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Found primarily in fields, roadsides, and near water in North America, Europe and Southwest Asia, giant hogweed is a rather large perennial weed that can grow 15 to 20 feet tall. Stems have characteristic purplish spots and white bristles, while leaves are compound and deeply lobed with toothed leaflets. Small white flowers grow in umbels which can measure 2 ½ feet in diameter, and generally appear in summer. The seeds of the giant hogweed are elliptic shaped and flattened, with winged ribs covered in short hairs.
Heracleum mantegazzianum, better known as giant hogweed, produces toxin filled sap that can cause severe skin irritation when it comes into contact with skin. Your horse could be affected by burn-like lesions that can blister, and in extreme toxicities, could become fatal.
Giant hogweed primarily affects the skin, with direct contact resulting in phototoxic dermatitis, a severe skin irritation that worsens with exposure to sunlight. Simple rashes and blisters can cause infections, or in severe cases, can even result in amputation or death. Symptoms can include:
The cause of a giant hogweed poisoning is the plant’s toxic levels of furanocoumarins. These toxins are found in high concentrations in the leaves and roots, but it is generally the clear, watery sap that makes contact with a horse’s skin. The furanocoumarins contained within giant hogweed include bergapten, xanthotoxin, isopimpinellin, imperatorin, angelicin, and sphondin, some of which are carcinogenic, teratogenic, meaning they can cause congenital malformations, and are photoactive.
Photoactive toxins are enhanced when exposed to ultraviolet radiation, such as from the sun. Once contact is made with the skin, these toxins induce photosensitivity in your horse. When skin integrity becomes compromised by these toxins, a heightened reaction occurs when the skin is then exposed to sunlight, causing severe lesions that appear as burns.
While darker skin contains higher amounts of melanin, a skin pigment that protects against photosensitization, lighter colored skin has the least amount of protection from UV exposure, and can be more at risk for severe skin damage.
A diagnosis can be made from a physical exam, symptoms, and a history of exposure to giant hogweed. If you know your horse has had contact with this plant, bring a sample to your veterinarian so that a positive identification be made.
A diagnosis can be more difficult if you do not know that your horse has had contact with giant hogweed, although the rapid and severe appearance of the dermatitis will clue your veterinarian in to a possible plant poisoning. Your vet will need to run some tests to discover which plant may be involved, as well as if the plant is also affecting your horse’s liver. Testing can include blood work, serum testing, a urinalysis, and microscopic examination of skin samples. Various toxins and compounds can be detected with methods such as gas chromatography and mass spectrometry.
Treatment for any plant poisoning begins with removing the plant from the reach of your horse. Check your fields, pastures, trails, and fence lines, as well as your horse’s feed for giant hogweed, and ensure that your horse does not have any further contact with this plant.
As with any photosensitizing plant, keep your horse completely out of the sunlight while he is recovering. The affected skin is then gently cleaned with a mild iodide solution. Topical treatments may also be prescribed. Antibiotics are administered, particularly if a secondary bacterial infection has developed.
Recovery is good in less severe cases. You will be given a cleaning routine to perform, and possibly creams, lotions, or medications to administer. Skin and hair generally regrows within 2 to 3 months, while skin that has become hyperpigmented can take several weeks to clear. Horses that have had more severe trauma may be at risk for more serious conditions. In these cases, your veterinarian will discuss recovery for your horse’s particular case.
Control giant hogweed on your property with herbicides. If you have a small area of this toxic plant, you can remove by hand with tools, but be sure to wear gloves and appropriate clothing to protect yourself from the sap. Refrain from using a weed whacker, as it can spray the sap uncontrollably.
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