Mud Fever

Under The Skin: The Truth About Mud Fever

As the name suggests mud fever is most frequently seen in horses during wet, muddy conditions but this is not always the case. Other names frequently used for this condition are greasy heel and cracked heel. The condition is more common on the hind legs and tends to affect the back of the pastern causing an inflamed and scabby area which may or may not be itchy or painful. Pastern dermatitis is more common in the feathered breeds but can affect any horses, ponies or donkeys.

Signs

mud fever

Initially, the skin at the back of the horse’s pastern or heel becomes inflamed and thickened before progressing around and up the leg. Usually, the lesions will only go as far as the fetlock but in extreme cases the cannon can be affected as far as the knee or hock. At the start, there may be some hair loss and crusting but as the infection progresses, it becomes more pronounced within the horizontal skin folds. There may be a wet look to the leg (greasy) where serum oozes from the affected area which may progress to a white, pussy discharge. These discharges will dry and harden into thick crusts that harbor the bacteria within. The more severe cases will become very painful and hot and the whole lower limb may become swollen and the horse may become lame.

Rain Scald - photo by Farmers Weekly

Occasionally, the mud fever bacteria may also infect skin over the quarters and along the back of a horse with typical “paintbrush” scabs, this is known as Rain Scald and is treated in a similar way to uncomplicated mud fever.

Skin structure

Diagram from Merck Vet Manual

The skin is made up of an outer epidermis, a middle dermis and an inner subcutaneous layer. The outer epidermal layer acts as a physical barrier to the environment. If it is damaged this allows bacteria to infiltrate the skin starting an infection and inflammatory reaction. The skin is also protected by hair follicles which grow from the dermis. They also have sebaceous glands which produce natural oils to help waterproof and protect the skin.

It is normal for the skin of a healthy horse to have many commensal microorganisms (bacteria, fungus) living on its surface causing no harm. When the skin barrier is damaged these “normal” bacteria can get into the deeper layers and multiply.

Causes

Traditionally mud fever was blamed on a bacteria called Dermatophilus congolensis; however it is now recognized that there are many factors that can contribute to the symptoms.

If the skin is damaged in any way microorganisms can take a hold, multiply and start to cause problems. Apart from Dermatophilis congolensis which is found in soil, other bacteria can also be cultured from mud fever lesions such as Staphylococcus and pseudomonas species as well as mites and fungi.

Leg mites (Chorioptic mange) can create damage and cause thickening of the skin, predisposing to bacterial entry. They will irritate the skin and lead to self-trauma, allowing bacterial infection to take over. Horses with leg mites are often very itchy, and stamp their feet. They may even bite and scratch their own limbs. The mud fever will not improve if the mites aren’t treated as well. Mites are common in breeds with feathered legs and a diagnosis of mite infestation can usually be suspected from the breed type and clinical signs but definitive diagnosis is made by taking deep dermal skin scrapes from the edges of lesions.

Wet conditions cause the skin to soften. Mud will rub against this softened skin causing abrasions to the surface. Certain soils and pastures appear to be more likely to cause skin damage than others. Lots of work in certain sandy arenas and schools can be quite traumatic to the skin.

Horse's legs close up

Lots of work in certain sandy arenas and schools can be quite traumatic to the skin.

Additionally, pastures with copious, rough vegetation or spiky weeds can cause trauma to the skin. Excessive leg washing will weaken the skin and remove the natural grease that acts as a barrier resulting in cracks which can allow the entry of bacteria.

Certain equine bedding can act as an irritant both physically (straw) or chemically (high ammonia from urine in deep litter).

Boots and bandages incorrectly placed or inappropriately used can also cause damage to the vital skin barrier.

Pastern and cannon leukocytoclastic vasculitis are immune-mediated conditions which can look very similar to mud fever and can predispose a horse to infection with bacteria. They are usually seen in the summer and are thought to be allergic reactions to either sunlight or something in the environment such as buttercups, other flowers/plants or bedding. They cause reddening (inflammation) of the skin around the pastern above the heel bulbs and tend to be seen on white legs and causes oozing of serum and scabs which are often very painful to touch. Strong sunlight on white legs can cause sun burn particularly on certain horse pre-disposed to photosensitization.

A very rare condition called pemphigus foliaceous can also cause crusting and scabby legs with or without itching, often around junctions of skin and horn i.e. at the coronet band and around chestnuts. This is an auto-immune condition where the body attacks itself and can be diagnosed with a biopsy. It is very rare but has been mentioned for completeness.

It is important to recognize that systemic diseases such as Cushing’s can reduce the horse’s immune system making it easier for bacterial to gain a hold and undoubtedly, certain breeds of horse are genetically pre-disposed to mud fever.

Diagnosis

Your vet will want to examine the horse and will ask you lots of questions regarding its management and if there are other horses on the yard that are affected, or have mites. Diagnosis can often be made on clinical signs alone with a detailed history. However it is deemed appropriate your vet may want to take acetate t-ape impressions or hair samples to look for fungal or bacterial infection. They may collect some of the scabs for examination or culture for bacteria, or want to take a skin scrape using a scalpel blade to look for mites which burrow through the skin.

If the problem is not responding or if the vet is suspicious of a neoplastic (tumour or sarcoid) lesion, or an immune mediated condition (pastern and cannon leukocytoclastic vasculitis or pemphigus foliaceous) they may decide to take a full thickness skin biopsy to send off for histopathology where the structure of the skin cells and layers will be examined under a microscope by a specialist.

Treatment

Treatment varies depending on the cause of the mud fever. There are many treatments available and no one is a “cure-all”. The basis is to treat any underlying conditions such as mite infection or contact allergy, remove infection and allow the skin’s natural barrier to heal.


Usually this will involve stabling the horse as this removes the horse from mud contamination and the wet-dry cycle which damages the skin barrier. Turnout in an arena may be possible if it is dry but sand can be an irritant and the horse may still get “chapped” skin.

Heavily feathered horses will benefit from having their legs clipped. This has several benefits including allowing easy visualization of the area affected, permitting the hair and skin to dry faster. In addition, scabs will come away from the affected part and topical treatments can reach the necessary areas rather than being stuck in the hair.

Removal of scabs can be a controversial area. Traditionally, bacteria are thought to live within the scabs and therefore removing them will remove bacteria from the site. Because Dermatophilus congolensis is an anaerobic bacteria (it cannot survive in the presence of oxygen) scab removal allows oxygen to bathe the affected area.

Hibiscrub (chlorhexidine) has excellent antibacterial and some antifungal properties if used appropriately. It should NOT be used NEAT – the ideal dilution is 0.1% and your vet can advise you how to dilute it appropriately as it can be bought in different strengths.

Standard recommendations are that you soak the legs in warm water and dilute Hibiscrub and remove any scabs that come away easily without forcing them. It is suggested that you repeat this every 3 to 4 days (every day will dry out the skin too much) and it is very important to rinse well with warm water and to use a clean towel to dry the legs thoroughly afterwards (ideally a clean towel for each leg so as not to spread any infection around!).

Antibacterial cream such as Silver Sulfadiazine (Flamazine) may be applied daily and after washing the legs.

Other treatments that may be prescribed on a case by case basis are

  • Systemic antibiotics; courses can vary from 3 days to 3 weeks and will be determined by your vet.
  • Topical antibiotics
  • Pain relief/anti-inflammatories- are important if the horse has painful scabs. Making the horse more comfortable is important for their welfare but is also likely to make them more compliant when it comes to picking off the scabs or applying treatment!
  • Mite treatments- these are a whole topic in their own right and will be discussed by your own vet.
  • Stabling during the day or using ultraviolet, turnout socks for horses with light-sensitive lesions.
  • Topical or systemic steroids for horses with immune mediated conditions.

Prevention & Prognosis


Mud fever is traditionally associated with mud coating the legs, however many out-wintered horses and ponies who live in muddy fields continue through the whole winter without developing any signs of mud fever. This concludes us to believe that it is not merely the mud but instead the constant wetting and chilling of the skin that is at the heart of the cause.This theory is supported by the fact that mud fever can often be a real problem on those yards where the horses’ legs are washed on a frequent basis throughout the winter months and left wet for long periods of time compared with those yards that never wash the horses legs and suffer little problem with mud fever.

Rather than daily washing, a better course of action is to leave the mud to dry naturally on the legs and then brush it off the following day when dry.

Leg bandages can be applied over the mud and these bandages will act to ‘wick’ away the moisture allowing the mud to be brushed off the next day. Traditionally leaving the lower legs unclipped was thought to protect the legs from infection but we now realise that mud fever is more common in hairier legs due to the fact they take longer to dry out and that it is very difficult to identify early lesions of mud fever. Waterproofing the lower limbs, particularly before turn out or exercise, is good practice and barrier creams such as ‘Sudocrem’ are effective for this. However, it is important to ensure the legs are clean and dry before application otherwise the cream will merely trap infection in the legs.

The Rotation of paddocks to avoid poaching and the use of electric fencing to block off particularly muddy areas such as those around gateways will aid to further minimise the risk of mud fever. Once infection has become established the affected horse should be removed from the muddy conditions and stabled until clinical signs have resolved. For primary mud fever cases the prognosis for a full recovery is very good providing that the appropriate treatment is carried out and rigorous, ongoing management is instituted including the preventative measures discussed.

However, if there is an underlying cause for the mud fever then resolution of infection will not occur until this cause has been addressed and treated such as leg mites or Cushing’s disease for example.