Rainscald & Mud Fever

Does a change in weather change the risk of colic?

Dematophilosis (commonly known as rainscald, rainrot and mud fever) is a common skin infection of horses, characterised by pustules and crusting. The disease is caused by a bacteria called Dermatophilus congolensis. The bacteria infects areas of unhealthy or damaged skin and the spread of its spores are stimulated by excessively moist or wet conditions.

Common causes of skin damage which enable infection include biting flies or insects, prickly vegetation, damage from gear or tack and other inflammatory skin diseases.

Sources of moisture encouraging infection are rain, wetting and sweating under blankets or harnesses. Other predisposing factors are high ambient temperatures and humidity, poor nutrition, poor hygiene, stress and coexisting medical conditions (e.g. unmanaged Equine Cushing’s disease).

The main lesions in dermatophilosis are tufted raised bumps (papules) which rapidly merge and ooze. This often results is large (up to 5cm diameter) ovoid groups of hairs becoming matted together. On closer inspection these hairs are imbedded in thick crusts and when removed, areas of erosion or ulceration are revealed.

Active areas of infection contains thick, creamy pus which sticks to the skin surface and underside of the crusts. Horses with shorter, summer coats tend to have smaller and less oozing lesions. When active, affected skin areas may be painful but the condition is rarely itchy.

The healing or chronic form of the condition appears as dry crusting, scaling and hairless areas.

The lesions are usually found:

• Rump and topline
• Saddle area
• Face and neck
• Pasterns, coronets and heels

Secondary infections with opportunistic skin bacteria can occur and may produce more severe and painful lesions.

It is important to appreciate that the disease is contagious and may be spread from horse to horse. Spores from the bacteria can live in skin crusts for up to 3.5 years in favorable conditions. So crusts on infected animals and in the environment are important potential sources of infection. A horse exposed to infection can develop clinical signs from 24 hours to 34 days after exposure, however the average time from exposure to infection is 2 weeks.

In general, there are no breed, age or gender predilections for the disease and infections may occur in one or up to 80% of horses in a shared herd.

Diagnosis

Whilst dermatophilosis infection has a characteristic appearance, there are multiple other skin diseases that can look similar; such as ringworm, staphylococcal infection and mite infestations.

It is important to confirm the diagnosis as these conditions require different treatments.

Physical examination of the lesions is suggestive of dermatophilosis infection. However, definitive diagnosis is based on sample collection and testing; including cytology, skin biopsy and culture.

Treatment

• Keep the hair and skin dry.
• Remove and dispose of skin crusts. (Soaking the crust with chlorhexidine shampoo for 5-10 minutes may help loosen crusts and facilitate removal).
• After crust removal, topical application of 1-4% chlorhexidine to lesion area.
  
  • This can be used as a whole-body shampoo or rinse for 5-7 consecutive days and then once weekly until healing has occurred. Note: It is important to dry the skin thoroughly after application.
  • For small areas, daily topical application of a chlorhexidine ointment or spray is often effective.

Most cases (particularly mild ones) will spontaneously regress within 4 weeks if the animal can be kept dry and out of predisposing conditions. More severe cases require aggressive topical therapy and in some cases, systemic treatment is sometimes needed.

Management

• Improve hygiene, overall nutrition and the management of any underlying diseases (e.g. Equine Cushing’s)
• Avoid mechanical trauma to the skin (e.g. ill fitting tack).
• Keep the skin dry. In prolonged wet weather either keep horses undercover or use weatherproof rugs to keep the skin from remaining damp.
• Where relevant, control exposure to biting insects (e.g. insect repellents or covering with rugs)

Rainscald is a contagious disease, so separation of infected and non-infected animals is advised. Proper management and disinfection of shared tack, blanket and grooming supplies is crucial in controlling infections on farm (disinfection with bleach or Virkon products are recommended).

It is also important to note that previously affected horses do not develop significant immunity against the disease, so can become reinfected. Whilst rare, dermatphilosis can be transmitted to humans through direct contact with an infected horse. Use of gloves and handwashing can limit transmission.