E770
Skin Disease

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Introduction: Skin problems in horses can be a fairly common occurrence. The challenge occurs when a problem is discovered and a diagnosis of what is causing the problem is required. Because it can be challenging to arrive at a diagnosis, a thorough history is necessary. The following is a list of questions a horse owner might be asked by a veterinarian when dermatologic (skin) problems occur:

1. How long has the problem been going on?
2. Do the symptoms come and go?
3. If there is more than one lesion, where did you notice the first one?
4. Do other horses, animals, or humans in close proximity have similar problems?
5. Have there been any changes in the horse’s environment, diet, or supplements?
6. Have there been any medications or vaccines given recently (orally, topically, or by injection)?
7. Does the problem seem to be associated with certain seasons or types of weather (sun, rain, wet/humid)?
8. Does the problem seem to be associated with any external parasites such as flies or biting insects?
9. When was the last time you de-wormed your horse, and what product did you use?
10. Have you noticed a change in hair coat?
11. Does the problem seem to cause itching (pruritus)?
12. Do the lesions seem to be associated with certain color patterns on the horse?

Causative Agents: The following discussion will identify four major categories for the problems that cause skin disease. Specific causes are listed under each major category.

1. Hypersensitivity - These "allergic" type reactions often result from administration of, or exposure to, an allergy causing substance.

1. Certain drugs and vaccines
2. Some ingredients in the diet
3. Insect bites and irritation - flies, ticks, fleas, bees/wasps, mites, etc.

2. Environment

1. Chemicals and pesticides
2. Photosensitization (reaction to the sun)
3. Burns and frostbite

3. Infectious agents

1. Fungal, bacterial, and viral infections
2. Internal and external parasite infections

4. Other

1. Toxicity - excess of iodine or selenium and certain plants
2. Tumors - sarcoids, melanomas, and squamous cell carcinomas
3. Congenital and genetic disorders - Albinos, and epitheliogenesis imperfecta (this is where the skin does not form properly)
4. Hormonal diseases (hyperadrenocorticism)

Clinical Signs: As can be noted from the above list, there are many different causes of skin disease. In general, most of the causes of skin disease manifest themselves in up to six different ways (see the following list). However, it is important to realize that any one disease can cause one or all of the signs listed.

1. Pruritus (itching) - This can be localized to one area or can be all over the body
2. Lumps, bumps, and swellings
3. Changes in hair coat, including hair loss (alopecia), increased length, and changes in color
4. Erosions/ulcerations - These can appear as irritated, damaged skin, with little or no hair
5. Pustules and vesicles - These are raised lesions containing fluid or infection in them
6. Dry, scaly/crusty skin

Diagnosis: There are many tools available to a veterinarian to help with a diagnosis. The essential starting point is a complete history of the problem (see Skin Disease Introduction). Once a thorough history is taken, the animal is then examined. All areas of the horse should be evaluated, even if they do not seem to be involved. If specific lesions are identified, samples are often taken for additional evaluation. These samples can include portions of the hair, a skin scraping of the area, samples taken directly from a pustule or vesicle, and a skin biopsy (this is a full thickness sample of the skin). These samples can be evaluated using a microscope, culture, and/or examination by a dermatological expert. Once every piece of the puzzle is in place, a diagnosis can usually be made.

Treatment: Once a problem is identified, treatment can be initiated. Some hypersensitivity reactions require immediate treatment with anti-inflammatory agents, epinephrine, and continual monitoring (see page E195). The infectious problems may require antibiotics, anti-fungals, de-worming, or topical parasite control products. Skin diseases that result from exposure to some substances require that the exposure be eliminated or reduced. For example, in cases of photosensitization (reaction to the sun), the horse may require sun screen and shade. Once the problem has been specifically identified, the local veterinarian can usually begin the proper treatment.

Prevention: First time occurrences of many of these skin diseases are difficult to prevent. For example, it is unlikely a horse owner will know if his/her horse will have a reaction to a vaccine or insect bite. Once a hypersensitivity reaction has occurred, however, that product, insect, or ingredient should be avoided at all times. Some of the infectious causes of skin disease can be prevented by maintaining a good de-worming and vaccination program (see pages A905 and A622). Along with these programs, it is always advisable to avoid situations where a horse is in contact with other horses that are infected with a skin disease. It is also important to remember that many of the infectious skin diseases can be transmitted from animal to animal by exposure to contaminated hands and equipment.

1. Hypersensitivity reactions (allergic reactions)

Cause: Hypersensitivity reactions in horses can be caused by many different things. Following is a list of the most common:

• Inhaled allergies (pollens, molds, smoke, dust, plus others)
• Drugs and vaccines (penicillin, sulfonamides)
• Ingested items (additives, medicines)
• Contact allergies (chemicals, some plant and animal materials)
• Infections (bacterial, fungal, viral and parasitic infections)
• Insect bites (rare)

Clinical Signs: Most of the hypersensitivity reactions result in hives (urticaria). Urticaria is a reaction where the skin or mucous membranes swell because of fluid accumulation (edema). These swellings are usually raised and small, but occur in large numbers. They are observed most commonly on the neck and head. They often "pit" when an indention is made in them with a finger. In severe hypersensitivity reactions, anaphylaxis can result. Anaphylaxis is a life-threatening reaction that can cause hives, an increased heart rate, sweating, colic, and difficulty breathing (dyspnea).

Diagnosis: Most of the time these hypersensitivity reactions can be identified by physical examination. The fact that most hives or urticaria "pit" with pressure, can help differentiate them from other lumps or masses. The difficulty comes in determining what is causing the reaction. Changes in environment and diet may need to be made to help determine the cause.

Treatment: For inhaled allergies and some of the others, hyposensitization therapy can be considered. Hyposensitization therapy involves skin tests that identify what is causing the allergic reaction. A set of injections is then formulated that desensitize the horse’s immune system to the substances causing the reaction. The frequency, dose, and duration of these injections are determined by the response of each individual horse. Most horses require that some type of therapy be given for life.

For mild hypersensitivity reactions, corticosteroids are often used. See the anti-inflammatory section of this manual for additional details on specific products. For severe (anaphylaxis) reactions, refer to page E195 under allergic reaction.

Prevention: All of the hypersensitivity reactions can be prevented if the exact cause for the reaction is known. Once it has been determined what the horse is reacting too, access to this substance should be avoided. Hyposensitization therapy is also an option is some circumstances.

Cause: Ringworm in horses is a fungal infection of the skin caused most commonly by the organism Trichophyton equinum. Ringworm in horses can also be caused by other Microsporum and Trichophyton species.

Clinical Signs: Ringworm causes hair loss and crusts to form on the skin. There is usually more than one lesion present and the lesions can form almost anywhere. However, the head and around the eyes or neck seem to be the most common areas that are involved.

Disease Transmission: This skin problem is spread from animal to animal by direct contact. It can also be spread from animal to animal when contact is made with a contaminated object (equipment, trough, feeder, etc.). Young animals and horses that are sick, exposed to damp environments, or have poor nutrition are at an increased risk of developing the disease. Animals that are kept indoors or away from adequate sunlight are also at risk.

Diagnosis: The best way to diagnose this problem is by performing a fungal culture. This is accomplished by taking hair samples from the outside edge of a few of the lesions. The hair is then placed on fungal specific media and any dermatophytes are allowed to grow over a period of days.

Treatment (choose one of the following):

• Chlorhexidine (diluted 1:4 in water) applied three times a day until resolved.
• Clorox (diluted 1:10 in water) applied twice a week until resolved.
• Captan, a plant fungicide (mixed 1 ounce of the 50% powder to a gallon of water) applied daily for three days and then weekly until resolved.

* Allow the animal as much exposure to sunlight as possible.

4. Lice

Introduction: Lice infections in horses are caused by Bovicola equi and Haematopinus equi. They are most common in the winter months.

Clinical Signs/Diagnosis: Most lice infections of horses cause itching (pruritus) and dry, scaly skin. The infestations most often occur in the mane, neck, and shoulder areas, but can spread across the body. Areas of hair loss (alopecia) and ulceration can also occur. In severe infections, weight loss may result. These infections are diagnosed by identifying the parasite and common clinical signs.

Treatment: An effective treatment program must involve an insecticide to kill the parasite on all infected/exposed animals, and treatment of the animal’s environment (saddles, halters, ropes, bedding, blankets, stall, etc.). Most topical products must be administered for two or three treatments to effectively break the parasite’s life cycle. Blankets, brushes, and ropes can be soaked in boiling water, while saddles and other leather items can be treated by wiping on a topical insecticide. Use a product that is labeled for use in horses (like Ectiban by Durvet), and test a small area of the leather article before applying to the entire surface. Some reports indicate that oral ivermectin can also be effective.

5. Mites (mange and trombiculidiasis)

Introduction: Mange in horses can be caused by four different types of mites: Psoroptes equi, Chorioptes equi, Sarcoptes scabeii, and Demodex equi. Most mite infections in horses are rare, and many infestations are reportable to the state veterinarian. Trombiculidiasis ("chiggers" or harvest mites") is caused by Eutrombicula alfreddugesi and Trombicula mites. Mites are spread by direct contact between animals or by contaminated equipment.

Clinical Signs/Diagnosis: Mite infections usually occur at the base of the mane or tail and cause a red, raised bump (papule). These infections can progress and cause areas of hair loss, crusting and irritation. In some infections itching (pruritus) and weight loss can also result. Diagnosing a mite infection involves skin scrapings and identifying the specific mite.

Treatment: Refer to the lice treatment information.

6. Culicoides Hypersensitivity (Queensland itch or equine sweet itch)

Introduction: This problem is caused by the Culicoides gnat (punkies, no-see-ums, biting midges). These horses have a hypersensitivity reaction to the saliva of the gnats as they feed on the horse’s blood. The gnats breed and congregate around marshes, swamps, and ponds.

Clinical Signs/Diagnosis: Affected horses experience a great deal of itching (pruritus) and irritation. Areas of hair loss, crusting, and scaling can be found on the head, neck, shoulders, mane, and tail. This problem is diagnosed based on the history, clinical signs, location of the lesions, and response to treatment.

Treatment: Culicoides hypersensitivity can be controlled by moving the horse at least 1/2 mile from breeding areas for the gnats. This should be accompanied by the use of fly sprays and other topical insect repellents. The key to treating this problem is reducing the horse’s exposure to the gnats. In some problem cases, topical and/or oral steroids may be required.

7. Dermatophilosis (Rain scald, streptotrichosis)

Cause/Clinical Signs: Dermatophilosis is caused by the bacteria (actinomycete) Dermatophilus congolensis. This problem causes areas of hair loss, matting, crusting, and scab formation over the back areas. Small tufts of hair can easily be removed, exposing a raw, sometimes infected lesion.

Disease Transmission: This problem is spread by direct contact from animal to animal. It can also be spread when contaminated equipment is not cleaned properly between animals or when insects move between animals. Wet, humid, rainy environments and any situation that causes the surface of the skin to be damaged, increase the chances of developing the problem.

Diagnosis: A diagnosis can often be made based on the appearance of the lesions. If this is not possible, a culture or microscopic examination is required.

Treatment: For effective treatment, four things should be done:

• Clip the hair from the problem areas
• Disinfect the lesions with dilute betadine or betadine shampoo
• Keep the skin dry and exposed to sunlight
• In severe cases, administer penicillin

Prevention: Isolating any diseased animals and controlling biting insects are two of the most common preventative measures.

8. Warts (Papillomatosis)

Cause: Warts in horses are caused by a virus.

Clinical Signs: Warts can be found on the head, neck, and ears. The lesions are white to grey, firm and raised. These lesions are harmless in almost all situations, except where pain is involved.

Disease Transmission: Warts are spread by direct contact from animal to animal. They can also be spread when contaminated equipment (halters) is not cleaned properly between animals.

Treatment: Small warts can be crushed, pinched off, frozen (cryosurgery), or surgically removed.

Prevention: When an animal has a wart infection, it should be isolated from other animals. Any infected animal should also be kept from rubbing on feeders and posts that may be used by other animals. Cleaning of equipment with a product like chlorhexidine is essential for preventing the spread of warts.

Cause/Disease Transmission: The cause of equine sarcoids is not exactly known. Many scientists suggest that sarcoids are the result of a combination of trauma, genetics, and an infection with a viral agent. Additional research will need to be performed before the exact cause and method of transmission can be identified.

Clinical Signs: Equine sarcoids are often classified as either verrucous (wart like), fibroblastic (proud flesh like), or as combinations of the two. Some sarcoids are small and inactive, with flat areas that are hairless, dry, rough, and grey in color. Other sarcoids are raised, large, and active, often with the skin still intact. These tumors can occur anywhere on the body, but are most commonly found on the head, face, legs, and abdomen.

Diagnosis: A diagnosis can often be made based on the appearance of the lesions. If this is not possible, a biopsy or complete surgical removal can be performed. It is thought that some verrucous sarcoids change into more aggressive fibroblastic tumors when they injured or traumatized. Because of this, partial biopsy of any verrucous tumor is not recommended.

Treatment: Because many equine sarcoids often recur after they are removed, treatment can be challenging. The following are some of the most common treatments (often more than one of the following are used on the same sarcoid):

1. Surgical removal - Complete surgical removal of the tumor is often difficult and many sarcoids return. This method is often combined with some of the other techniques.
2. Chemotherapy - Placing cisplatin in the tumor is the chemotherapy most often used. This treatment requires special training on the use of chemotherapy for both the veterinarian and horse owner.
3. Immunostimulation with certain medications.
4. Cryotherapy - This method destroys the tumor by using extremely cold temperatures. Like surgical treatment, cryotherapy is often combined with other techniques to achieve a better outcome.

Introduction: Melanomas are skin tumors found in all animals, but are most common in grey horses. Reports indicate that melanomas are rare in horses less than 6 years of age and approximately 80% of all grey horses over 15 years of age have this tumor.

Cause: Melanomas are tumors that result from over active melanoblasts (immature melanocytes). Melanoblasts/melanocytes are cells that produce melanin which is a dark pigment of skin, hair, and some tumors. In horses a melanoma is thought to appear when abnormal melanin production occurs.

Clinical Signs: Melanomas are firm, round, grey to dark, tumors that can occur individually or most commonly in numbers. These tumors can occur anywhere on the body, with the areas around the tail head, anus, and vulva being the most common. Melanomas are also seen on the limbs, penis, prepuce, udder, eyelids, and head. Most of these tumors are slow growing and do not metastasize (spread). However, some tumors are fast growing, malignant, and can spread to the lymphnodes, liver, lungs, and spleen.

Diagnosis: A melanoma can be identified by a pathologist based on microscopic observations and by the presence of melanin pigment. Often the entire tumor is removed and sent in for evaluation.

Treatment/Prevention: Melanomas that are found in older animals are often left untreated unless they interfere with normal body functions (urination, defecation). Tumors that are determined to be aggressive or are causing problems, can be surgically removed. Cryotherapy (freezing the tumor) is also an option. No matter the method that is chosen, it is important that the entire tumor is removed. Cimetidine (Tagamet) can be used to help prevent additional tumors from forming and also limit the size of current tumors. The dose of cimetidine is 115 mg/100 lbs. of body weight administered 3 times a day for 3 months. A lower dose of cimetidine may be required for long term treatment and prevention of this tumor.

1. Albinism - This is a genetic defect present at birth where melanin production is defective. These horses have white hair, white skin, pink eyes, and are sensitive to sunlight (photophobic).
   
   
2. Arabian fading syndrome (Arabian leukoderma) - This is also a defect in melanin production. It is most often seen in young horses between the ages of 6 months and 2 years. This disease results in white pigmented areas of the muzzle, anus, and around the eyes. This condition can come and go, get worse, or return to normal.
   
   
3. Equine staphylococcal cellulitis - This is a severe Staphylococcus bacterial infection that spreads through tissue planes. These infections often occur in the limbs causing swelling, lameness, and skin damage. Treatment for these infections should be aggressive and may involve antibiotics (penicillin), anti-inflammatory agents (bute), cold water soaks, and support wraps.
   
   
4. Pemphigus Foliaceus - This is an autoimmune disease that causes destruction of normal skin cells. In a general sense, an autoimmune disease is one where the immune system of the animal recognizes normal cells and tissue as foreign. In the case of pemphigus foliaceus, the horse’s immune system attacks normal skin tissue causing changes to skin cells. The destruction of skin cells occurs first on the face, belly, or limbs and can progress to involve the entire body. Initially, the affected areas are inflamed and develop fluid filled vesicles. These vesicles eventually rupture, leaving a break in the skin that begins to scale and crust. This problem can be diagnosed by taking a biopsy of an affected area and having that sample examined by a pathologist. These horses require high doses of steroids to suppress the immune system. Some animals require treatment with products like aurothioglucose, while others may require lifelong therapy.
   
   
5. Reticulated Leukotricia - This is mainly a disease of yearling quarter horses. This problem begins with areas of crusting in a "cross-hatched" pattern over the back and shoulders of the animal. In time the crusts and hair fall off, and white hair regrows. There is no known treatment.
   
   
6. Cutaneous Onchocerciasis - This disease is caused by a nematode worm called Onchocerca cervicalis. The worm is spread from horse to horse when the flying insect Culicoides takes its blood meal. Once the worms enter the horse’s body, they migrate to the connective tissues and skin of the neck and head. Affected horses are thought to develop a hypersensitivity reaction to the dying worm, resulting in skin and eye lesions. Skin lesions are scaly, hairless, and commonly form a "bull’s eye" like pattern in the forehead. This condition is diagnosed by taking a biopsy of a lesion and observing the Onchocerca. Oral ivermectin at a 0.2 mg/kg dose is the treatment.

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