This week we look in more detail at two conditions in horses – mud fever and rain scald – that are both caused by the same bacteria. We touched loosely on these problems when discussing winter equine problems late last year. Over the next two weeks we will learn about them in slightly more depth because the wet and warm weather we’ve generally had over the past few months have provided the perfect conditions to make both infections more prevalent.
The technical name for both conditions is Dermatophilosis because the type of bacteria which causes them is called Dermatophilus congolensis. When the skin infection is mainly found along the horse’s back it’s known as rain scald and when the area affected is localised to bottom part of the legs, usually between the top of the hoof and the fetlock region, it’s called mud fever.
What both areas have in common is that they are the parts of the horse that are most likely to get wet or muddy, and stay like this. Even horses that have rugs on can get a type of rain scald if they become damp and remain so from being sweaty under the rugs.
The warmth and dampness encourage the bacteria to multiply by releasing spores, which, when combined with the skin being more prone to damage due to being wet for a prolonged period, creates a perfect environment for the bacteria to really take a hold. Once the skin’s integrity has been broken, due to chapping, for example, it allows the bacteria a much greater opportunity to cause infection.
The bacteria are commonly passed from a carrier horse that has a long-standing, chronic condition to other horses by either direct (horse to horse) or indirect contact (via a brush, for example).
The classic clinical sign when looking at an affected area of skin with either of these conditions is described as “paintbrush” clumps of hair. The appearance is caused by the matted tufts of crusted hair stuck together with exudate (sticky discharge).
If removed, the skin underneath these scabby lesions often has a small crater, and pus, varying from grey-green to yellow-orange in colour, may be found underneath. The affected area will not normally be itchy but can be painful. In more chronic cases, the lesions are more subtle but include hardened areas with raised hairs and flaking skin. The hair often stays in longer than it normally would, which causes a rough appearance. In the early stages, or in more subtle chronic cases, you may feel the crusty lesions more easily than see them. If not picked up and treated, the scabs can become deeper with more chapping and cracking allowing the infection deeper into the tissue. This can lead to secondary problems that can be enough to make the horse lame and run a temperature. Areas of white hair (either along the back or on the limbs) seem to be more commonly affected.
Next week we continue looking at these conditions by moving on to discuss how they are diagnosed, treated and prevented.