Equine pastern dermatitis – also known as mud fever or greasy heel – is a reaction or infection that occurs on the skin of the pastern. 
In mild forms, it is characterized by itchiness, redness and hair loss on the back of the pasterns and heels. In more severe cases, skin lesions form and produce oily secretions.
Pastern dermatitis is not a single condition, but rather a skin reaction that has multiple causes. It may be caused by underlying diseases or infection by microorganisms penetrating the epidermal layer of the skin.
Physical assessment, blood testing, skin scraping, cell culture, and biopsy are diagnostic strategies used to confirm a diagnosis of equine pastern dermatitis.
Successful treatment of mud fever requires addressing the cause of the condition. Treatments typically involve medications to stop the infection and alleviate irritation within the skin so it can heal.
Horses that are vulnerable to pastern dermatitis must be managed carefully to protect their skin from damage.
What is Equine Pastern Dermatitis?
Equine pastern dermatitis has many names among horse owners including scratches, mud rash, mud fever, greasy heel, dew poisoning, cracked heel and grapes.
Horses with pastern dermatitis experience skin inflammation and develop redness, hair loss, ulceration, scabbing, scaling, and crusting on their pasterns.
In advanced cases, the skin secretes exudate (pus) and becomes thick, resulting in tissue granulation (proud flesh).  Horses can experience significant pain and may become lame.
The condition is more likely to affect the skin of the hind legs on the back of the pastern. It also more commonly affects non-pigmented (white) skin. 
Verrucous pastern dermatitis is a severe type of dermatitis that can occur in cold-blooded and heavy horses. It is associated with a greasy, odorous coating of the skin and calluses and wart-like growths in its advanced stage. 
The skin is comprised of an outer epidermis, a middle dermis, and an inner subcutaneous layer.
Hair follicles located in the dermis grow hairs that protect the skin. Sebaceous glands, also located in the middle layer of the skin, produce natural oils that protect and waterproof the skin.
When intact, the outer epidermal layer of the skin serves as a physical barrier to the environment.
If the epidermis becomes damaged, bacteria can permeate the outmost layer of skin. This can result in inflammation and infection as bacteria multiply in the deeper layers of the skin.
Equine pastern dermatitis can cause irritated and itchy skin that becomes very uncomfortable for affected horses. As the condition progresses, lesions become severe and painful. 
In some horses, the condition can progress to a serious bacterial infection of the soft connective tissues in the leg (cellulitis) if left untreated.
Equine pastern dermatitis is a commonly reported health problem. In a research study of 1,861 Swiss warmblood horses, 16% had experienced pastern dermatitis. 
Donkeys, horses, and ponies of all breeds are susceptible to developing this condition. However, breeds with feathered legs are at a greater risk of the skin reaction. 
Mud fever develops more frequently during the summer season when the skin is exposed to wet and muddy environmental conditions. However, it can develop at any time and may be precipitated by multiple factors.
Signs & Symptoms
In the initial stage, the skin at the back of the pastern becomes inflamed prior to lesions forming. Lesions typically develop on the pastern and may extend up onto the skin of the cannon bone or as far as the knee or hock as the infection progresses.
As lesions develop, hair loss can occur and the skin may appear crusted as scabs thicken. Serum or pus may ooze from affected areas causing additional crusts to form.
If the condition is allowed to progress, the skin can become warm as the body attempts to fight against the infection and blood flow to the area increases. The condition can result in swelling in the lower limb that causes severe pain and lameness.
Causes of Pastern Dermatitis
Mud fever most commonly results from a microbial infection. It can also be preceded by health conditions that cause skin damage, subsequently enabling infectious microorganisms to enter the skin and promote disease.
Multiple types of bacteria including Dermatophilus congolensis, Staphylococcus aureus , Streptococcus , and various species of Pseudomonas are associated with the development of equine pastern dermatitis.
Fungi and Oomycetes
A wide variety of fungal organisms including those from the phylum Zygomycota and the genus Malassezia are commonly associated with pastern dermatitis in horses.
Pythium is a soil-borne organism classified as a fungus-like eukaryotic microorganism (oomycete). It is also associated with pastern dermatitis.
Chorioptes are a type of parasitic mite known to cause pastern dermatitis. This type of mite causes damage to the skin (chorioptic mange), which allows microorganisms to enter.
Irritation from chorioptes mites causes the skin to become thickened and itchy. Horses with infestations of these mites are driven to inflict trauma on their own skin by rubbing, scratching, and biting to relieve irritation. Chorioptes mite infections are most likely to affect breeds with feathered legs. 
Pelodera strongyloides and Strongyloides westeri are species of parasitic organisms also associated with equine pastern dermatitis.
Photosensitization is a type of dermatitis caused by increased sensitivity of the skin to damage by ultraviolet light. Photosensitive chemicals in the skin react with sunlight and result in skin damage.
Equine pastern dermatitis may be caused by photosensitization in two different ways.
Primary photosensitization takes place when a horse consumes plants containing a photosensitizing agent that travels to the skin. Types of plants that can cause primary photosensitization include St. Jon’s Wort, buckwheat, and perennial ryegrass. 
Secondary photosensitization results from liver impairment. If the liver is unable to metabolize certain types of chemicals (from the diet or medication) properly, they accumulate in the blood and react with UV light in unpigmented areas of the skin.
Pastern Leukocytoclastic Vasculitis (PLV)
An immune-mediated condition, PLV can predispose a horse to skin infections.  PLV is believed to be caused by an allergic reaction to either sunlight, plants, or bedding.
The allergic response in horses with PLV leads to inflammation in the skin of the pastern and heel bulbs and the development of lesions that ooze serum. Once lesions form, they are vulnerable to invading microorganisms.
Pemphigus Foliaceus (PF)
A rare autoimmune condition in which the body attacks skin tissue, PF causes blisters and pustules to form on the skin that can provide an entryway for microorganisms.
Symptoms associated with the condition include itching, oozing, crusting, and scaling skin on the legs.
Chronic Progressive Lymphedema (CPL)
A systemic disease of the lymphatic system involving poor lymphatic flow and drainage, CPL promotes the development of skin lesions on the lower limbs. These lesions can become infected by microorganisms, leading to dermatitis.
Mud fever usually occurs because of one or more predisposing factors. The condition most commonly results when the skin has been weakened or damaged, thus allowing microorganisms to enter.
Several conditions can promote skin injury including the following:
Wet conditions: Mud and moisture cause the skin to soften, leaving it vulnerable to injury. When dried, mud may rub against the skin and cause damage.
Work in sandy footing: An abrasive substance, sand can irritate the skin and cause lesions.
Turn out areas with rough plants: Vegetation with sharp leaves or branches can wound the skin.
Contact with alkaline soils: The high alkalinity of certain soils – such as clay – can harm the skin.
Excessive washing: Frequently washing the skin weakens it and removes the natural oils that serve as a protective barrier.
Dirty bedding: Contact with wet and soiled bedding is harmful to the skin. For example, contact with bedding saturated with a high amount of ammonia from urine promotes skin irritation.
Incorrect placement of boots and bandages: Inappropriate use of or poorly fitted equine legwear can cause damage to the skin barrier.
Insect bites: Bites from ticks and flies break the skin and allow bacteria to penetrate. Insects can also promote infection by transmitting bacteria between horses.
Diseases that impair the immune system: Horses with systemic diseases – such as Pituitary Pars Intermedia Dysfunction (PPID) – have reduced immune function, which makes them susceptible to infection. 
Genetic predisposition: Although a specific gene has not yet been identified, certain breeds of horses – such as drafts – may be genetically predisposed to developing equine pastern dermatitis.
Photosensitization: A skin reaction that occurs with direct exposure to sunlight, photosensitization can cause lesions to develop on the skin after plants containing photosensitizing chemicals are consumed.
Thick and long pastern hair: Breeds with a large amount of pastern hair are more susceptible to skin problems as moisture is trapped close to the skin.
A veterinarian will make a diagnosis of equine pastern dermatitis based on a physical examination and consideration of clinical signs. Your vet will also evaluate your horse’s health status and management situation when diagnosing the condition.
Accurate diagnosis may require collecting a sample of microorganisms from the skin to assess under a microscope.
Acetate tape impressions are a minimally invasive method of collecting microorganisms from the skin and hair. This method involves using a piece of tape to sample hair or tissue, staining the sample, and viewing it under a microscope.
Hair and crust/scab samples may also be collected to culture cells of bacteria or fungi. This can help determine what types of microorganisms are inhabiting the skin and propagating infection.
A superficial skin scraping may be taken to determine the presence of mites on the skin. The material collected from the scraping is placed on a slide and examined under a microscope.
If your horse is not responding to treatment, your veterinarian may take a full-thickness skin biopsy. This type of biopsy is used to examine the structure of the skin cells and layers.
A complete blood count and chemistry panel may be necessary to rule out underlying metabolic conditions that could be suppressing immunity. Blood tests can also detect liver diseases that could be contributing to photosensitization disorders.
Multiple factors can make a horse susceptible to equine pastern dermatitis. Successful treatment involves identifying and addressing each contributing factor.
The goal of treatment is to stop the cause(s) of skin damage and infection, allowing the skin’s natural barrier to heal.
Horses recovering from equine pastern dermatitis should be kept in a dry environment until their skin heals. Affected horses should only be turned out to a dry area with non-abrasive footing that won’t cause injury or irritation to the skin.
Horses with light-sensitive lesions can be turned out to a dry area if wearing UV-blocking boots, bandages, or socks. Alternatively, they can be turned out at night in a dry area.
Horses with excessive hair on their legs may benefit from having them clipped. Removing hair enables the skin to dry faster and facilitates application of topical treatments.
Clipping hair in affected areas should be done carefully to avoid further injury to the skin. Scabs should not be forced away from the skin as doing so could promote infection.
The affected areas of the legs should be soaked in warm water and washed with an appropriate antibacterial and antifungal cleanser such as Betadine or Hibiscrub (chlorhexidine). Ensure chemical washes are properly diluted before use and rinsed off the skin before drying it.
Affected areas should only be washed every few days to avoid weakening the skin and excessively drying it out. Each leg should be dried with a separate, clean towel to prevent spreading the infection.
Removing scabs from skin affected by equine pastern dermatitis should be done gently to avoid additional pain for the horse. Scabs should only be removed once they have been softened from washing and if they slough off the skin easily and without force.
Medications may be required to combat infection, alleviate pain and support the immune system. Topical treatments are commonly applied directly to the pastern.
Your vet will prescribe an appropriate medication depending on the cause of the skin infection.
After affected areas of skin have been cleansed, topical medications can be applied.
Topical applications can be used to provide antibacterial, antifungal, and anti-inflammatory activity. Commonly prescribed topical medications include:
- Silver Sulfadiazine (Flamazine): An antibacterial cream that can be applied daily
- Fipronil, Ivermectin: Antiparasitic creams that may be prescribed if mites are causing infection
- Miconazole, Enilconazole: Antifungal treatments
Your vet may prescribe a course of oral or injectable antibiotics such as Trimethoprim Sulfa, Enrofloxacin, or Procaine penicillin G to treat the infection.
Depending on the severity of the infection, some horses are administered Phenylbutazone (Bute) or Flunixin Meglumine (Banamine) to help with the pain.
Corticosteroids may be prescribed in a topical, injectable, or oral format for horses with immune-mediated diseases such as vasculitis.
Treatment for liver disease is necessary if photosensitization is the underlying cause of the skin infection.
Sun-creams may be applied over areas of skin that develop lesions due to UV exposure.
The prognosis for horses with pastern dermatitis depends on addressing the underlying cause that led to the condition. If the underlying cause is not addressed, recurrence is likely.
The following strategies can help to prevent pastern dermatitis from recurring in your horse:
Minimize exposure to moisture and mud
Turn your horse out after the morning dew has dried. Don’t turn your horse out into an area with deep mud or standing water. If turn out is necessary, block off muddy areas if possible. Alternatively, keep your horse in a dry stall during wet weather.
Removing excessive hair on the pasterns can help to reduce moisture retention that may promote the proliferation of bacteria and fungi.
Don’t wash the skin too frequently
Avoid washing the skin daily as this weakens the skin. Allow mud to dry naturally on the legs and then brush it off when it dries.
Use a waterproofing agent
Use a barrier cream, ointment, or powder that is designed to provide protection for the skin against microorganisms and moisture. A barrier product should only be applied to clean and dry skin to avoid trapping microorganisms on the skin.
Treat conditions that impair immunity
PPID is an endocrine disease associated with poor wound healing and impaired immune function. Horses with PPID require treatment, typically with the drug Pergolide.
Avoid allergenic bedding
If skin allergies to a specific type of bedding are suspected, use an alternative. Treated and aromatic types of wood shavings contain chemicals that may induce skin hypersensitivity.
Block UV light
If Pastern Leukocytoclastic Vasculitis is present, avoid UV light exposure by stabling your horse during the day or using wraps.
Feeding a balanced diet including zinc and vitamin E to support skin health. Zinc helps immune cells respond to infections. 
A zinc deficiency in horses can cause skin abnormalities such as slow wound healing and may leave them susceptible to equine pastern dermatitis.
Omega-3 fatty acids
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