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.
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