Bit & Bridle

Equine Rain Scald (Dermatophilosis)

Other Names

  • Rain Rot
  • Dermatophilosis
  • Streptotrichosis

What is Rain Scald in Horses?

Rain scald — also known as rain rot or dermatophilosis — is a common bacterial skin infection affecting horses, caused by the organism Dermatophilus congolensis. It typically affects the back, rump, and hindquarters — areas that receive the most direct exposure to rain — resulting in matted hair, crusted scabs, and underlying raw skin. In horses with white markings, the white areas of skin are often more severely affected.

Rain scald is closely related to mud fever (pastern dermatitis) — they are caused by the same organism, but affect different areas of the body. Rain scald affects the upper body (back, quarters, neck, head) while mud fever affects the lower legs. The same management principles apply to both.

While rain scald is rarely life-threatening, it causes significant discomfort, is unsightly, and if left untreated can become widespread and leave permanent scarring. It is particularly prevalent in the UK during prolonged wet periods in autumn and winter.

What Causes Rain Scald?

Dermatophilus congolensis is unusual in that it behaves like both a bacterium and a fungus — it produces branching filaments (hyphae) and motile zoospores that are released when the organism is reactivated by moisture. Under normal conditions, the organism lives dormantly on the horse’s skin or in the environment without causing disease. Activation and infection occur when the skin barrier is compromised by:

  • Prolonged moisture: Persistent rain, high humidity, or continual surface dampness softens and damages the outer layer of skin (stratum corneum), allowing zoospores to penetrate and germinate.
  • Weakened skin: Insect bites, wounds, skin abrasions, or existing skin conditions create entry points for the organism.
  • Compromised immune function: Horses with underlying health problems, nutritional deficiencies, or immunosuppression are more susceptible.
  • Ectoparasites: Ticks, mites, and biting flies can mechanically transmit the organism between animals and disrupt the skin barrier.

Why Some Horses Are More Susceptible

Horses with very fine or sparse coats have less natural protection. Horses in poor condition with compromised skin integrity are more vulnerable. Some horses appear individually more susceptible than others kept in identical conditions, suggesting individual variation in skin immunity plays a role.

Signs and Symptoms

  • Matted coat and tufts of hair: The earliest sign — hair becomes stuck together in distinctive “paintbrush” tufts or mats as the skin exudes serum. This gives the coat a spiky, patchy appearance, particularly over the back, quarters, and neck.
  • Crusty scabs: As the condition progresses, yellowish-grey to brown scabs form. Lifting these scabs reveals raw, moist, red skin underneath, sometimes with a purulent surface.
  • Hair loss: When scabs are removed, hair roots are often embedded in the underside, leaving bald patches. These areas heal in time but the coat may take weeks to return.
  • Pain and sensitivity: Affected areas can be sore to the touch — horses may react when the back or quarters are brushed or touched over active lesions.
  • Discomfort and irritability: Horses with widespread or established rain scald may show behavioural signs of discomfort — reluctance to be groomed, tacked up, or touched in affected areas.

Rain scald must be distinguished from ringworm (a fungal infection), which causes circular, scaly lesions and is highly contagious, and sweet itch, which primarily affects the mane and tail base and is caused by hypersensitivity to insect bites.

Diagnosis

Diagnosis is usually clinical, based on the characteristic appearance and seasonal pattern. Laboratory confirmation is occasionally needed:

  • Microscopy and culture: Scab material or surface swabs can be stained and examined under microscopy, revealing the characteristic branching filaments of Dermatophilus congolensis. Culture confirms the organism.
  • Skin biopsy: Rarely required but may be used in atypical or treatment-resistant cases to rule out other skin conditions.

Treatment

Step 1: Shelter and Dryness

Remove the horse from wet conditions. Stabling, rugging, or providing adequate shelter reduces the moisture that perpetuates the infection. This is the most fundamental aspect of treatment — without it, other measures will have limited effect.

Step 2: Soften and Remove Scabs

Soften the scabs with a gentle antiseptic wash — dilute chlorhexidine scrub (Hibiscrub) or a medicated shampoo (such as a selenium sulphide or antifungal shampoo) is commonly used. Allow it to sit for several minutes before gently working the scabs loose. Do not scrub aggressively, as this damages already compromised skin. The scabs must be removed to allow topical treatment to reach the infected skin underneath, and to expose the area to oxygen, which the causative organism cannot tolerate.

Dispose of removed scabs carefully — they contain viable organisms. Do not use the same brush on other horses.

Step 3: Dry Thoroughly

Allow the skin to dry completely after washing. A clean, dry towel followed by air-drying or gentle use of a hair dryer on a low heat is effective. Do not rug a wet horse.

Step 4: Topical Treatment

Apply an antibacterial topical preparation to the affected areas. Options include:

  • Dilute iodine or chlorhexidine solution
  • Antibacterial creams or sprays
  • Zinc oxide preparations, which create a protective barrier while providing antibacterial action

Systemic Antibiotics

In moderate to severe cases, or cases that do not respond to topical treatment, systemic antibiotics are required. D. congolensis is susceptible to penicillin, trimethoprim-sulphonamide combinations, and tetracyclines. A full course must be completed.

Isolation

While rain scald is not highly contagious between horses in normal circumstances, infected horses should not share grooming equipment, rugs, or saddle cloths with other horses. The causative organism can be transmitted via shared equipment or insects.

Prevention

  • Provide adequate shelter: Field shelters, trees, or access to a stable during extended wet periods significantly reduce skin exposure to sustained moisture. Horses should always have access to some form of weather protection.
  • Appropriate rugging: A waterproof turnout rug during wet weather protects the back and quarters. However, rugs that are too heavy, poorly fitting, or remain wet underneath can themselves cause or worsen skin conditions — check regularly under rugs and remove in dry weather to allow the coat to breathe.
  • Regular grooming: Brushing the coat regularly stimulates circulation, removes surface debris, and allows early detection of any developing skin changes.
  • Control insects: Fly spray, fly rugs, and insect deterrents reduce the transmission of organisms via insect bite activity.
  • Do not share equipment: Dedicate grooming kit, rugs, and brushes to individual horses, particularly during wet seasons.
  • Monitor nutrition: Horses in good nutritional condition have healthier skin with better natural resistance. Ensure adequate protein, zinc, and vitamin intake.

When to Call the Vet

Contact your vet if:

  • Rain scald is widespread, severe, or not responding to 10–14 days of topical treatment
  • The horse shows signs of systemic illness (fever, depression, loss of appetite) alongside skin lesions
  • You are uncertain whether the lesions are rain scald or another skin condition (particularly ringworm, which is contagious and has different treatment implications)
  • The horse appears in significant pain or shows lameness related to skin involvement

Frequently Asked Questions About Rain Scald in Horses

Is rain scald contagious to other horses?

Rain scald has low direct contagiousness between horses in normal circumstances — the organism is present in the environment and on many horses’ skin without causing disease. However, the organism can be transmitted via shared grooming equipment, rugs, and potentially through fly activity. Good hygiene (not sharing equipment, isolating severely affected animals from horse-to-horse contact during grooming) is a reasonable precaution, particularly on busy yards.

Should I rug a horse with rain scald?

This depends on the location and severity of the lesions. If the horse needs protection from further rain exposure, a lightweight, well-fitting waterproof rug can help — but the skin under the rug must be clean and dry before rugging, and the area must be checked and aired regularly. A damp rug over active rain scald will make it worse, not better.

Will the hair grow back after rain scald?

Yes, in most cases the hair grows back fully once the infection has resolved. Recovery time depends on the severity of the lesions and the individual horse’s hair growth cycle. Mild cases may look normal within a few weeks; severe cases with deep skin involvement may take several months for full coat regrowth. Occasionally, extensive scarring can affect regrowth in severely affected areas.

Can rain scald affect horses in summer?

Yes. Although rain scald is most common in autumn and winter in the UK, it can occur at any time of year when conditions are persistently wet and humid. Horses in irrigated arenas or those that are regularly washed and not properly dried can develop rain scald outside the traditional winter season.