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Skin deepby Dr Chris Heislers B.V.Sc. (Hons)

A characteristic of skin disease is that no matter what the cause of that disease, the symptoms often look remarkably similar (some or all of; hair coat changes, scale, scabs, pain, swelling, itch and lumps). Thus a combination of an open minded approach to diagnosis, careful evaluation of history and clinical signs, and the use of various pathology tests (skin scrapings, hair plucks, histopathology etc.) is often needed to effect a diagnosis and effective treatment plan.

Whereas treatment of many diseases is theoretically straight forward, logistical and financial constraints sometimes complicate that. And some diseases are difficult treat; in fact in some cases treatment is really aimed at control rather than absolute cure.

In these circumstances a full and accurate diagnosis is needed in order to

have confidence in the treatment / management path, and a patient consistent approach to treatment with realistic expectations is essential. Unfortunately, too often, owners are influenced by the inaccurate opinions of others which can result in an inconsistent approach and bad outcomes. And other times logistical and financial constraints result in poor and frustrating outcomes.

So, the message; if skin disease in your horse is not resolving quickly with simple symptomatic treatment, consult a veterinarian who you have confidence in. They will, via an objective and systematic approach arrive at an accurate diagnosis and treatment plan. Follow that plan and keep communicating with your veterinarian!

In particularly difficult cases your veterinarian has ready access to veterinarians and pathologists who have particular expertise with skin disease and can thus assist with diagnosis and treatment. And if you expect a good and timely outcome, don’t allow logistical difficulties and short term financial frustrations to get in the way of that outcome.

Categories of skin diseaseSkin disease generally falls into one of various categories, all of which are common...

• Infectious (bacterial, fungal, viral)

• Parasitic (lice, mites, flies)• Allergic• Immune mediated• Neoplastic (tumors, cancers)• Miscellaneous

Remember that a disease from one category may initially look like one from another category. For example some sarcoids (neoplastic) can look like ringworm (infectious; fungal), parasitic or allergic disease.

Following is a rundown of some of the more common skin conditions. Note that this list is far from exhaustive.

Generally, remember that ‘common things occur commonly’. And if treatment for a suspected disease is not effecting a rapid cure then further investigation often including samples for pathology is needed.

Infectious skin diseases (An ‘infection’ or caused by infectious organisms).

‘Rain Scald’ refers to infection usually along the along the back, sides and rump usually caused by a bacteria (NOT a fungus!) called Dermatophilus. It is characterised by scabbing of tufts of hair that can be painful and pull away to reveal a bald and sometimes moist purulent piece of skin. It is caused by humid conditions (long

Skin diseases of horses in southern Australia.As with all species, skin disease in horses is common. Whereas sometimes symptomatic treatment of the clinical signs results in a successful outcome, many cases are more complex and so good treatment outcomes rely on an accurate diagnosis and a multifactorial treatment and or management approach. In this article Dental Vet Dr Chris Heislers outlines the most common skin diseases affecting horses in Southern Australia.

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Good treatment outcomes for skin diseases rely on an

accurate veterinary diagnosis and a multifactorial treatment and management approach

wet hair or moist under a rug) that allow this ‘normal’ bacteria to flourish.

Poor body condition and poor health predispose to the disease. It is potentially contagious via the spores that the bacteria produces and transfer is via infected rugs etc. Treatment should be simple; remove all scabs, antiseptic shampoos, ensuring dry skin. Occasionally penicillin will be required.

‘Rain Scald’ is an infection caused by the Dermatophilus bacteria that thrives in humid conditions such as long wet hair or under a humid rug. Poor condition and health predispose the animal to infection.

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‘Mud Fever’ or ‘Greasy Heel’ (pastern dermatitis).

This is a classic example of multifactorial skin disease. i.e. it is usually not just an infectious disease; there are often other elements to the disease, and unless all those elements are treated effectively the disease will continue to frustrate. And to further complicate, these two names are ‘lay terms’ that have no consistent medical meaning. i.e. two cases of ‘mud fever’ may be quite different diseases that just look similar. Having said all that, most cases of this disease are due to soggy wet pasterns that due to abrasion from grass, mud etc. crack microscopically and exude serum (body fluid) which forms scabs. Bacterial infection is an important secondary problem that exacerbates the problem and contributes to further pain and spread. Treatment involves keeping the lower limbs dry (often requires stabling), scab removal (often requires bandaging to soften scabs), antiseptic shampoos, antiseptic / moisturising creams and often antibiotics. Treatment (primarily maintaining dry conditions) needs to be prolonged enough to ensure full skin regeneration before the horse is allowed access to the offending conditions again. Not uncommonly, uncomplicated pastern dermatitis can become a very frustrating immune mediated disease.

Bacterial folliculitis

Presents as painful small nodules, often with a small surface scab, often after clipping or some other skin trauma. Most cases require antibiotic therapy; and in non responsive cases resistant bacteria need to be considered as a cause. These cases may require pathology to assist a treatment plan; sometimes that will be just choosing a specific antibiotic that the bacteria are sensitive to.

Fungal skin disease (‘ringworm’)

usually appears as patches of dry scaly skin, spreading outwardly. Most common around the face and neck due to the fact that it is transmitted often via contaminated feed bins or via horses grooming one another. Most common in very young horses but also seen in immunocompromised individuals. The disease is usually self limiting; though as it is contagious treatment is worthwhile via appropriate antifungal preparations. Note that it is common for owners to misdiagnose; suspecting ‘ringworm’ when in fact the disease may be sarcoid, lice or rain scald. Of course ‘ringworm’ treatment will do nothing for sarcoid or lice.

Parasitic skin diseaseBy far the most common in southern Australia is lice. Almost always it occurs in late Winter, and by mid Spring once the weather warms up it is self limiting. It appears usually as large patches of lair loss particularly around the face and neck. The disease is very pruritic (itchy); horses rub their hair out. Of course that needs to be differentiated from the normal hair moult as horses come into their Summer coat which can cause some degree of itch and transient alopecia (areas of baldness). Lice can normally be seen hiding at the skin level under the hairs; approximately the size of a flea scurrying away from light. The most likely area to find them is the mane; though may be found anywhere on the body. The lice eggs (‘nits’) are seen as multiple tiny very round whitish spots attached to near the base of the hairs; particularly of the mane and forelock. Treatment is via effective anti-parasitics, repeated later to kill new lice that have hatched from the eggs (which are usually resistant to treatment from the anti-parasitic chemicals). Note that growth regulator treatments alone will not kill live lice; they will only ‘sterilise’ the lice so that their newly laid eggs will not be viable.

Mites

Also known as ‘mange’, mites are the other main parasites, however in horses are rare compared with lice. They usually cause extreme pruritis. Sarcoptes, Demodex and Psoroptes are all quite rare but Chorioptes (leg mange) is fairly common in the feathers of feathered breeds which causes leg stomping and biting of lower limbs. Treatment of this type of mange is relatively easy via anti-parasitics although treatment for Sarcoptes and Demodex is more difficult.

Another parasitic skin disease worthy of mention is...

HabronemiasisThis is caused by the larvae of a stomach worm and it causes tumor like lesions usually on the conjunctiva or the mucosa of the penis. It is easily treated with ivermectin based dewormers. If ivermectin treatment does not resolve the lesion within a couple of weeks then think cancer and call your veterinarian immediately. Note that overall, cancerous lesions are more common than Habronemiasis.

Neoplastic skin disease

There are many neoplastic skin diseases in horses that vary from life threatening malignant cancer to benign growths of little consequence. However unfortunately the early appearance of a lesion generally gives little assurance as to the potential danger of the lesion; hence biopsy of all lesions that don’t disappear rapidly on their own is strongly recommended. Only then can an accurate diagnosis, prognosis and treatment plan be developed. The most common neoplasias that we see in horses are melanomas, sarcoids and squamous cell carcinomas (though there are many others).

Sarcoid Theses are not truly malignant (they don’t travel to internal organs) but they can and often do spread from the original lesion to surrounding skin, or may be transmitted by biting insects to skin on another part of the body. There is evidence of a viral component to these tumors’ aetiology. Sarcoids vary in appearance from a hairless scaly patch to a skin nodule or a warty granulomatous growth. They are always potentially serious. Even small ‘benign’ appearing sarcoids may sit unchanged for many years before becoming very angry and even life threatening. Thus all sarcoids should be evaluated early and aggressive removal considered. Note that removal is often difficult and dedication to aggressive repeat treatments if required is essential.

Treatment options vary from creams /ointments for superficial ‘occult’ lesions to cryosurgery, chemotherapy or immunotherapy. Depending on the position and type of sarcoid, your veterinarian will devise an appropriate plan.

Melanomas are most common in grey horses and are most common under the tail, on the penis, within the mouth (lips) and the lymph nodes behind the mandible. They are quite different to dreaded melanomas in humans and not as

aggressively malignant; however they are cancers that in some untreated cases will result in performance limitation (mouth lesions), difficult nursing management and premature death from lesions around the anus and tail or from colic (intra abdominal lesions). So, don’t treat them too lightly. Like most tumors; early removal is usually straightforward and will remove the possibility of them causing major problems in years to come. Of course in a prone horse (greys) more melanomas will arise with time, so it is common to present these horse annually (or as needed) for tumor removal. Cimetidine has been proposed as a management option. There is no firm evidence as to the true effect of this, however anecdotally some reasonable results have been obtained.

Squamous Cell Carcinomas

These appear as ulcerated lesions or warty granulomatous growths (see image on page 15); particularly involving eyes, eyelids, penis, vulva and the nose (non pigmented or pink areas of skin). Also they occur occasionally at the site of a wound (as with sarcoids); so consider them with non healing wounds. Treatment is via wide surgical excision, cryotherapy or chemotherapy. Treat these early and aggressively as they can be very nasty malignant tumors when untreated.

An acute case of ‘mud fever’ or ‘greasy heel’ an example of a multifactorial and painful skin disease that in most cases

occurs when horse’s legs are exposed to wet, muddy conditions.

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Left: Mites are parasites that affect horses. Above: Mites in the tail.

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Squamous Cell Carcinomas are lesions or warty granulomatous growths that require early and aggresive treatment as they can be nasty malignant tumours when untreated

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Allergic skin disease.As with all species, allergic skin disease presents in the horse as pruritis (intense itch). Obviously that symptom is also seen with external parasites, biting insect irritation, moulting, pin worm, infections etc. So diagnosis relies on excluding each of those causes.

The cause(s) of skin allergy is many and varies from horse to horse but includes airborne pollens and moulds (atopic dermatitis), biting insects and feed components.

It is impossible to determine the cause of the allergy from symptoms so treatment is symptomatic and includes prednisolone, oatmeal colloidal shampoos / conditioners, oatmeal colloidal or triamcinolone spray preparations and of course avoidance.

However avoidance requires a careful elimination trial approach (frustrating and often unrewarding in horses; consult your veterinarian) or the assistance of intra dermal (ID) skin testing which can be performed by veterinarians with particular expertise in dermatology. Once results are obtained from ID testing, desensitisation therapy (immunotherapy) can commence to target specific allergens; this is the best long term management option for severe atopic dermatitis.

Another common presentation of allergic skin disease is ‘heat lumps’ that most commonly occur as nodules within the skin under the saddle region of the back. Ultimate diagnosis (to distinguish from tumors, bacterial folliculitis etc) is via biopsy and treatment is via intra lesional corticosteroids. If left untreated or unresolved, these lumps can become calicified and thus permanent; requiring surgical removal if they are considered unsightly or interfering with saddle comfort.

Other skin diseasesAlopecia

or areas of baldness in the true sense is rare in horses, but occasional primary cases are seen and most often presents as non pruritic loss of tail and mane hairs. Diagnosis is ultimately via biopsy.

Secondary skin diseases

This refers to skin or coat abnormalities due to internal health problems. Obviously in such cases there will be no resolution of the skin / coat changes unless the primary disorder is corrected. Some examples are:

Photosensitisation

It is severe ‘sunburn’ of all exposed non pigmented (pink) areas of skin usually due to underlying liver damage, such as from ingested toxins. It presents as raised hairs to full superficial painful skin sloughs.

Hirsuitism Horses with Equine Cushings Disease often display hirsutism (excessively dense, long and curly coat with reduced Spring moult) with excessive sweating or transient alopecia.

ABOUT THE AUTHOR: Dr Chris Heislers, a 1989 University of Melbourne graduate, is a founding principal veterinarian at The Veterinary Surgery; Yarrambat and North Warrandyte. This in an eight - vet practice on the northeastern fringe of Melbourne that provides veterinary services to all species in the local area and into the Plenty and Yarra Valleys. Horses comprise 60% of the practice’s workload. The equine hospital facilities at Yarrambat help Chris and his team of vets and nurses to provide best practice case management to all aspects of horse veterinary practice. Particular areas of Chris’s interest and expertise include reproductive services, lameness and dentistry. With the assistance of Dr Robert Hilton, a highly respected veterinarian with specific interest

and expertise in dermatology, the practice provides primary and referral dermatology services. The practice can be viewed online at www.thevetsurgery.com.au or contact via email nurses@thevetsurgery.com.au or phone on 03 9436 1699.

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Probably the most important points to be taken out of this article are;

Many different skin diseases with different cause and different treatment requirements will present remarkably similarly. So if initial symptomatic treatment is not resolving the problem quickly then seek a veterinary consultation and be open to pathology testing. Otherwise there is a real possibility of the disease worsening to the point of being dangerous to the horse or requiring long term, difficult and expensive treatment.

Particularly in the case of skin lumps / tumors and immune mediated disease, early accurate diagnosis and treatment is essential to reduce the risk of severe and sometimes life threatening complications.

Skin disease is sometimes just a symptom of another underlying health disorder; the skin symptoms may alert to that disease earlier and thus facilitate diagnosis / treatment. And of course the skin will not resolve unless the primary disease is treated.

An accurate diagnosis and a proactive systematic approach to treatment / management is important to the resolution of most skin diseases. Avoid listening to too many conflicting non qualified opinions and ‘old wives tales’. Instead obtain the assistance of a veterinarian who you trust; follow their optimal advice; and don’t be afraid to ask for pathology tests. For particularly complicated cases, referral to a veterinarian with specific expertise in dermatology cases may be worth while.

Selenium toxicity

Usually occurs from over supplementation and may present as areas of baldness.

Diagnosis is by ruling out more common causes and then via biopsy and specific blood testing.

The information in this article is a long way from being an exhaustive list of skin disease in horses; however it covers many of the most common conditions that as veterinarians we see regularly in practice.

Luckily some cases are relatively straight forward and even without an exact diagnosis they will respond to a common sense approach to management and symptomatic treatment of the clinical symptoms.

Cases that don’t respond quickly however, require a more proactive approach to an accurate diagnosis, and evidence based management and treatment.

Take home message

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Urticaria is an allergic reaction that presents as raised welts on the skin that itch intensely.

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Immune mediated skin disease.Occasionally the immune system inappropriately targets normal body tissues, including the skin. It needs to be considered in any skin disease involving pruritis, scabs and hair loss that does not respond to treatment for uncomplicated infectious dermatitis. Accurate diagnosis is important and relies on histopathology (biopsy). Secondary infection is common and often requires long term antibacterial treatment. Environmental management (avoiding saturated conditions, UV light etc.) and treating the cause are important. A common presentation of immune mediated skin disease that we see is Pastern Leukocytoclastic Vasculitis; often as a sequelae to treatment of unresolved ‘mud fever’. It is a frustrating condition that requires accurate diagnosis and determined systematic treatment with medications such as pentoxifylline, corticosteroids and tacrolimus along with careful environmental management / avoidance of triggering chemicals.