Abstract

Mud fever, also known as scratches or pastern dermatitis, is a group of diseases of horses causing irritation and dermatitis in the lower limbs of horses. Often caused by a mixture of bacteria, typically "Dermatophilus congolensis", and "Staphylococcus spp", mud fever can also be caused by fungal organisms (dermatophytes). Photosensitization, chorioptic mange mites, contact dermatitis and other conditions also contribute to some cases. This condition is also known as "dew poisoning," "grease heel," or "greasy heel".

Mud fever affects most horses and ponies during winter and early spring, resulting in painful sores and scabs, which in severe cases can make a horse lame. Mud fever most commonly affects the pastern and heel area but can also affect the upper leg, the belly, and in some cases the neck area (also known as Rain Scald). Non-pigmented skin tends to be more severely affected.

Cause

Mud fever is caused by an infection of the skin by bacteria, including "Dermatophilus congolensis", and often "Staphylococcus spp." Dermatophytes (fungal organisms such as "Malassezia" or "Trichophyton") can also contribute, as can chorioptic mange mites. Photosensitivity or irritant contact may contribute in certain cases. Rarely, vasculitis can cause continued inflammation.

Under normal circumstances the skin acts as a protective barrier, preventing microorganisms from entering the horse's system and doing any damage. However, the integrity of the epidermis can become compromised through the abrasion of soil grit, sand or stable bedding on cold, wet skin. The continual wetting of the skin causes a breakdown of the protective barrier of the epidermis, allowing the bacterium to enter and cause infection.

Shared boots, wraps, grooming supplies, and horse-handler's hands can all serve as fomites, carrying the causative organism(s) from one individual to another. For this reason, mud fever should be considered a contagious disease, and general hygiene steps should be taken to limit cross-contamination.

Horses and ponies standing for hours in muddy, wet paddocks and at gates are a common sight during the winter months and early spring. It is in these conditions that mud fever thrives. Generally, those horses and ponies with white socks are more prone to the condition, although Mud Fever will affect horses of all breeds, ages and colours.

As with any bacterial infection, mud fever can become a very serious condition very quickly. The legs can become swollen and sore and open sores can become quickly infected. Often, such is the level of damage to the skin that these open sores can become very difficult to heal and can result in proud flesh, permanent hair loss and in severe cases the need for skin grafts.

In draft horses, particularly Clydesdales, Shires, and Belgians, a similar-appearing, but more serious condition occurs called chronic progressive lymphedema. This condition appears to be genetically-linked, starting early in life and progressing, causing thickening, fibrosis, and predisposing to secondary infections. This disease can progress enough to require euthanasia.

Clinical signs

Mud fever is a chronic but progressive dermatitis. It affects all breeds of horses, but it is most common in heavy draft horses like the Clydesdales. It often starts as a small red ulceration of the skin in the plantar pastern region of the legs. The lesions then grow and develop scaling with the formation of a crust, hair loss, edema, oozing and the release of a malodorous exudate. Skin fissures and papillomatous lesions can develop in chronic cases.

Prevention | Paddock management

Preventative action should be taken as soon as the paddocks start to become wet and muddy. Rotation of paddocks keeps horses from having to stand in wet, muddy ground. Electric fencing may also prevent horses from standing for long periods in the deep mud that collects in high traffic areas.

Prevention | Stable management

The horse should be kept in a clean, dry indoor barn stall with wood shavings for bedding to prevent a moist environment around the legs.

Prevention | Grooming

Washing the infected area with an antiseptic solution is part of the treatment for mud fever. However, washing a horse's legs repeatedly can remove the natural oils in the skin and may allow the condition to become established. The legs should be dried thoroughly after washing with antibacterial shampoo using paper towels,

Prevention | Care products

There are many products available to help protect the skin from the constant wet by forming a barrier between the mud and the leg. However, barrier creams have the disadvantage that the horse's legs are still covered in mud when they come in from the field. Alternatively, some form of covering for the leg may be used such as Anti mud fever boots.

Treatment

Effectively treating Mud fever is a three-step process.

1. Eliminate the predisposing factors.

2. Topical Care

3. Systemic Therapy (only for most severe cases)

Treatment | Eliminate predisposing factors

One of the most important factors in successfully treating scratches is moving the horse to a clean and dry environment.

- Use paddock and pasture rotation and fences to keep horses from standing in wet and muddy ground for long periods of time.

- Keep the horse stall in the barn clean and dry.

- Avoid using leg wraps, tack items or devices that could increase moisture retention.

Treatment | Local topical care

Steps to Prepare area for treatment:

1. Cut the hair in the pastern area with scissors or clippers if necessary. Be careful not to break the skin when clipping around the affected area as to not irritate the skin.

2. The scabs, crusts, dried serum and dirt should be removed very gently by using mild soap or natural mild shampoo, a mild chlorhexidine (maximum 2%) or povidone-iodine wash and a small amount of lukewarm water or physiological saline. Other washing methods include using a cleansing cloth or pre-moistened aloe vera wipes.

3. The area should then be patted dry thoroughly and carefully using clean towels or paper towels. The area is now ready for topical treatment.

Treatment | Topical therapy

The major objectives of the topical treatment is to kill any bacteria or fungi and to reduce the inflammation.

Treatment | Topical therapy | Anti-inflammatory

Good success has been noted by some with topical steroids such as triamcinolone and betamethasone applied to the lesions in conjunction with other treatments containing Kunzea.

Treatment | Topical therapy | Antiparasitic

Antiparasitic treatment is necessary only if mites or mange have been identified or if there is a need to repel insects such as Culicoides. Permethrins at concentration greater than 2%, fipronil spray or ivermectin orally have been used successfully. The essential oil of lemongrass (Cymbopogon citratus) is a good choice for a natural insect repellent.

Treatment | Systemic therapy

Systemic therapy is only for the most severe cases under the supervision of a veterinarian.

Treatment | Systemic therapy | Antibacterial/ antifungal

Enrofloxacin and trimethoprim potentiated sulfonamides are amongst the antibiotics commonly prescribed by veterinarians to treat severe bacterial infection of the pastern. Systemic antifungal therapy is rarely necessary, but ketoconazole, itraconazole, and fluconazole are sometimes recommended with variable success.

Treatment | Systemic therapy | Immunomodulatory therapy

In a few cases an immunosuppressive dose of steroids (dexamethasone or prednisolone, gradually tapered down) may be necessary to control immune- mediated equine pastern dermatitis and vasculitis.

If treated properly and early a full recovery from scratches can be expected. It does require moving the horse to a dry area, cleaning the lesions properly and using the correct products to control the infection and promote healing.