Equine Health Library

Pleasure Horse

Other Concerns

Skin Conditions | Other Conditions | Infectious Diseases

Dermatophilosis (rain rot)

Dermatophilosis goes by many names. You may have heard it called rain rot, mud fever, or dew poisoning. This common skin disease characterized by crusting and pustules is caused by an Actinomyces-type of bacteria (Dermatophilus congolensis) that infects broken, abraded or otherwise traumatized skin. Horses that are in poor condition, under stress or have a compromised immune system are also more at risk. Infections are more common during moist, humid weather.

Lesions begin as bumps or papules that progress to weeping pustules covered by matted hair and crusts. The rump, top of the back, and face are the body regions most commonly affected – places exposed to rain and prolonged moisture.


Treatment includes keeping the horse dry and clean and protecting the affected areas from biting insects. Topical antibacterial and keratolytic shampoos can help remove the crusts. Severe cases should be seen by a veterinarian and may require systemic antibiotics. To avoid spreading this skin disease between horses do not share brushes, tack, or blankets and practice good hand hygiene after handling affected horses.

Equine pastern dermatitis (scratches)

Equine pastern dermatitis, also known as scratches, mud heel, grease heel and cracked heels, is one of those skin conditions that nearly every horse owner deals with at one time or another. It’s an infectious disease with a variety of causes that include Staphylococcus aureus (bacteria), Chorioptes mites (external parasite) and Dermatophilus congolensis (bacteria).

Scratches is a moist dermatitis that sometimes includes pustules and typically affects the skin along the back and sides of the pasterns. Non-pigmented (white) limbs are more commonly affected.


Therapy includes clipping the affected areas followed by use of an antiseptic cleanser and application of an astringent and antimicrobial ointment. For severe cases, please call your veterinarian.

Prevention of scratches, particularly for horses prone to this condition, includes keeping horses’ legs dry and clean.


Melanomas are malignant tumors that can be locally invasive, as well as metastatic (spread internally to distant sites). These tumors are often progressive and can have an unpredictable pattern of growth. Common sites for external melanomas include the corners of the lips, the external genitalia, under the tail, and surrounding the anus. External melanomas can vary in appearance from flat to wart-like to pedunculated (attached to the skin by a stalk).

Horses also can have melanomas inside their bodies in areas such as the abdominal cavity, lymph nodes, salivary glands, mammary glands, the vertebral column, and the eye.

Owners of gray horses take note; there’s a good chance your horse will develop melanomas at some point in its life. Some statistics suggest that up to 80 percent of gray horses older than 15 years of age will develop at least one melanoma, while the overall incidence among the general horse population is only 3-15 percent. Melanomas in gray horses are usually slow growing but can cause the horse problems as the tumor gets larger and/or metastasizes. For that reason, melanomas should be addressed when they are small. Melanomas do occur in non-gray horses and are often more aggressive in those horses.


Call your veterinarian if you notice a mass or swelling on your horse. Your veterinarian may recommend one or more of the following: surgical excision, immunostimulant vaccines, intralesional chemotherapy and/or an autologous vaccine made from your horse’s own tumor. Melanomas, even small ones, should not be ignored. Your veterinarian will help determine the most appropriate course of treatment.

Sarcoid tumors

Sarcoids are the most common skin tumors in horses, donkeys, and mules. They are benign, locally invasive, and non-metastatic. Although sarcoids can occur on any part of the horse’s body, they are more commonly found on the head (especially around the eyes and ears), ventral abdomen (especially around the sheath), axilla (armpit), groin, and legs. Sarcoids are not usually life-threatening but can be performance-limiting or cause cosmetic problems depending on their size, location and rate of growth. Bovine (cow-origin) papillomaviruses are believed to play a role in the development of sarcoids in horses, but the exact mechanism remains uncertain.

Sarcoid appearances can vary and include a combination of:

  • Verrucous: Wart-like with a dry, hard, cauliflower-like appearance
  • Fibroblastic: Fleshy and aggressive, may have a pedicle or stem attachment to underlying tissue
  • Occult: Slow-growing and flat in appearance
  • Nodular: Discrete, firm nodules under the skin

Therapeutic Options 

  • Surgical removal
  • Radiation therapy
  • Topical treatment
  • Photodynamic therapy
  • Intralesional therapy

Call your veterinarian if you notice a mass or swelling on your horse. Your veterinarian will help determine which combination of therapies is best suited to your horse depending on the type of sarcoid and location.

Equine Recurrent Uveitis (ERU)

If your pleasure horse is an Appaloosa, you’ve probably heard of Equine Recurrent Uveitis (ERU), also known as moon blindness or periodic ophthalmia. ERU is the most common cause of blindness in horses and most often occurs in Appaloosas, draft breeds, and European Warmbloods, although it can strike any breed.

ERU is an immune-mediated disease of one or both eyes. The inciting cause can vary and as mentioned above, there is a known genetic predisposition in some breeds. Most affected horses experience periods of quiescence with few or no clinical signs, followed by bouts of active inflammation of the inside of the eye. That’s how moon blindness got its name – early horsemen thought the intermittent eye disease was related to the cycles of the moon.

Clinical Signs

  • Frequent blinking or squinting
  • Photophobia (sensitivity to bright light)
  • Excessive tearing
  • Reddened conjunctiva
  • Blood-shot or injected sclera (white of the eye)
  • Constricted pupil
  • Cloudy, opaque cornea (surface of the eye); yellow glow to the interior of the eye
  • Recurrent bouts of uveitis frequently lead to scarring, cataract formation, and blindness.


There is no cure for ERU. The goals of treatment are to minimize the horse’s discomfort and prolong vision. Successful management requires early recognition and aggressive treatment. Treatment options include topical and systemic anti-inflammatory medications to reduce inflammation and pain. Surgical implantation of a small disc containing immunosuppressive medication just under the surface of the eye can help to decrease frequency and severity of ERU flares and help slow the progression of the disease.

Monitor your horse’s eyes closely, particularly if he or she is a breed with an increased risk of ERU. Call your veterinarian immediately if you suspect an eye problem of any sort.


Owning a mare comes with its own set of challenges, namely hormones. Some mares can be docile and even-keeled, while others squeal, kick and make a fuss when they come into heat. If you need ways to manage your mare’s hormone-related behavior problems, talk to your veterinarian about hormonal treatment options, such as altrenogest.

You can help reduce undesirable changes in behavior and mood by suppressing your mare’s estrus (heat) cycle with Regu-Mate® (altrenogest). Within three days of the start of treatment, Regu-Mate will effectively suppress estrus in 95% of mares.

For more information, Download the Better Mare Management Brochure.

Click here for more information provided about Mare Care.
Click here for more information provided about Foal Care.

Important Safety Information

Regu-Mate® is contraindicated for use in mares having a previous or current history of uterine inflammation. Natural or synthetic gestagen therapy may exacerbate existing low-grade or “smoldering” uterine inflammation into a fulminating uterine infection in some instances. Do not use in horses intended for human consumption. Avoid skin contact. Regu-Mate® is absorbed through unbroken skin, and exposure may result in serious side effects to both women and men. Wear vinyl, neoprene, or nitrile gloves when handling or administering Regu-Mate®, or when touching contaminated surfaces or equipment. Latex gloves are not protective. PREGNANT WOMEN OR WOMEN WHO MAY BE PREGNANT SHOULD NOT HANDLE REGU-MATE®. WOMEN OF CHILDBEARING AGE SHOULD EXERCISE EXTREME CAUTION WHEN HANDLING THIS PRODUCT.  Regu-Mate® is readily absorbed through the skin and could lead to a disruption of the menstrual cycle or prolongation of pregnancy. Accidental spillage on the skin should be washed off immediately with soap and water. Keep out of the reach of children. For complete safety information, refer to the product label.


At your horse’s annual physical exam, your veterinarian will listen to your horse’s heart to make sure there are no signs of underlying heart disease.

Loud murmurs, rapid heart rates, and irregular heartbeats are potential signs of underlying disease and can be performance limiting. If abnormalities are detected, more sensitive diagnostics such as an echocardiogram (ultrasound of the heart) or electrocardiogram (ECG) might be indicated to determine the cause and severity of the heart dysfunction.

Toxic plant diseases

If your horse has access to pasture, it’s imperative that you learn to recognize toxic plants. While most horses will not readily consume these plants, they may ingest them if other forage sources are lacking.

  • Red Maple toxicosis follows ingestion of wilted or dried red maple (Acer rubrum) leaves. The toxin causes destruction of red blood cells inducing a hemolytic anemia. Clinical signs include weakness, loss of appetite, low-grade fever, mild colic, elevated heart and respiratory rates, and passage of red-discolored urine (due to the destruction of red blood cells). There is an increased incidence during the late summer and fall months when red maple leaves are wilting and falling off the tree. Therapy includes intravenous fluids and blood transfusion. Oxygen administration may be beneficial. The prognosis for horses with red maple toxicosis is guarded to poor.
  • Slaframine toxicosis is caused by a fungal mycotoxin that infects legume forages, especially red clover and, less commonly, alfalfa and white clover. Ingestion of the offending toxin produces “slobbering” or varying degrees of profuse salivation in affected horses. Increased salivation can occur within an hour of exposure to the fungus-infected forage. Removing horses from infected pastures or discontinuation of feeding infected hays is the primary treatment.
  • Alsike clover toxicosis can cause chronic liver failure, photodermatitis, and acute neurologic impairment. Horses with liver failure typically show loss of appetite; progressive weight loss; dry, poor hair coat; head pressing; excessive yawning; icterus (yellow discoloration of mucous membranes); and death. Horses experiencing the neurologic syndrome display a variety of signs, including alternating periods of depression and excitement, head pressing, yawning, aimless wandering, teeth grinding and coma preceding death. Toxicity can occur following ingestion of alsike clover on pasture or in hay. Some horses that develop liver disease may also experience signs of photosensitization characterized by reddening of the skin accompanied by itching and hair loss.
  • Many ornamental and landscaping plants – including rhododendron, azaleas, yew, and oleander – also are toxic to horses. For example, only 10 to 20 ounces of yew leaves can be lethal for an adult horse. Oleander contains cardiac glycosides that cause colic, loss of coordination, profuse sweating, difficulty breathing, muscle tremors, abnormal heart function and death in horses that consume even a small amount of this flowering shrub.
  • Black walnut tree bark also is toxic for horses and can cause severe laminitis if ingested or if black walnut bark is used as bedding.

Walk your pastures on a regular basis to ensure there are no potentially toxic plants. During periods of drought when grasses die back, weeds and other plants may suddenly become more appealing to your horses. Create a buffer zone between pasture fence lines and nearby plant growth to reduce the risk of your horses reaching over and nibbling on potentially hazardous vines, shrubs, and trees. Your local extension agent is often a good source of information on pasture grasses and plants.

Call your veterinarian right away if you suspect your horse ingested a toxic plant.


Also known as equine granulocytic ehrlichiosis (EGE), anaplasmosis is caused by the organism Anaplasma phagocytophilum, which is transmitted to horses by ticks and infects white blood cells.

Clinical Signs

  • Fever (102 to 106 degrees Fahrenheit)
  • Depression
  • Loss of appetite
  • Swollen limbs
  • Unsteady gait (ataxia)
  • Yellow discoloration of the gums and sclera (white portion of the eye)

Without treatment illness may last 10-14 days and relapses can occur. Anaplasmosis typically occurs during fall, winter, and early spring. Any age horse may be affected, but clinical signs may be less severe in horses younger than four years of age.

This disease has been reported in horses residing in states along the west coast, northern Midwest, northeast and Mid-Atlantic regions of the country. Cases have also been reported in Canada, Brazil and northern Europe.

Diagnosis and treatment

Diagnosis is based on clinical signs and blood test results. Therapy includes administration of antibiotics effective against A. phagocytophilum such as intravenous oxytetracycline and oral doxycycline. Because there is no vaccine against anaplasmosis, prevention focuses on tick control using permethrin-based repellent products and prompt removal of ticks already attached.

Lyme disease

Lyme disease is caused by the spirochete bacteria Borrelia burdorferi. The disease circulates in deer and the white-footed mouse. Two types of ticks, Ixodes scapularis and Ixodes pacificus, feed on infected deer or mice and then transmit the organism to horses. Many horses living in the northeastern states, Mid-Atlantic states, and northern Midwest have been exposed to infected ticks – but it is unknown how many exposed horses actually develop clinical disease. Unfortunately, Lyme disease is not as well defined in horses as it is in humans and dogs.

Clinical Signs

  • Low-grade fever
  • Lethargy
  • Vague, shifting leg lameness
  • Stiff gait
  • Behavior changes
  • Muscle tenderness
  • Neurologic disease


In many cases, diagnosis is difficult and includes a thorough physical exam, exclusion of other possible diseases, and testing the horse’s blood for antibodies against the disease.


Therapy includes a prolonged course of treatment with specific antibiotics (intravenous oxytetracycline for several weeks, if possible, followed by oral doxycycline or minocycline).

Because no equine vaccine exists, prevention focuses on tick control using permethrin-based repellent products and prompt removal of ticks already attached.

Pigeon Fever

Also called dryland distemper, pigeon fever is an infectious and contagious disease caused by Corynebacterium pseudotuberculosis (bacteria). C. pseudotuberculosis is transmitted by flies and infects horses through small abrasions or wounds in the skin, usually along the ventral midline. The organism lives in the soil and outbreaks of pigeon fever are more common during dry, hot conditions. Reports of pigeon fever used to be confined to horses residing in western and southwestern regions of the country, but drought conditions have resulted in reports of the disease throughout the United States.

Clinical Signs

The incubation period (time between exposure and signs of disease) is three to four weeks. The most common form of the disease is characterized by external abscesses along the ventral abdomen and/or prepuce and across the chest or pectoral region. Swelling and abscess formation in the chest region resembles a pigeon’s breast, which explains the common name, pigeon fever. External abscesses can also form elsewhere on the horse’s body.

Two less-common forms of the disease:

  • Internal abscesses involving various organs (including the lungs, liver, spleen and kidney)
  • Infection of the limbs leading to swollen legs and lesions that ulcerate and drain, called “ulcerative lymphangitis”


Diagnosis of pigeon fever is usually based on clinical signs and culture of the fluid within the abscesses. Ultrasound of the affected area can also be useful in determining a diagnosis.


Treatment for horses with external abscesses focuses on opening, draining, and flushing visible abscesses. Drainage from the abscess can infect other horses, and strict hygiene and biosecurity should be observed when opening and flushing external abscesses. Good fly control is vital to reduce the risk of transmission.  Horses with internal abscesses and/or ulcerative lymphangitis require prolonged courses of antibiotic therapy.

No vaccine is available. Strict hygiene, good sanitation, and effective fly control are essential in preventing the spread of the disease.

Potomac Horse Fever (PHF)

Caused by the rickettsial organism Neorickettsia risticii, PHF was first identified in the late 1970s in horses residing along the Potomac River in Maryland. Since then, PHF has been reported in 43 states, South America, India, Europe, and parts of Canada.

The disease occurs near freshwater streams and rivers or on irrigated pastures with peak incidence during the summer and fall months. Horses can become infected with N. risticii by ingesting several different aquatic insects, e.g., dragonflies, mayflies, and caddisflies, that are infected with the rickettsial organism.

Signs of PHF

  • High fever (102 to 107 degrees Fahrenheit)
  • Depression
  • Mild colic
  • Loose manure, diarrhea
  • Edema along the ventral abdomen
  • Laminitis (founder) that frequently affects all four feet
  • Pregnant mares who have recovered from PHF earlier in their pregnancy can subsequently abort around seven months of gestation


Diagnosis of PHF is usually based on clinical signs, time of year, environmental risk, and blood testing (PCR and/or antibody titer).


Therapy includes intravenous oxytetracycline, intravenous fluid therapy, anti-diarrheal medications, anti-inflammatory drugs, and preventive therapies for laminitis.

Vaccines against PHF are available, but protection is often incomplete. Talk to your veterinarian about whether vaccination is recommended for your horse.


Botulism is caused by toxins produced by the anaerobic (requires an environment with no oxygen to survive), spore-forming bacteria, Clostridium botulinum. C. botulinum is found in the soil in certain parts of the country – particularly throughout the Mid-Atlantic region of the country, including Kentucky and Tennessee.

Horses and foals can be exposed to the botulinum toxin through ingestion of contaminated feed (ex. improperly fermented feeds, rotting vegetation, or carrion-contaminated feedstuffs) or through wounds (including the umbilicus of foals and castration sites). The toxin produced by C. botulinum disrupts nerve and muscle function leading to a variety of clinical signs.

Clinical Signs

  • Decreased tail, tongue, and eyelid tone
  • Inability to prehend food and swallow
  • Severe muscle weakness
  • Eventual paralysis leading to recumbency and inability to stand
  • Decreased ability to breathe leading to respiratory arrest and death

Foals affected with botulism have been called “shaker foals” because of the muscle weakness and trembling that occurs when they attempt to stand.


Diagnosis of botulism is usually based on history, clinical signs, and exclusion of other possible diagnoses. Diagnosis can be aided by testing of suspected contaminated feedstuffs for presence of the botulinum toxin.


Treatment involves intravenous administration of botulinum antitoxin along with antibiotics effective against C. botulinum. Good nursing care is essential to provide fluids and nutrition for foals and horses unable to eat and swallow. If you live in a high-risk area, ask your veterinarian about vaccination as a preventive measure.

Broodmares are vaccinated during late pregnancy with the goal of increasing anti-botulinum antibodies in their colostrum, which, in turn, will be ingested by their foals. Plan ahead if your broodmare will be shipping to an at-risk area for foaling and/or re-breeding.