are exposed to UV sunlight, a hyper-reaction occurs resulting in the tissue “burning”. Tissues that have little hair are especially vulnerable such as ears, muzzles, genitals and perianal area. The burned tissue blisters and splits, causing open sores. Damaged tissue may die and become infected. Eyes are especially vulnerable, as the photosensitivity extends to these delicate organs causing cloudy corneas and conjunctiva keratitis, which may cause permanent scarring and blindness.

Your veterinarian will perform a complete physical examination of your cat and examine skin sores, burns and lesions as well as perform a complete optical examination to determine the extent of the condition. A medical history will need to be provided. If your cat has been outdoors in an area where cow parsnip may be present, provide this information to your veterinarian. A sample of the plant that your cat was exposed to should be provided, if possible. Take care in handling the plant, as the toxins can be absorbed through the skin. Gloves should be worn and the plant should be bagged or contained so as not to cause further contamination. Your veterinarian may perform blood and urine tests or take skin scrapings, cultures or biopsies to rule out other conditions, including parasites, cancer or infection. If other conditions are not present and there has been exposure to cow parsnip, a diagnosis of cow parsnip poisoning in your cat will be made.

If your cat is suspected of, or diagnosed with, cow parsnip poisoning, immediately remove your cat from UV sunlight exposure. If ingestion has occurred recently, the cat's mouth should be flushed with water and vomiting induced to dispel any remaining plant material. Your veterinarian may alo perform a gastric lavage and administer activated charcoal to bind with toxins and pass them through the gastrointestinal system without further absorption. Your cat will need to be kept away from sunlight while the effects of photosensitivity continue, which may be a few days.

Steroids may be administered to reduce reactivity and swelling. If skin infection has occurred as a result of bacteria entering open wounds, antibiotics will be prescribed. Steroid and antibiotics appropriate for eyes will also be administered and medication to counteract photosensitivity prescribed. Preventing further contamination of wounds with insect repellent suitable for wounds may be recommended. 

If treatment is received in a timely fashion, skin sores should heal within several days. If damage to the eye has occurred, however, scarring and damage may be permanent and your cat's vision may be permanently impaired.

If treatment was not received until after tissue necrosis has become advanced, permanent scarring may occur.

Cats with cow parsnip photosensitivity should be kept out of sunlight until after the toxins have left the body, which may take several days.