Pastern dermatitis is also known as mud fever or greasy heal. It is not a specific disease but is a group of inflammatory skin conditions affecting the horse’s pastern and canon regions.
The dermatitis is initiated by mechanical injury. One of the most common causes of this in the UK is chronic moisture or frequent cycles of wetting and drying that occur during wet weather. Once the skin is damaged inflammation occurs and bacteria and fungi can invade. Infection leads to further damage, resulting in the redness, crusts, scales, erosions and scabs that are typically seen.
Pastern dermatitis occurs in all breeds and ages but is most common in heavy horses. Chronically moist conditions, alkaline soil, abrasion from riding arenas and poor stable hygiene all predispose horses to it. Hind limbs are affected more commonly than fore limbs, probably because of increased contact with faeces. White legs seem to be more often affected suggesting exposure to UV light is a factor. Overzealous frequent washing and scrubbing with strong antiseptics to try and prevent mud fever can also damage the skin and actually increase the likelihood of its occurrence.
Initiating factors can include trauma e.g. boot rubs, parasites such as feather mites and harvest mites, an allergic reaction to something the skin has contacted or immune-mediated conditions such as leukocytoclastic vasculitis.
An examination can help ascertain whether the case is one of common mild pastern dermatitis, where wet conditions have led to infection with bacteria such as Dermatophilus congolensis (the same bug that usually causes “rain scald”). Where more serious infections such as Staphylococcal bacteria, which cause much more pain, are suspected swabs may be taken. Scrapes and hair plucks may be used to look for fungal organisms or parasites such as mites.
In more serious cases, or those not responding to treatment, a skin biopsy may be taken to check for underlying causes such as leukocytoclastic vasculitis or other conditions such as pemphigus foliaceus (an autoimmune skin disease) that may resemble mud fever.
Treatment includes both management and medical measures. Obviously any underlying causes, such as feather mites, must be dealt with.
The horse should be kept in a clean, dry environment. Although hair and feathers can help protect the skin from injury, once the skin is diseased it is sometimes necessary to clip the air off in order to gain access to the skin so that topical treatments such as creams or sprays can be applied.
If the infection is restricted to the superficial layers of the skin then topical treatment is usually sufficient. The area is clipped, washed and dried. Once the area is clean try not to keep washing it unless it is soiled by faeces or discharge. Use only dilute antiseptics and rinse them off well, or use salt water (a teaspoon in a pint). A preparation containing antibacterial ingredients, and sometimes a corticosteroid (which is anti-inflammatory), can then be applied until the condition resolves. If the scabs are very stubborn and cannot be removed a “sweat dressing” may be used. The scabs are smothered in plain cream and the limb loosely wrapped in cling film and covered with a bandage for 12 hours. After this is removed the scabs can be easily washed off. Topical treatment can then be more effectively applied, rather than putting it on top of the scabs.
If there is significant pain or limb swelling (see image on right) present this may indicate deeper infection. Injectable or oral antibiotics with or without injectable or oral anti-inflammatories may be required to treat these cases.
Field management to reduce poaching and fencing off wet areas may help. There is some debate on whether it is best to hose off horses’ legs when they are brought in from the field, or to leave them to dry and then brush the mud off. The best course of action likely depends on how feathered the horse is and the soil type. If legs are hosed towel drying them may be helpful.
Some barrier creams and oils which reduce water contact with the skin also prevent the skin from “breathing” so they should not be used all the time. They can be useful in the treatment of horses that cannot be kept stabled during treatment or when horses are first turned back out after an episode of mud fever.
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