Anatomy and Function
The skin is the largest organ in the body – it keeps the outside away from the insides! It is the interface between the body and the environment.
- Protects the body from external injury
- Prevents excess loss of water, electrolytes and other macromolecules
- Regulates body temperature
- Controls blood pressure
- Allows the body to secrete and excrete via sebaceous and sweat glands
- Sensory perception
- Protects the body from solar damage through pigmentation
- Stores water, electrolytes, fat, proteins,vitamins and other elements
- Acts as a health indicator
- Prevents disease
The skin of the horse is a complex biological structure with different tissue types present for response, defense and repair.
The response of the skin to various agents, diseases or conditions is dictated by the types of cells and structures contained within the skin but the ways it can respond to these are limited. The skin responds to various stimuli and produces lesions. These lesions include scaling, crusting, creating a lump or bump (nodule, plaque or papule), changing hair quality or quantity, losing hair (alopecia) or ulcerating. As there are a limited number of responses, it is often difficult to diagnose conditions based only on the lesions seen – other information is required. Many infections, allergens or toxins will produce lesions of a similar appearance.
For example, the classical lesion below shows loss of hair (alopecia) and crusting. This is not a diagnosis and the causes can be many. The list below shows possible causes (in no particular order) of such a lesion – difficult isn’t it?!
- Dermatophyte (Ringworm)
- Dermatophilosis (Rain Scald)
- Insect hypersensitivity – Queensland Itch
- Occult Sarcoid
- Rub marks
- Pemphigus foliaceus
- Leukocytoclastic vasculitis of skin
- Parasite burden e.g. fleas or lice
Diagnosis of skin conditions.
Skin conditions provoke many responses for owners – annoyance. alarm and frustration being the most common. They can be concerning because they can alter the appearance of the horse or may interfere with performance. Some skin conditions are a sign of underlying disease, some are contagious and some are transmissible from horse to people.
The skin is one of the easiest body organs to examine because it is so accessible and abnormalities are easily seen. Yet diagnosis can be difficult and nothing replaces the experience of seeing a lot of cases and a veterinary surgeon’s clinical acumen.
The diagnosis of skin conditions involves being systematic rather than utilising random treatment and diagnostic tests. Sometimes the information that the vet asks about may seem irrelevant to you but is part of the exclusion process. Provide as much information as you can even if you doubt that it is relevant! The following summarises the process in diagnosing skin conditions:
THE DIAGNOSTIC PROCESS.
~ General History.
Including general health, environment, bedding, recent travel, feeding, contact with other horses, medications, worming, parasite control etc
~ Skin History.
– How long has there been a problem, onset of lesions, previous injury or trauma to the area, what they were like, how they changed, any itching, rubbing or biting, seasonal condition, any patterns, any treatments been used and the response
~ General Physical Examination.
– General health check – sometimes skin conditions may be associated with other disease processes.
– Examination of lesions (classifying type of lesion) and checking elsewhere including eyes, coronary bands and mouth.
After this the veterinary surgeon will have an initial list of possible diagnosis in their head. They will then suggest one of the following:
~ Initial course of treatment
– Based on diagnosis. If the treatment does not work, the case will be re-evaluated and the next level instigated.
~ Diagnostic tests.
– These are selected systematically rather than randomly with every test performed.
– Tests may include blood sampling, skin scraping, biopsy, bacterial or fungal cultures, allergy testing.
– Another opinion is sometimes useful – either as a fresh look at the case by another veterinary surgeon or by referral to a specialist or to a vet with an interest and experience in skin cases.
COMMON SKIN CONDITIONS.
Now we will have a look at some of the skin conditions that we see in equine practice. There are obviously many diseases that can occur but the following are the more common ones.
HIVES / URTICARIAL REACTION.
The presence of these often causes much alarm in the owner as they can look very severe and can come up very quickly.
Hives are a localised soft swellings (wheals) of the skin varying from small circular spots to large lesions, they may join up together to form huge lesions. The face sometimes develops swelling particularly around the yes and muzzle. They usually represent an acute allergic reaction of the body to something ion the environment or ingested. Common cases include insect bites or stings, exposure to toxic plants, pollen, dust, drugs, contaminants in feed or bedding or chemical agents that have come into contact with the skin. It is not always possible to work out what the cause is.
Usually it is only the skin that is involved but occasionally there may be a more severe response such as sweating, difficulty in breathing and laminitis.
Treatment usually involves removing the possible cause and the use of short acting but potent corticosteroids, some vets may use antihistamines too but the benefit is questionable. Most reactions regress over 24 – 48 hours.
Other reactions may be more chronic and long lasting when there is continued exposure to the allergen. Careful diagnostic investigation is warranted to find out why it is occurring.
Also called “bumps”, nodular collagenolytic granuloma or nodular necrobiosis. The horses present with small firm nodules or bumps, often on the back, neck withers and on the body. The lumps are usually 1 -3cm in diameter and may occur singularly or in clusters. The lumps are not painful or itchy.
The cause is not precisely known and there is always much speculation. It is generally widely accepted that they are due to a localized trauma to the skin and a reaction to insect bites seems to be the most common trigger.
Biopsy shows abnormal staining collagen surrounded by granulomatous inflammation containing eosinophils, lymphocytes and histiocytes.
Treatment depends on the number, size and site of the lesions. Single ones may be surgically excised or injected into the lesion with corticosteroid. Multiple ones may require a course of oral corticosteroids.
Warts, grass warts or papillomatosis is a common skin condition of younger horses. It is caused by equine papilloma virus. Lesions occur principally on the nose, around the eyes and occasionally in the body and legs. The lesions have a classical wart like appearance and are usually slightly pink. Sometimes many of them all join up together. They may be very unsightly!
The condition is usually self limiting and the warts usually disappear within 3 to 6 months. They disappear when the horse builds up immunity. They are contagious between horses by direct contact but not to people. They do not require treating but many people paint them with all sorts of homemade remedies! It is however important to maintain the health of the horse that had the warts so they have a good level of immunity.
Queensland ITCH – Insect Hypersensitivity/dermatitis.
Now for one of the biggies!! This is probably one of the most common skin conditions that we are called to see in horses, the Samford Valley area is a particularly bad area. It is also known as Sweet Itch, Summer Itch and Insect Dermatitis.
What is it?
It is a hypersensitivity reaction (allergic reaction) to bites by various insects. The hypersensitivity reaction is believed to be due to an allergy to a protein in the saliva of the biting insects. Many different insects can be triggers including Culicoides (midges), stable flies, hornflies, horse flies, deer flies and black flies. There may be a genetic predisposition in some breeds. Also other environmental allergens may contribute to the issue.
What are the signs?
The characteristic sign is the constant ever present itch (pruritis) – horses scratch, rub and bite at the affected areas resulting in hair loss and scabs. Initially there may be small nodules and raised lumps but these very quickly progress as the horse rubs and scratches. Over time with chronic cases the hair may never grow back and the skin becomes very thickened and hard.
Classically the lesions are found on the neck, around the head, down the back and at the tail head. However lesions can occur elsewhere including the abdomen and in really bad cases the whole body. Different insects bite in different places and also it depends upon what part of the body might be exposed to the insects.
Sometimes there may be secondary infection of lesions.
Treatment and Management.
As with most allergic diseases treatment is ongoing and no cure can be expected. Usually combinations of management and treatment strategies have to be used and are tailored to the individual horse and owner. There isn’t a magic cure and these cases can be hard work and time consuming to manage.
These include the following:
- Avoid areas and times where the insects are.
- Reduce exposure to biting insects.
- Stable horses between sunrise and sunset
- Fly mesh
- Keep horses away from standing water (breeding grounds for the insects)
- Keep horses away from manure piles and containers.
- Use physical barriers to stop insects from reaching the skin e.g. masks, rugs, boots etc.
- Apply insect repellents to the horse. Use according to manufacturers direction. Permethrin products appear to be the most effective. They have to be applied at regular intervals
- Topical agents.
- Iodine based shampoo – to help control secondary infection and remove crusting.
- Oatmeal shampoo to help soothe itch and scurfy skin.
- Topical anti-inflammatory drugs
- Topical corticosteroids may ease some of the itch.
- Systemic Corticosteroids
These may be required to help stop the intense itch. It is not a permanent treatment. The common ones used are dexamethasone or prednisolone. Corticosteroid drugs are powerful and effective drugs but they do have potential side effects (including laminitis, immune suppression and fertility issues) and not every horse can have then because of their risk. Veterinary surgeons will not prescribe them without seeing the horse.
THE KEY TO QUEENSLAND ITCH MANAGEMENT, IS REDUCING EXPOSURE TO INSECTS, SO TO GAIN CLINICAL CONTROL
If you have any queries about equine skin conditions – please do not hesitate to contact your large animal veterinary surgeon.