Equine Skin Tumors

1. I have a grey 5-year-old Arab/Azteca mare. She is greying pretty slowly and I am wondering if horses that grey more slowly are more or less likely to develop melanomas as they age. (View Answer)

   Melanocytic tumors in gray horses are thought to arise because of a disturbance in melanin (or pigment) transfer from dermal melanocytes to follicular cells. This accumulation of pigment and eventual transformation to a melanoma has been documented in melanoma-prone locations.  Interestingly, common sites of occurrence are typically the areas that first begin to show loss of pigment as the horse ages.

   The majority of melanomas are located around the perineum and base of the tail, with lesions around the head (lips, eyes, parotid region) and other sites less frequently reported. Melanomas have also been reported in the foot, meninges, thorax, ocular structures, and abdominal cavity. These unusual locations appear to be associated with a poor prognosis.

   Logically you might argue that horses that lose pigment or “gray” more slowly would also be slower to develop tumors but data is lacking.  In reality the vast majority of gray horses will ultimately develop melanomas.  In some reports it appears that “flea bitten” grays are less likely to develop melanomas compared to other gray coated horses.

   There is good news to be had despite this information!  The majority of melanomas in horses are benign meaning they rarely metastasize or cause systemic problems. The biggest issues with melanomas relate their location and size.  Melanomas around the lips can interfere with training, melanomas around the vulva can impact a mare’s ability to carry a foal.  When melanomas get large (called dermal melanomatosis) they can ulcerate, attract flies and cause more discomfort.  Malignant melanomas are very rare and usually detected in aged horses.  

   In a study looking at gray horses at a Lipizzaner stud farm they reported that melanomas were more common in horses >15 15 years old (75%) verses all horses (50%).  The vast majority of horses were clinically unaffected by the melanomas. 

   Should your horse develop melanomas there are many options for treatment. Depending on the size, growth rate and location your veterinarian can discuss the best approach to ensure your horse remains comfortable and functional. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

2. I have an 18-year-old Thoroughbred in otherwise good health but has suffered with severe "scratches" and edema on one hind leg for nearly a year. We have repeated antibiotics and prednisone that helped somewhat but have not resolved the problem. Potassium iodide solution administered orally over two months (so far) has been most effective, but the problem still lingers. Although the skin is less dry, some unusually thick, protruding scabs are still apparent with dry, scaly hoof growth from the coronet. He currently lives out on good mixed grass pasture as I do not feed grain. What else can I be doing to eliminate this frustrating condition? (View Answer)

   Scratches can be a very frustrating condition to resolve.  Once the skin and subcutaneous tissues gets damaged and scarred the tissues are always more prone to recurrence.  This is, in part because the normal skin barrier against infection is permanently damaged.  In addition, the subcutaneous scarring effects lymphatic drainage.  There are always small abrasions that occur in our skin allowing bacteria or other contaminants to enter the tissues.  These are usually carried away by the lymphatics where white blood cells can destroy and clear them away.  Because this mechanism is impaired there is a greater risk of recurrence or “flair up”.

   Based on your description I am not completely sure if you are describing scratches or cellulitis.  The latter is usually a more severe bacterial infection and often results in greater scarring and increased risk of recurrence. Scratches typically occur around the pastern and are thought to be triggered by multiple factors, moisture (wet grass), breaks in the skin barrier due to small abrasions/trauma, presence of dirt or debris and then secondary bacterial or fungal invasion.  Draft breeds can have “foot mange” which will also exacerbate the lesions.

   As you have said it can be difficult to treat and fully resolve.  Depending on the condition of the skin (is it moist and exudative or dry and cracking) as well as some of the other factors I mentioned above, treatment is tailored to an individual case.  Making sure his diet is adequate in vitamins and micro minerals is also important as these can impact skin and hoof wall growth.  Many veterinarians have treatment recipes designed to control bacterial, fungal or parasitic organisms as well as to calm down the inflammatory process that develops.  In some cases keeping the area wrapped with soft clean cottons can help resolve the lesions.  Chronic edema increases the risk of recurrence so wraps can help by preventing edema accumulation and potentially decrease the number of recurrences.

   Potassium iodide is an old treatment for chronic infections and has been shown to be effective in control.  Unfortunately, given your description I don’t have any magic cure.  Your goals are going to have to continue to be management directed; preventing the edema accumulation, keeping the area clean and dry and the tissues healthy and treating any recurrences as quickly as possible. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

3. I have an acquaintance with a 2-year-old AQHA filly that has severe skin issues. She has these ‘welts’ all over her back, belly, legs, rump, etc., and they have become painful at all times and itch like crazy. Her hocks are swollen so largely some days. The filly had an emergency veterinarian visit at one point when she was laying down and profusely sweating. After the veterinarian's examination, he indicated it could be cutaneous larva migrans. They administered steroids, antihistamines and antibiotics for weeks straight. They also dewormed her every Thursday for a month straight. This has not helped. They recently trailered her to Mizzou University and the attending veterinarian said he has no answers except that it is a rare condition. (View Answer)

   I wish that I could give you a definitive answer but unfortunately it sounds like this filly has a somewhat unusual skin disease.  When faced with skin problems, dermatologists typically recommend a series of diagnostic tests including skin scraping, skin culture, and skin biopsy +/- special stains to examine the tissue.  It is important to take biopsies of both the early lesions (new ones) and older ones as the pathology changes over time and examining the new/recent lesions is helpful in determining the underlying cause.  Taking pictures of the lesions when they first develop as well as the changes that occur with progression is also helpful.  I would strongly recommend that your acquaintance work with their veterinarian to find and consult with an equine dermatologist (I would be surprised if they did not see one at the Missouri location) to determine the best approach to diagnosing the filly’s skin disease.  To get an answer they need to be willing to potentially perform multiple tests described above. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

4. My horse always gets sweet itch April-Oct despite using fly sprays. Fly sheets cause problems since even the lightest ones cause him to sweat. He is on night turnout during most of these months but night/day doesn’t seem to make any difference. Do you have any advice? (View Answer)

   Sweet itch is a hypersensitivity to Culicoides midges, which are small insects that are in the environment during the spring and summer.  An allergic reaction to the insect’s saliva results in inflammation and pruritus (itchiness) which caused the horse to rub and traumatizing the skin, typically the tail, mane and dorsal and ventral midlines.

   Control of sweet itch is multifactorial.  The best method is to prevent the insect bites by using insect repellants, protective clothing (fly sheets and masks) and avoiding turn out during the time of day the insects are most active.

   Immunotherapy or desensitization (allergy shots) has also been used to try to decrease the hypersensitivity response to the insect bites with some success.  This type of treatment attempts to modulate the allergic response.

   Nutritional supplements may be helpful though are unlikely to control the symptoms alone.  Fatty acid supplements have been suggested as another method to decrease symptoms though statistical data is lacking.

   Control of symptoms with medication can be helpful.  The use of antihistamines or corticosteroids can dampen the allergic response and decrease the pruritus.  These drugs are not without side effect particularly corticosteroids and you should consult with your veterinarian prior to use.  If a secondary infection develops antimicrobials (antibiotics) treatment may be indicated.

   Complementary therapies have also been recommended though proof of efficacy is lacking.  Herbal therapies, topical Aloe Vera, soothing bath solutions such as oatmeal may also decrease the pruritus though alone are unlikely to resolve the problem. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

5. Are there any difference in melanoma incidence between horses that grey at an early age versus those that grey later in life? More specifically, do horses that grey slower have less incidence of melanoma than horses that grey faster, or are they likely to develop them at a later age in life than those that grey faster? I am speaking of a horse that is mostly grey by age 3 to 5-years-old versus a horse that is only moderately grey by age 5 and will probably take another 5 years to fully grey. (View Answer)

   Melanocytic tumors in gray horses are thought to arise because of a disturbance in melanin (or pigment) transfer from dermal melanocytes to follicular cells. This accumulation of pigment and eventual transformation to a melanoma has been documented in melanoma-prone locations.  Interestingly, common sites of occurrence are typically the areas that first begin to show loss of pigment as the horse ages.

   The majority of melanomas are located around the perineum and base of the tail, with lesions around the head (lips, eyes, parotid region) and other sites less frequently reported. Melanomas have also been reported in the foot, meninges, thorax, ocular structures, and abdominal cavity. These unusual locations appear to be associated with a poor prognosis.

   Logically you might argue that horses that lose pigment or “gray” more slowly would also be slower to develop tumors but data is lacking.  In reality the vast majority of gray horses will ultimately develop melanomas.  In some reports it appears that “flea bitten” grays are less likely to develop melanomas compared to other gray coated horses.

   There is good news to be had despite this information!  The majority of melanomas in horses are benign meaning they rarely metastasize or cause systemic problems. The biggest issues with melanomas relate their location and size.  Melanomas around the lips can interfere with training, melanomas around the vulva can impact a mare’s ability to carry a foal.  When melanomas get large (called dermal melanomatosis) they can ulcerate, attract flies and cause more discomfort.  Malignant melanomas are very rare and usually detected in aged horses.  

   In a study looking at gray horses at a Lipizzaner stud farm they reported that melanomas were more common in horses >15 15 years old (75%) verses all horses (50%).  The vast majority of horses were clinically unaffected by the melanomas. 

   Should your horse develop melanomas there are many options for treatment. Depending on the size, growth rate and location your veterinarian can discuss the best approach to ensure your horse remains comfortable and functional. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

6. This is the second season that our 27-year-old mare has flaky, scaly skin on her right side and only on her back, loin, croup and rump. This year, we helped her shed out with consistent brushing and loose hair removal in these areas - hair would easily pull out in small clumps with the roots showing as white follicles. We’ve been washing and rinsing these areas with an anti-bacterial soap, and it is better than last year, but still present. This seems like persistent rain rot, and it is uncomfortable to the touch. How do we rid her of this for the season? (View Answer)

   What you are describing does sound quite similar to “rain rot” (which is a superficial infection with an organism called dermatophilus congolenses).  This generally occurs when moisture and dirt are present on the hair and skin for a period of time. The skin’s protective layer becomes damaged by the moisture and the infection develops. It is easy to resolve with keeping the area clean (brushing) and dry and antibacterial shampoos.  The fact that your mare is 27 might be a clue that her immune system is not as “effective” as it should be.  I would consider having your veterinarian test or treating her for Equine Cushing's Syndrome if her clinical signs are suggestive of this syndrome.  Treating her may be the key to helping her resolve the dermatitis you describe. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

7. My 13-year-old rescued Morgan gelding came with a whole host of severe skin conditions- lice, mud fever, rain rot, along with a low body score, when removed from his previous situation. With vet care and good nutrition he has drastically improved over the last two years. I didn't even have to blanket him this past winter. We moved this past November and the change seemed to cause him a fair amount of stress. He lost weight and looked rather moth eaten on the inside of his front and back legs. The bare patches would start as warm, raised areas and then become scabby. However, there was no sign of rain rot or mud fever in the usual places and was unsure what this skin issue might be. Along with this, his entire body seemed to have a bad case of dandruff all winter - large flakes of skin from almost every inch of his body. Blood tests last August revealed his mineral levels were all in normal range, however a bit low, so we increased his probiotic. He also receives a 1/2 cup of freshly ground flax seed in his soaked/rinsed beet pulp along with soy hull pellets and alfalfa pellets. I feed local-soaked hay. (Feed rations based on NRC tables in relation to hay analysis). Any ideas what the skin issue on the inside of the legs could be? Staph infection? Fungal infection? The cause of the dandruff? (View Answer)

   Skin conditions can be very difficult to definitively diagnose without specific testing which would include skin scraping, cultures and biopsy.  The lesions you describe on the insides of his legs sound a lot like hives, which usually result from an allergic reaction or an insect bite. They typically begin as raised welts that then exude serum, which results in the scab that you see.  The causes are numerous, he may be allergic to something in his new environment.  I would recommend your veterinarian examine him, hopefully when he has early lesions present. That would help to determine if the lesions are consistent with hives.  If they appear to be allergic reactions trying to elucidate the cause of the allergy is the next step. This involves removing/changing environmental suspects (in some cases diet) to see if the symptoms resolve. Alternatively, skin testing (allergy testing) can be helpful to determine the culprits. Once the allergen is identified immunotherapy may help decrease his reaction.

   It is hard to know the underlying cause of the generalized change in the skin.  Given the timing is associated with a move, I would wonder if it could be related to dietary change.  Soaking your hay helps decrease dusts and molds that trigger recurrent airway obstruction (RAO) or “heaves” but also leeches out other important nutrients.   Nutrient content of forage can vary widely depending on where it is grown. It is unclear to me if you have had your hay analyzed or are basing your nutrient content on what is published for standard forages.  I would talk to your regular veterinarian to determine if you need to consider adding additional supplements such as a micro mineral or vitamin supplement.  Adding some omega 3 fatty acids to his diet may also help improve his overall hair coat. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

8. My horse has Horner's Syndrome affecting the left side of his face. Since his injury, he has had some improvement in the droopiness of his eye lid but when he gets upset, his face sweats uncontrollably. Even with rinsing the sweat seems to eat away the fine hair on his face and that side of his face appears even bald sometimes, though the hair continues to try and grow back. Fortunately, he has black skin and it doesn't noticeably burn. Is there any other way to maintain the health of his skin to promote hair retention? (View Answer)

   Unfortunately, there is no magic method to prevent the hair loss you are seeing with Horner’s syndrome.  The good news is that, over time, the sweating response should decrease significantly as the sweat glands down regulate their response.  Though you do not say the type of injury or how long ago it occurred, it is also possible that the Horner’s Syndrome may also resolve with time.  In my experience, it depends on the cause.  Accidental perivascular drug administration is the most common reason and these frequently resolve over time.  The general rule of thumb is that you need to give a nerve injury 6 to 12 months before knowing if it will resolve. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

9. In 2016, my horse had a sarcoid tumor on this back leg right before it went around to the inner thigh, heading toward the sheath. My veterinarian took a biopsy and settled on a treatment of bloodroot ointment. After a few months of a nasty looking mess, the tumor was gone. Now there is just a bare spot. Is there a chance it will return? And could a tumor erupt somewhere else on the body? My horse is a 14-year-old Appaloosa gelding. (View Answer)

   Unfortunately, the answer is “Yes” to both your questions.  While it is great to hear that the tumor appears to have resolved with topical therapy, it is still possible that there is some tumor remaining.  Sarcoid tumors are recognized to occur in multiple clinical forms one of which can look like simply a bald patch of skin.  In addition, horses that have had a sarcoid are at higher risk for developing additional tumors compared to horses that never have had a sarcoid.  To ensure that you catch any new tumors while they are small it is recommended to perform a full skin exam on a regular basis.  If you see changes in the skin contact your veterinarian so he/she can evaluate and determine the best approach. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

10. My gelding has odd moles. He has one on his back, one on the inside of his hock and there is one new one on the upper jaw close to his throat latch. Sometimes these moles secrete a liquid and scab over without any damage being done to them. I can’t tell if they are sarcoids or something else entirely. Any ideas? (View Answer)

    It would really help to see a picture of your horse’s “moles”. There are many different possibilities.  Certain sub-types of sarcoid can look like moles.  Other tumors including squamous cell carcinoma, mast cell tumors, melanoma and lymphoma are all possibilities.  It is also possible that they are not a tumor at all as horses can develop bumps do to trauma or insect bites etc.  I would encourage you to take some pictures (with a ruler) and have your local veterinarian look at them.  Unfortunately, just looking at them does not always tell you what they are and in some cases a biopsy may be necessary.  Whether or not to perform a biopsy would depend on the location of the nodules and if they are growing.  I am always more worried about bumps that are growing as they may become more of a problem over time.  Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University

11. My 2-year-old Arabian gelding has developed small raised bumps on his neck, and a few on his sides since March of this year. My veterinarian examined him and noted they are most likely fly bites as reflected by the "pattern of bumps". We did an allergy panel and flies are low on the panel. Things that are BL and BL-P are Pyrethrum fly spray, fire ants, wasps, noseums, Bahia grass, Johnson grass, bluegrass, nettle, rye/fescue, cedar mix, mesquite and maple. He lives in north central Texas (close to Dallas) in a stall part-time w/shavings. He can go outside on pasture a couple of hours per day, which is a sandy loam soil. He eats 2x/day Heritage pellets 14% plus alfalfa and coastal hay (free choice). I use a natural fly spray now, and recently started feeding him MLS equine minerals & vitamins tub w/Clarifly (free choice). I plan to put a fly sheet w/neck cover on him (although I don't like having him in any horse clothing). I would like to know alternative methods to reducing his bumps other than allergy shots. Is there a possibility he will grow out of this? (View Answer)

    It is always difficult to know exactly why a skin lesion develops without additional testing.  Given the seasonal onset insect bites or allergic response is certainly a possibility.  In some cases, insect bites can cause a lump without an allergic reaction; the trauma and inflammation associated with the bite can result in temporary swelling and oozing.  Horses that are recently moved to an area with a large insect population will often have more severe reactions to insect bites the first season or two in their new environment and gradually become less reactive the following seasons.  This is in contrast to horses that develop an allergic reaction which usually results in multiple bumps or hives not always at the site of injury. 

    If it is truly an allergic response then allergy testing can help determine the cause of the hives.  I am curious about the allergy panel you had performed. Was it a blood test or did you skin test your horse?  Skin testing is the most reliable test available. Blood testing is less so and somewhat dependent on the individual laboratory.  If it is truly an allergy skin testing would be the best way to effectively determine the allergens and desensitization therapy.

    The first question I would try to answer is if your horse is truly having an allergic response or are the bumps the result of the bite and the inflammation.  This is important because the latter would not necessarily respond to desensitization therapy.

    If your horse is truly having an allergic reaction to insect bites there are only two effective ways to try to control the reaction.  One is to avoid the allergen (housing indoors with fans, fly sprays and fly sheets) the second is to try to decrease the allergic response. Desensitization with “allergy shots” is one way to decrease the response. There are also medications that can decrease the response as well but they can have potential long term consequences. Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University