The horse’s skin – its largest organ but often overlooked

We don’t usually think about it as such, but the horse’s largest organ is its skin. It may not seem like an actual organ in the usual sense of the word, but it is the primary interface between the body and the environment. The skin can also tell a lot about a horse and its general health and it a very sensitive organ, housing nerve endings that respond to the external stimuli such as touch and temperature.

The skin also produces vitamin D through the oils naturally present in the horse’s skin. A series of chemical reactions take place when these oils are exposed to sunlight and vitamin D is produced in the kidneys. Oils also give the skin a waterproofing capability which is important to horses living outside. Vitamin D has a major role in the level of calcium in the blood. So when a horse gets deficient, this can lead to poor muscle contraction, bone deformities or even fractures. So a few factors which can interfere with this are:

  • frequency of bathing (stripping natural oils from the coat)
  • coat sprays
  • fly sprays
  • time spent indoors versus outdoors
  • exposure to the sun

Functions of the skin

The structure of the skin also allows it to perform important functions for the horse. The main one being a shield to protect the body from external injury and to prevent excess loss of water, electrolytes and other macromolecules.

Some other very important functions include:

  • controlling heat loss or heat gain to regulate body temperature
  • controlling blood pressure
  • protection against the environment
  • protection against microbes, insects and parasites
  • regulating the immune response (for example repelling toxins from plants)
  • gathering and transferring information (sensory perception)
  • protecting the body from UV damage through pigmentation
  • acting as an indicator of general health

The location of the skin (between the environment and the inner body) and its size (a large area exposed to various forms of attack) make it one of the most important organs for defense against disease. Problems of the skin are also one of the first things a horse owner will notice and can be a sign of underlying issues and health problems. However, the skin has a limited amount of ways to respond to various stimuli. It can therefore be very challenging to find out what the visible signs or changes in the skin are trying to tell us. Recognizing the onset of a skin condition, which can be indicated by a change in the horse’s behavior such as scratching, rubbing or biting or relating the start of the problem to any changes in the horse’s feed (grains, processed or whole foods) or their environment (bedding, dust, herbicides, new pasture) can be very helpful in determining the causing factor of the skin problem.

Structure of the skin

The skin of horses is a complex and organized biological structure that has a number of built-in methods for response, defense and repair. It basically consists of tree layers:

Epidermis or outer layer: It is covered with hair and its main activity is to produce two types of protein: keratin (the hard part of the skin) and melanin (the pigmentation of skin). It is the superficial layer in the epidermis which is the most important as it is also continuously being shed and replaced. Hair provides mechanical protection, filters UV light and has an insulating function.

Dermis or ‘collagen’ layer: This layer contains blood vessels for temperature control and nutrient supply. This layer further contains the follicles of the hair and sweat glands, which play part in the temperature control as well as in the loss of water, salts and electrolytes. It also contains the oil glands where present in the body.

Subcutis: This layer contains the subcutaneous muscle (twitching muscle) and fat for insulation and is a shock absorber. It is also a reservoir for fluids, electrolytes and energy.

Fascia is a connective tissue which connects the skin to muscles or other underlying tissue. It plays an important role in movement of the body itself and the tissues and organs within.

Some of the most common skin conditions


A very commons condition of the skin which usually present as a localized swelling of the skin varying in size from small circular spots to large lesions and often represent an acute allergic reaction of the body to something in the environment or something ingested. It can be caused by insect bites, stings, exposure to toxins or toxic plants, drugs, contaminants in feed or bedding or chemical agents that came in contact with the skin. The acute reactions can be accompanied by more severe allergic reactions such as sweating, trouble breathing, swelling of eyelids and/or nostrils and sometimes laminitis.

Insect-induced dermatitis or Sweet itch

Another very common cause of dermatitis in horses are midges or Culicoides (no-see-ums). These insects feed on various locations on the horse causing lesions to appear on the mane, saddle area, rump or along the ventral midline of the horse. The culicoides trigger a hypersensitivity response in the horse’s skin through antigens contained in their saliva. This sensitivity may be seasonal or year round.Other insects that can cause itchy dermatitis include stable flies, horn flies, horse flies, deer flies and black flies. Sometimes the considerable itching may lead to ulceration of the skin surface due to continuous scratching by the animal affected.


Scratches represents a group of conditions, rather than one specific one. Bacterial folliculitis often presents as encrusted area’s of hair loss. Dermatophilosis is a bacterial infection, while sporotrichosis is a fungal infection affecting the deeper tissues of the skin. Scratches generally affect mature horses and produce lesions confined to the lower extremities that lack pigment. the lesions are usually multiple and show redness before oozing and crusting open sores develop. The affected limb can then develop swelling. UV radiation may play a role in this disease or an immune component may be possible as well.

Rain Rot

A form of dermatitis caused by a fungus and mostly common in wet, humid climates. The fungi can live in dormancy within the skin for some time and become active when the skin is compromised in some way. (high humidity, long rains, high temperature, attacks by biting insects).There are many more conditions of the skin that can affect a horse, but these are the most common ones.

It suffices to say that the skin plays an important role in the health of the horse, and since it is a big part of the horse, it needs the proper care and attention to keep the skin in a healthy and self-regulating state. Shampoo’s, sprays, conditioners… all play a role as well as proper nutrition to provide the skin with the quality protein it needs and all the immune boosting nutrients it can get, without stripping it from its naturals oils through use of harsh chemicals and potions.


Feed Your Horse Like a Horse: Optimize Your Horse’s Nutrition for a Lifetime of Vibrant Health , by Ph. D. Juliet M. Getty