Equine pastern dermatitis is the name given to an inflammatory condition of the lower limbs in horses and usually appears as painful scabs and crusts. It is often termed mud fever. There are several known causes of this condition, including bacterial or fungal infection, environmental factors such as wet or muddy conditions or a disease called Equine leukocytoclastic vasculitis or ELV.  Cases of pastern dermatitis will often resolve with treatment and environmental or management changes. When cases fail to respond to first line treatment, the possibility of ELV should be considered. 

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What is Equine leukocytoclastic vasculitis?

ELV is typically characterised by lesions on white skin which tend to ooze and form crusts. Less commonly, lesions can occur on pigmented skin and tend to look scaly with areas of hair loss. Sunlight is thought to play a role in the development of this condition as it is seen on the unpigmented areas of the skin. However, there is often no evidence of exposure to a sensitising agent, so the cause can be unclear. In addition to this, not all of the unpigmented skin is affected, with lesions more commonly seen on the outside and inside of the legs, which can indicate that the condition is not caused by sun exposure alone. 

Equine leukocytoclastic vasculitis is a poorly understood disease. It mainly affects adult horses and usually occurs in the summer. It’s thought to be an immune mediated condition, where the immune system starts to attack the blood vessels. 

It may, in some cases, be triggered by factors such as sunlight, infections or a reaction to a drug. The possible causes of leukocytoclastic vasculitis can be differentiated into idiopathic (or it arises spontaneously with no known cause), photosensitisation, bacterial infection (with Staphylococcus or Dermatophilus), or secondary vasculitis (due to drugs or trauma).

How do we diagnose the condition?

Usually cases of ELV will have been first treated with environmental management and topical therapies for pastern dermatitis without a specific cause, without success. Cases of pastern dermatitis often respond very well to these treatments. It is quite rare to have a case that fails to respond to intensive, appropriate therapy and management. However, when the condition is still present after first line treatment, your vet may suspect ELV. 

A diagnosis is reached by carrying out a skin biopsy

Your veterinary surgeon may collect these biopsies in a hospital setting or often on your yard. The biopsies are taken under standing sedation and local anaesthetic to ensure your horse does not feel any pain during the procedure. The vet will usually take multiple biopsies to improve the chances of getting a diagnosis; but these biopsies are small and well tolerated by the horse. In preparation for the biopsy, your vet will usually ask that you don’t apply any medications or topical ointments to the biopsy area; as this could interfere with interpretation of the samples. Some vets may choose to treat with antibiotics first if there is a secondary bacterial infection present; but this is not always necessary. The biopsies are usually sent away to be examined and interpreted by a specialist pathologist with experience in equine skin disease.

How do we treat and manage cases?

Once a diagnosis of ELV is reached, unfortunately the outlook for resolution is not favourable. Even those that do resolve, may only do so temporarily. As it is difficult to determine an underlying cause, it is hard to prevent further episodes. We suspect that once a horse has suffered from this disease, they become more sensitive to other triggers such as sunlight; which may not have caused the disorder in the first instance. 

Treatment usually involves the administration of steroids at immunosuppressive doses

Initially treatment is with steroids given orally or injected. But if the lesions respond well, sometimes steroid creams or lotions alone can be used to keep the condition under long term control. Another drug that has shown some promise in research settings is Pentoxifylline. This drug is mainly used for older dogs, but it has some immunomodulatory properties. It can help to reduce the level of steroid given to the horse. 

There is a possible link between the administration of steroids and laminitis in some horses, particularly those that are overweight, in addition to other side effects. While these risks don’t always preclude the use of steroids, we always like to keep the dose to a minimum if possible. Another important factor in treatment is reducing exposure of the limbs to UV light. This can be difficult to achieve; but as it is thought that sunlight increases the risk of a flare up, it is certainly worth considering.

Your vet will make an informed decision based on all the medical history and risks and benefits of treatment to decide upon the most appropriate treatment for your horse. Treatment plans will vary from case to case.

What is the prognosis?

If your horse has been diagnosed with ELV, it is understandably a worrying time. It is always useful to keep a diary of events to monitor the lesions and to quantify flare ups. Always keep in contact with your vet: it is often a very painful condition. However, if your horse’s welfare is suffering and treatment has been unsuccessful, then euthanasia may need to be considered. Some cases will respond well and others may need long term care. 

Please keep in mind that cases of ELV are rare and are only diagnosed once more common conditions have been ruled out. Most mud fever is, fortunately, just mud fever!

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