Why do spots and bald patches appear?

Pyoderma appears mainly as spots and bald patches, sometimes associated with itching. It is a very common disease among horses and can affect any part of the body. The age, sex or breed of the horse does not affect its likelihood of contracting the disease.

Pyoderma includes all skin infections caused by pyogenic bacteria (bacteria that cause pus to be produced), the most common of which are staphylococcus, streptococcus and dermatophilus congolensis (the cause of dermatophilosis in horses). Pyoderma can be a primary disease with no apparent cause or a secondary disease caused by trauma such as scratching, irritation, insect bites or parasites, which may be linked to poor environmental conditions (humidity, heat, etc.) or internal disorders.


Pyoderma most commonly manifests as a secondary infection, which may be caused by various factors:

    - Superficial causes, related to physical changes in the skin barrier: friction sores, harness sores, lesions from scratching (particularly scratching due to allergies or parasites); weakening of the skin by fungi; insect bites.

    - Immunological causes: weakening of the immune system following a virus, illness or physical or emotional trauma.

    - Metabolic causes: diabetes, frequent or inappropriate use of corticosteroids, food allergies or imbalance.

Secondary pyoderma is reoccurring: it can flare up repeatedly as long as the underlying cause remains untreated.

Primary pyoderma appears on healthy skin, where no particular trauma has been identified. It is usually caused by staphylococcus bacteria. Primary pyoderma is different from secondary pyoderma as it does not reoccur.

  Lesions and their development

There are various types of lesion :

- Pyoderma remains superficial at first : it is limited to the epidermis and affects primarily the hair follicles (follicular pyoderma). Papules then form, due to a build-up of inflammatory cells in the follicle. Bumps and areas of raised hair appear on the surface of the skin, sometimes with scabs at the base of the hair. These areas are usually very sensitive and swollen. Although such lesions may disappear without treatment thanks to the body’s immune response, it is more common for them to gradually spread. Bald patches appear, giving the coat a moth-eaten appearance. Lesions of this kind can appear anywhere on the body. When associated with harness sores, however, they are most frequently found around the girth, the neck, the back and the withers.

Dermatophilosis is a type of superficial pyoderma, which is caused by the bacteria dermatophilus congolensis. It proliferates in humid external conditions. The lesions caused by the bacteria appear most frequently on areas exposed to humidity or rain and on areas in which water accumulates, such as the croup, the back, the neck, the head and the legs. Horses kept in paddocks or on damp bedding are more likely to develop the disease. As with other superficial forms of pyoderma, the lesions appear first as areas of raised hair with scabs at the base. Hairs come out in clumps when pulled, which are held together by scabs at the base.

- Pastern pyoderma, also known as mud fever (not to be confused with mange). It manifests first as itchy micro-lesions that appear around the heel or pastern, where the skin is thin and sensitive. These micro-lesions become infected with the bacteria and subsequently worsen, creating cracks that ooze and weep. If left untreated, it can lead to lymphangitis (an infection of the lymphatic system), which requires veterinary attention and treatment with antibiotics. The most common causes of pastern pyoderma are environmental humidity, mud and repeated washing without drying, as well as abrasions caused by sand and photosensitivity around the pastern.

-  If left untreated, superficial pyoderma can develop into deep pyoderma, which affects the dermis. Infected hair follicles are destroyed, forming pus-filled micro-abscesses. This is known as furunculosis. The furuncles may then merge, leading to necrosis or skin ulcers (a stage of cellulite). At this point, the overall well-being of the horse is severely affected and it will appear despondent. 


In all cases, fast action is required to prevent complications. The infected area must be cleaned and dried. It must then be treated with an ointment or a bacteriostatic or antibacterial hydrating lotion (derfen cream ; derfongen, natjely).

To prevent the spread of the bacteria at the root of the infection, the environment and all equipment that comes into contact with the horse must be kept perfectly clean; bedding, grooming equipment and tack must be cleaned and, if possible, disinfected regularly. Avoid sharing equipment between horses to prevent cross-contamination.

As mentioned above, in most cases pyoderma is a secondary condition caused by another injury. The underlying injury must be identified in order to resolve the problem, otherwise the infection will reappear.

Changes in or weakening of the skin: what are the causes?


  • Skin allergies: DERE, photosensitivity, insect bites... Skin weakened by these factors can become a breeding ground for bacterial infections. Moisturise the area of skin in question and keep an eye on any wounds that appear. Regularly inspect socks, the nose and all other unpigmented areas, as the skin in these zones is particularly sensitive to UV rays. Certain plants, medicines and topical products can make the skin more sensitive or reactive to UV rays. Ensure that all medicines and products that you use on your horse do not increase photosensitivity. If the reaction is not caused by an unsuitable product or medicine, take a look around your paddock: you might find one of these plants (photosensitivity file)
  • Abrasions and irritants: Poorly fitting or worn equipment or tack, dirty bedding... Check that all equipment (including stitching and padding) is in good condition and fits well. Prevent rubbing. Ensure that any areas of skin that are in contact with the equipment are not bald, damaged or irritated. Seek advice from your peers or a professional if in doubt. Replace all poorly fitting equipment immediately — your horse will thank you for it.
  • Humidity and mud: If possible, move drinking troughs and hayracks regularly to prevent areas of trampled ground from appearing around them. Raise the floor level of all shelters to prevent running and stagnant water. Do not use rugs if the horse is wet or damp: humidity and heat encourage the formation of dermatophilosis. Equally, avoid leaving the horse in a rug that is too warm or in a waterproof rug in the sun, as this encourages sweating and leaves the epidermis damp, which increases the risk of pyoderma. If the ground is muddy or sandy, clean the horse’s legs regularly. Above all, remember to dry legs and pasterns thoroughly after washing. It is also important to ensure that the horse’s living areas (shelter, stable, etc.) are well ventilated, as poor ventilation encourages humidity, sweating and the build-up of dust and urine vapours, which weakens the epidermis.

Internal Disorders:

  • Food allergy: be alert to changes in diet. If the appearance of skin disorders occurs after a change in diet or food, the allergy to one of the components is a track to dig.
  • Immuno-depression: disease, emotional shock, injury .. search in your horse's antecedents
  • Metabolic problems: leanness, obesity, diabetes, dull hairs (signs of dietary imbalance or deficiency), dental problems, gastric ulcers.

Indicative information - Does not replace consultation with a veterinarian

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