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Diagnosing Skin Conditions

December 2018 by Dr Robin van den Boom

The skin is the largest organ of the body, It protects from the environment, helps maintain body temperature, allows sensory perception and plays a role in the immune system.

Skin consists of a superficial layer, the epidermis, and a deep layer of connective tissue, the dermis. Hence, dermatology is the branch of medicine dealing with the skin. Underneath the dermis is a fat layer, which acts as a shock absorber and provides insulation, while muscle fibres within the fatty tissue allow the skin to move, for example, when shivering.

The skin contains pigment (melanin), which helps protect it against UV radiation, hairs, sweat glands and glands that produce sebum - a waxy substance to lubricate and waterproof the skin. It also contains numerous nerve endings, which allow the horse to sense touch, temperature and pain.

Skin diseases

A wide range of diseases can affect the skin and, broadly, these can be divided into infectious and non-infectious diseases. Infectious diseases are caused by bacteria, viruses, fungi or parasites. The non-infectious diseases can be present at birth or may develop later. They include immune and allergic disorders, environmental and nutritional diseases, endocrine problems (related to hormones), disorders of pigmentation and/or hair, and neoplasia (tumours).

Skin problems can sometimes be very easy to recognise and treat but, at other times, can be very difficult to diagnose and frustrating to treat. Being on the outside of the body, the skin is easy to see and the skin only has a limited number of ways it can react to infection, injury or disease.

Sometimes, this results in a pattern that is characteristic for a specific diagnosis and a vet (with some experience of dermatology) will be able to recognise this, even from a distance. At other times, the visible signs will not allow an immediate diagnosis to be made and further tests are necessary.

When dealing with skin diseases, it is important to realise the changes that we see can be primary - a direct result of the skin disease or condition, or secondary.

For example, an insect bite may cause a small swelling and itchiness, which can cause the horse to rub or bite the affected area, resulting in secondary damage, such as broken hairs, bald patches and even bleeding lesions.

The history

To gain a better understanding of what is going on, the vet needs to ask a number of questions that may reveal information about the possible cause of the skin problem or condition.

First, it is important to determine when the problem started. Some problems are seasonal, such as insect bite hypersensitivity, which occurs during Spring and Summer. Another example would be rain scald, a bacterial infection that occurs mainly on the horse’s back, which tends to start (as the name suggests) after a period of rain.

How and where (on the horse’s body) did it start? Fungal infections, like ringworm, often start where tack contacts the horse’s skin and can then spread from there. Some lesions, such as hives, develop very quickly; while others, such as skin tumours, generally develop much more gradually.

If the horse has already been treated, any change in the skin condition following this treatment can provide clues as to the cause of the problem. If skin lesions do not improve when washed with an antifungal medication, it is unlikely that a fungus is the cause.

Some equine skin diseases are paired with (severe) itchiness, or pruritus, while others are not. Sometimes, lesions that would be itchy in people are not in horses. An example of this is a fungal infection, like ringworm.

Protecting other horses

If horses don’t all have their own grooming equipment, infectious diseases, such as lice, can easily be spread from one horse to another and multiple horses will be affected. This can also be the case if horses share tack.
Further, the management of the horse is important. Feed, bedding, rugs, medication, shampoos, human skin products, etc. may all cause an allergic reaction in horses.

Allergies can develop on pretty much anything in the horse’s surroundings and it can be very difficult to determine to what the horse is allergic. Keeping a logbook or diary of everything the horse comes into contact with can help with diagnosis and treatment.

Another example of how the management may be important is the de-worming history. A certain type of worm, pin worm or Oxyuris equi, causes severe itchiness around the anus and rubbing of the tail, leading to broken hairs and irritation of the tail base.

Horses for courses

Information about the horses affected by the skin condition is also important as some diseases occur more often, or sometimes even exclusively, in certain types of horses.

There are diseases that affect young horses, such as papilloma, or warts, while others only affect older horses. An example of the latter is the curly hair coat associated with Pituitary Pars Intermedia Dysfunction (PPID), or Cushing’s disease.

The colour of a horse may predispose it to certain conditions. Melanomas - black skin tumours - mainly affect grey horses and most grey horses will develop these if they live long enough.

A horse’s breed can also determine what skin disease is likely to develop. Mites are often found on the legs of feathered-legged horses, such as Friesians and Shires, while hereditary or genetic disease usually affect one or several breeds. Extremely elastic skin occasionally occurs in Quarter Horses or Quarter Horse crosses.

Once this information about the horse and its history have been collected, a general clinical examination should be performed. This will reveal whether or not the horse is healthy, other than its skin disease. Some skin conditions may be the result of other diseases, such as sunburn, which can be secondary to liver disease in horses.

The examination

After a general clinical examination, an inspection of the skin and hair coat should be conducted. A general inspection will reveal if the horse has a Summer or Winter coat. Delayed shedding of the Winter coat may be a sign of Cushing’s disease.

Outside normal shedding season, it should not be possible to pull a horse’s hairs out easily.

If hairs are pulled out, they can be examined under a microscope to determine if they have been broken or have come out in their entirety. The hair coat of the horse should be shiny and a dull coat could reflect a severe systemic illness as the skin reflects the inner state of health.

A change in the colour of the hair may be the result of an old injury or trauma. White hairs often remain after a saddle sore, for example.

Following the general examination of the skin and coat, a closer, local inspection takes place. Some parasites, lice for example, can be seen with the naked eye, although a magnifying glass can also be used. If lesions are present, their type and distribution are observed.

The different types of lesions include lumps or bumps of various sizes, blisters, redness of the skin and crusts.

The presence of pus indicates infection, often caused by bacteria, while oozing is often seen when the blood vessels in skin are inflamed (vasculitis).

It is often useful to keep a record of the findings for future reference. This can be a written description of lesions or they can be indicated on a diagram. An alternative is to take photographs.

In this day of digital cameras and smart phones, it is easy to keep a digital record. This also allows these images to be sent to others, so a vet can have a look at the skin lesions even if she or he is a long way away. A series of photographs can be useful to document how lesions change over time.

Additional diagnostic tests

Sometimes, the general and local inspection of the skin will allow a diagnosis, or presumptive diagnosis, to be made. At other times, further tests will be necessary to confirm a diagnosis, rule out causes or discover what type of reaction is occurring within the skin.

A number of samples can be examined under a microscope. When hairs are observed under magnification, it is possible to see if the hair shaft is infected by fungi. Ectoparasites, such as mites, can also be seen and identified under the microscope. Skin brushings are collected, especially from the legs, and these are placed under the microscope.

Sometimes, mites will be seen straight away. If not, the sample can be left with the light of the microscope switched on. The warmth of the light will cause any mites present to start moving, making them easier to spot. The various mites that can affect horses have different shaped bodies and legs, and this allows us to tell them apart.

Another test is the sticky tape test. As mentioned before, certain worms (pin worms) can cause itchiness around the tail. These worms lay their eggs around the anus. By pressing sticky tape against the horse’s anus, these eggs can be collected. The sticky tape is then applied to a microscope slide and observed under the microscope, and the characteristic eggs can be seen.

If bacteria or fungi are thought to be the cause of a skin infection, their presence can be confirmed by performing a culture. This may also tell us which bacteria are present and what antibiotics can be used to kill them.

Care must be taken when interpreting results as normal equine skin contains many bacteria, which may not be the cause of disease.

Fungal culture is a slow process and it often takes 1 to 3 weeks before a result is obtained. An alternative approach, when a fungal infection is suspected, is to treat the horse with antifungal medication and, if this leads to a resolution of the symptoms, it is likely that it was, indeed, caused by a fungus.

A commonly used test in equine dermatology is the skin biopsy, whereby a sample of skin (including all layers) is obtained and thin sections are examined under a microscope. This may reveal a specific diagnosis or the type of reaction that is occurring within the skin, such as vasculitis, which is inflammation of the blood vessels in the skin.

Before taking a skin biopsy, the hair should not be clipped and the site should not be cleaned, as sometimes the ‘evidence’ is contained within the very superficial layers of the skin and could be removed by clipping and cleaning.

Often, the horse is sedated and/or a nose twitch applied before a biopsy is taken. Local anaesthetic can be used so the biopsy can be obtained safely (for the vet), although care should be taken that the anaesthetic solution is not injected into the skin to be biopsied. Usually, a punch biopsy is taken, using a 4, 6 or 8 mm diameter punch, depending on the site from which the biopsy will be taken.

The biopsy is placed in a fixative solution (formalin), very thin sections are cut, which are stained and examined under the microscope. The hole left after the biopsy is taken is usually not sutured and will generally heal in several days. A larger skin biopsy can be excised using a scalpel after which the wound is sutured.

Intradermal skin testing is sometimes performed to help in the diagnosis of allergic skin diseases. A very small volume of a number of allergens (a substance to which a horse may be allergic) is injected into the skin and the resulting swelling is then measured.

It can be quite difficult in horses to distinguish an allergic reaction from an irritant reaction. The relevance of allergens giving a positive reaction must also be taken into account.

If the horse reacts positive to an allergen to which it is never exposed this is unlikely to be the cause of the problem.

Occasionally, blood tests are used to try to diagnose allergies, although most dermatologists do not consider these to be reliable in horses.

Treatment

Treatment of skin disease is often started based on a presumptive diagnosis and treatment is continued or modified once a definitive diagnosis has been reached.

Infectious diseases are usually treated with specific drugs, such as antibiotics in the case of bacterial infections.

Immune-mediated or allergic reactions often require treatment with corticosteroids.

If the skin disease is the result of environmental factors, the horse may need to be stabled and, if the diet is contributing to skin disease, this will need to be changed.

Endocrine (hormonal) diseases, such as Cushing’s disease can be treated, after which the changes to skin or hair coat will often resolve.

Tumours often require surgical removal, but they can sometimes be treated by the injection of chemotherapeutic drugs. Ask your veterinarian for advice.