Some recent posts on social media advocate not feeding Biotin to horses or ponies who have “Mallenders” or “Sallenders” – a thickening of the skin that leads to scaling and “dandruff” and is often the result of chronic and repeated infection by Chorioptes equi. Its more common in heavy breeds and horses with heavy feathering on the legs and in Winter. Chorioptes are small mites which live on the skin surface. It is most common on the legs but can spread over the rest of the body if not treated.
Hyperkeratosis of the skin is caused by inflammation due to the movement of the mites and these areas may also become infected. Hyperkeratosis is an abnormal thickening of the outer layers of the skin and is a defect of epithelial cells. Epithelial cells shed at regular intervals (we know that we lose skin cells all the time) but in hyperkeratinisation this process is disturbed due to excess keratin formation and accumulation. Hyperkeratinisation which occurs because of chronic irritation is due to a higher rate of proliferation of the epithelial cells.
Keratin is a fibrous protein that is the main structure in skin, nails, hair, hooves, etc and is produced by epithelial cells. Keratin is also essential for wound healing. Biotin (vitamin H, vitamin B₇) is a water-soluble B vitamin involved in a wide range of metabolic processes in animals and in other organisms, primarily related to the utilisation of fats, carbohydrates, and amino acids. Biotin deficiency in people can lead to brittle nails and thin or damaged hair and inflamed skin. In horses, there is reasonable evidence that biotin supplementation can improve hoof horn quality.
Hyperkeratosis is most commonly caused by mites. Biotin supplementation will not worsen this condition. Biotin deficiency may contribute to worsening of the condition based on available knowledge. A study in chickens found that hyperkeratosis was a direct effect of biotin deficiency.
There is no benefit to removing biotin containing feeds or supplements from horses with this condition and long term this may actually make it worse. The only effective treatment is either with off licence moxidectin or ivermectin leg wash from your vet.
Further information on treatment is available here. https://www.liverpool.ac.uk/media/livacuk/equine/equinepractice/Chorioptic,Mites.pdf