When first contacting the giant hogweed, your dog will not have any symptoms at all and this is why giant hogweed poisoning is often mistaken for other dermatitis conditions. It takes exposure to sunlight within the next few days to cause the phytophotodermatitis reaction. The topical exposure is bad enough, but this plant can also cause blindness if your dog’s eyes absorbed some of the sap. Between the pain, the huge blisters, and the swelling, that should be enough. However, the giant hogweed sap is also capable of causing blindness if it comes into contact with your pet’s eyes. The giant hogweed can reach almost 15 feet tall. Like the cow parsnip, giant hogweed has an umbrella-type flower head with hundreds of tiny white flowers.

The giant hogweed (Heracleum mantegazzianum) is often mistaken for cow parsnip, and they do look similar and cause some of the same symptoms. However, the giant hogweed symptoms are much more severe, sometimes causing disfiguring blisters that take months to heal and may leave dark scarring that lasts for years. If the sap from giant hogweed touches your dog, this can produce a disorder called phytophotodermatitis, which is a severe photosensitivity to sunlight. It is the furocoumarins (psoralens) in the giant hogweed that cause the reaction when your dog is exposed to sunlight after contact with the plant. The psoralens bind to the DNA and RNA, killing the cell membranes (skin tissue) and causing edema (swelling from fluid retention). The chemical is photosensitive, which means the rash does not show up until the affected area is exposed to strong sunlight.

In the first few hours, the skin will be itchy and red, progressing to more severe symptoms within 12 hours.

 Types

Giant hogweed, or Heracleum mantegazzianum, is from the Apiaceae family in the order of Apiates and is related to, and sometimes mistaken for Heracleum maximum, known as cow parsnip. These two giant herbs look similar to each other, but the cow parsnip is shorter and has less blooms than the giant hogweed. In addition, the giant hogweed is much more toxic than the cow parsnip and the side effects of exposure are a lot worse. Some common names of giant hogweed are:

Giant hogweed contains furanocoumarins, which is a toxin that causes severe phytophotodermatitis. The sap is found throughout the entire plant, but the stems and foliage have the highest concentration of the toxin.

To get a definitive diagnosis for giant hogweed, your veterinarian will go by the symptoms your dog has, a skin biopsy and histology, and physical examination. A skin biopsy is a simple and painless process done by scraping the affected skin with a special tool. The veterinarian can look at the sample under a microscope and will be able to see the damaged cells. A physical examination includes body temperature, blood pressure, breath sounds, weight, reflexes, and overall coat condition. Blood work will be done for a blood count, and blood chemistry profile will be done to check the levels of bilirubin, phosphatase, gamma glutamyltransferase, and liver enzymes.

You may have already washed your dog to get rid of the sap, but the veterinarian will want to do it again with a special shampoo that will also soothe the skin. Some other parts of the treatment plan may include:

The veterinarian will apply a steroid cream or spray for pain, inflammation, and itching. A bandage may be applied for protection from dirt and germs and the veterinarian will probably want your pet to wear an Elizabethan collar to prevent licking the area or pulling the bandage off.

Prednisone (corticosteroid) may be prescribed to reduce the pain and swelling. Oral antibiotics will also be prescribed in case of infection.

Do not leave your dog outside unattended for a long period of time and provide plenty of shade.

Once your dog’s skin has had a case of phytophotodermatitis, that area will be sensitive to sunlight for several years. It is extremely important to protect your dog from sunlight. You can use sunblock for dogs or protective clothing and provide plenty of shade for your dog when outside. It is best to keep your dog in during the peak sun hours of 10am to 4pm if possible. Otherwise, the phytophotodermatitis may return, and leave permanent scars.