Mud Fever (Scratches or Pastern Dermatitis) in Horses

In most cases, mud fever starts with a wet, muddy environment. When the horse’s pasterns are continuously wet and muddy for a prolonged period, the skin becomes wet and porous and loses its ability to act as a protective barrier. Horses with feathers are particularly susceptible because the long hair traps moisture and dirt against the skin. Bacteria, viruses, parasites, or fungi that commonly live harmlessly on the surface of the skin begin to proliferate in this moist environment. These agents cause inflammation and damage to the tissue.

Mud fever can affect any horse, but particularly horses with feathers over the pasterns such as draft horses. Mud fever is most common in other breeds in the spring or whenever conditions are muddy or moist. Horses with allergies or photosensitivity are also susceptible.

Severity depends on the length of exposure to a muddy, wet environment, as well as the infecting agent. Severe cases may have additional symptoms such as:

• Swelling of the leg
• Lameness
• Development of nodules of firm tissue in the affected area
• Yellow, thick, foul-smelling discharge from the affected area

Mud fever may resemble or occur at the same time as other conditions affecting the lower limb, such as chronic progressive lymphedema or pastern leukocytoclastic vasculitis.

Mild cases of mud fever are manageable by experienced horse owners. Severe cases require prompt veterinary care.

Mud fever is often a complex condition with more than one contributing causal agent. Possible infecting agents include:

• Staphylococcus bacteria
• Dermatophilus bacteria
• Fungal infections such as ringworm
• Yeasts
• Chorioptes mite infection
• Chemical contact
• Photosensitization agents

In cases of photosensitivity, sunlight is the cause of mud fever. Underlying causes of photosensitivity include:

• White leg markings or thin hair that offer little protection from UV rays
• Ingestion or contact with photosensitizing agents such as clover pastures, St. John’s Wort, buckwheat or rye grass
• Liver disease

In cases of allergic reaction, contact with harsh chemicals from treated bedding or ingestion of allergens causes mud fever.

In cases where the horse is limping, has a severely swollen limb, or where management has not been sufficient to control the condition, veterinary care is required. Mud fever is sometimes multifactorial. In these cases, identifying underlying causes is difficult. Diagnostic tools include:

• Physical examination
• Hair plucks
• Skin scrapes
• Bacterial and fungal swabs
• Blood tests
• Tape impressions
• Biopsy
• Allergy testing

Treatment depends on diagnostic findings.

Depending on the causal agent, treatments may include:

• Antibiotics
• Antifungal treatments
• Antiparasitic treatments
• Corticosteroids
• Sunscreen or reducing sun exposure
• Treatment for underlying liver disease
• Using alternate bedding if contact allergic dermatitis is suspected
• Stalling the horse between 10 am and 4 pm to reduce sun exposure or using sunscreen in cases where photosensitivity is suspected

Keeping the horse's feet and legs clean and dry is crucial to recovery in both mild and severe cases. In cases of mild mud fever, management of the environment is often sufficient.

At-home treatments include:

• Cleaning with antibacterial or antiseptic soap and thoroughly drying the pasterns
• Avoiding over-washing and over-medicating the area

Prognosis with careful management and treatment is good. Recurrence is very common in circumstances where environmental changes are not feasible. Muddy or wet environments quickly reestablish the circumstances that lead to further outbreaks of mud fever.

If left untreated, serious conditions such as lameness and cellulitis are possible.

Strategies to keep the pasterns dry and reduce the risk of infection include:

• Avoiding muddy environments
• Keeping horses inside in wet weather
• Allowing the dew to dry before turning horses out
• Improving hygiene and reducing moisture in stalls
• Clipping or shaving feathers (often critical to managing mud fever)
• Removing leg wraps and boots when they are wet or dirty
• Disinfecting leg wraps and boots between uses
• Avoiding sharing grooming tools
• Using topical barriers (zinc oxide or petroleum jelly) to reduce moisture

Mud fever is extremely common in environments where the horse is muddy or wet for prolonged periods.

Management includes:

• Keeping the horse dry and clean
• Limiting sunlight, photosensitizing agents and allergens

Treatment includes:

• Antibiotic, antifungal and antiparasitic treatments
• Corticosteroids
• Sunscreen or sun avoidance
• Treatment for underlying liver disease