Dew Poisoning in Horses

Dew poisoning is one of several names given the crusty, painful skin condition involving the skin of the back of the pastern, sometimes extending up into the fetlock area. Whether you call it mud fever, greasy heel or scratches, it’s characterized by the formation of thick, tightly adherent crusts/scabs. The skin will often show deep cracks, even bleeding when the horse moves. The pain this causes the horse is probably most similar to a paper cut, or those cracks you can get on your hands or feet when your skin is extremely dry.


Dew: The name dew poisoning was coined when people saw problems start after horses were turned out to pasture, although dew’s role in this isn’t clear. In horses with long hair on their fetlocks, the combination of saturated skin and hair could lead to conditions favorable for growth of the ”rain rot” organism, Dermatophilus, similar to how long winter coats that repeatedly get wet by rain is a recipe for rain rot elsewhere on the body.

The organism can get into the area through any microscopic breaks in the skin. Since the skin along the back of the pastern is close to the ground and poorly protected by hair, this likely happens all the time. It’s been suggested that cycles of wetting and drying may somehow damage the skin, but there’s little support for that idea.

Mud: A thick mud pack sets up conditions at skin level that favor the growth of Dermatophilus and other organisms. As the mud dries, it may also set up conditions of mechanical irritation.

Mechanical Irritation: Horses exercised in sand, over stone dust tracks, or on dry, dusty terrain can trap small particles in the folds of skin along the back of the pastern when they move. The particles then irritate the area. This is probably the most common cause of pastern dermatitis in performance horses, and fits the name ”scratches.” Some riders that use these rings routinely rinse their horse’s legs after every ride to remove the tiny particles. Thistles and other plants with sharp edges may also damage the pastern skin of horses on pasture.

Plants/Photosensitization: Many people think clover causes pastern dermatitis. However, the only possible connection between plants and this skin condition is by photosensitization. Clover and its close cousin, alfalfa, have been suspected on occasion to have high enough concentrations of chlorophyll to cause photosensitization. Chlorophyll is converted in the body into another pigment called phylloerythrin, which is excreted into the bile. High levels of phylloerythrin cause photosensitivity.

However, usually the real problem causing the photosensitivity is a fungus growing on the plant. Some other plants, like St. John’s Wort and buckwheat, contain substances that are directly photosensitizing.

When there is an element of photosensitivity, the first change is a sunburn. Severe photosensitivity may even cause the skin to crack and ooze, but usually not to the point of forming the very thick crusts typically seen with dew poisoning. What it can do, though, is cause breaks in the skin that let other organisms in.

Foot Inflammation: Although this has not been formally studied, many trainers and owners have noticed pastern dermatitis is more common and/or severe on legs where the horse is known to have a foot/hoof problem. Why isn’t clear.

Mites: The Chorioptic species of mites typically produces weeping, crusting leisions on the pasterns and fetlocks of infected horses in the later stages of the infection. This problem is rarely seen in breeds other than drafts. Secondary infections on top of the mite irritation are likely.

Hygiene: Unsanitary conditions often get blamed. However, while it’s not good for the lower legs to stand bathed in urine or caked with manure, the truth of the matter is that the organisms usually involved are common inhabitants on the skin or in the environment in general and don’t cause problems unless the conditions that allow them to proliferate and invade the skin are present, i.e. moisture and/or irritation.


Moisture, low-oxygen tension at skin level and skin irritation are the conditions that allow pastern dermatitis to take hold. Reversing them is a critical first step to getting control:

• Clip any long hairs until they are below the surface of the crusts.

• Maintain the horse in a dry area.

• Use shavings for bedding instead of straw (high bacteria and fungal contamination) or fine sawdust (irritation).

• Get the crusts off using a wash.

Getting heavy crusts off is far easier said than done. They adhere tightly to the skin and trying to remove them is painful. Repeated washings with soap and water is suggested but rarely leads to a quick resolution, so you will still end up doing a lot of painful pulling.

Applications of oils or heavy ointments/creams may be recommended by some horsemen, but these products seal out oxygen and, if softening isn’t rapid, the condition may progress under the ointment. Using oils or heavy ointments/creams is most likely to be successful if the crusting is not excessively thick.

We would try an ointment that also has a good broad spectrum activity against organisms, such as Animal Legends Tea Tree ADE (, 800-635-2044). Never put straight tea-tree oil on the skin, especially skin that is already irritated.

If crusting remains a problem, our favorite solution is to use a tea-tree-based sheath cleaner, such as Triple J (, 888-778-8100), to soften and lift the crusts. If your horse is irritated by tea-tree oil, use a gentle Betadine surgical wash for cleansing, which can be found at your local pharmacy.

Gently rub involved areas slightly, add a little water to the cleaner in the palm of your hand then rub it into the crusts well. Allow this to sit on the skin for five to 10 minutes then rinse. One or two treatments usually are all that is needed to remove even thick crusts. The sheath cleaners are formulated to work on tough, thick deposits but also to be very gentle to the tissues.

Once the areas are open, a daily cleaning with the sheath cleaner will lead to rapid healing in most cases. Avoid working the horse or turnout until there are no more deep breaks in the skin. In the later stages of healing, and to treat any hint of it returning, condition the skin of the pastern by rubbing in Desitin (zinc-oxide diaper-rash cream), Corona Ointment (, 800-241-6996) or an aloe-based gel.

If swelling, redness and crusting continue to return almost as fast as you can stay on top of them, you have a tougher deep-seated infection. For this, try one of our homemade recipes (see page 21). Some cases benefit from the application of a cotton/gauze bandage over the medication. You can hold the bandage in place with Vetrap.

An Ounce Of Prevention

Pastern dermatitis is not only difficult to treat, it’s painful for the horse. There may not be much you can do about the conditions that favor this problem developing, but if you are vigilant about checking the back of the pasterns as part of your regular routine, you will catch it long before the serious problems develop.

When you check the pastern, be sure to p ull any overhanging fetlock hair out of the way so you get a clear view. Also, be sure to check the area with the hoof in its normal position on the ground, so that any problems developing in the folds of the skin that form when the foot is off the ground and flexed aren’t missed.

A puffy/mushy feeling to the back of the pasterns, redness, excessive flakiness and definitely any cracks in the skin are all early warning signs. If you see these early signs:

• Limit the time the horse spends turned out in wet grasses and always dry the pastern when he comes in.

• Wash, rinse, dry and inspect the pasterns after the horse is worked.

If roughening or crusting begins to develop, switch to washing with the tea-tree-oil-based sheath cleaner, and condition the pasterns after drying using Corona or Desitin. Rub these products in well so that there is no residue left on the skin to trap small irritating dirt particles in the creases.

Bottom Line

Dew poisoning requires that you not be too rough on the skin and start with as simple a treatment as possible. If you don’t see improvement within two to three days, you’ll need to change treatments.

If redness, cracking/flaking increases, try another remedy. Avoid treatments that cause further irritation. We’ve found lanolin and some herbals may cause skin irritation.

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