Complicated Equine Skin Diseases

Vesicular stomatitis, a viral infection, occurs sporadically in the southwestern United States and is thought to be transmitted by sand flies and black flies.

Photo: Courtesy Dr. Josie Traub-Dargatz

"The practice of equine dermatology is usually straightforward with clinical examination and diagnostic testing; it is a rare occasion for an equine skin condition to be considered an actual emergency," began Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, associate professor in the department of Large Animal Clinical Sciences at Michigan State University. However, complicated equine skin disorders do exist. During a presentation at the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., Rashmir-Raven reviewed complicated, catastrophic, and reportable skin diseases.

Vesicular Stomatitis

Rashmir-Raven first discussed vesicular stomatitis, a reportable disease that causes painful blistering and erosions in and around the mouth and around the muzzle, teats, or hooves of horses, cattle, goats, swine, deer, and some other livestock. She noted that vesicular stomatitis lesions look very similar to those caused by foot and mouth disease, a foreign animal disease in the United States.

Vesicular stomatitis, a viral infection, occurs sporadically in the southwestern United States and is thought to be transmitted by sand flies and black flies. Infected animals with open sores can expose herdmates to the disease through close contact or by sharing feed buckets or bits. As a precaution, all infected and susceptible livestock on a premise are quarantined until at least 30 days after all infected animals have healed.

"Diagnosis is based on antibodies to fluid samples from the animal or by isolation of the virus from swabs of lesions, blister fluid, and tissue flaps," she explained. There is no specific treatment for the disease other than supportive care, Rashmir-Raven noted, but affected horses generally recover in about two weeks.

"Veterinarians who suspect an animal may have vesicular stomatitis or any other vesicular disease should immediately contact state or federal animal health authorities," she stressed.


Caused by the organism Pythiom insidiosum, pythiosis is a complicated and potentially fatal skin disease with limited treatment options. As previously reported, pythiosis has been spreading from the traditional tropical climates to as far north as Washington state.

The causative agent resides in standing water or moist soil and finds its way into small wounds on a horse's legs, lower abdomen, chest, or face, causing large, tumorlike masses that ulcerate. These itchy lesions contain necrotic (dead) tissue with characteristic draining tracts, and they contain stony masses called "kunkers" at their core.

Rashmir-Raven explained that a presumptive diagnosis can be reached based on lesion characteristics and the presence of kunkers, and the diagnosis can be confirmed via serologic testing. "Lesions in horses are best treated with a combination of therapies including immunotherapy, radical surgical excision of the lesion, and topical application of antifungal solutions," she added.

The prognosis, she noted, is good for horses treated early and aggressively; however, it "declines rapidly" if the horse remains untreated within a month of developing signs.

Hepatogenous Photosensitization

Sometimes skin diseases really aren't just skin deep. Such is the case with hepatogenous photosensitization, Rashmir-Raven explained.

Hepatogenous photosensitization is a result of liver impairment, often caused by ingesting a toxic plant (such as ragwort or lantana). The process isn't simple: Forage plants contain chlorophyll (the part that makes plants green), which is converted to a porphyrin (a photodynamic agent that basically causes a toxic reaction when exposed to sunlight) called phylloerythrin. The bile ducts of the liver in turn excrete phylloerythrin from the body . In an animal with a liver problem that disrupts biliary excretion, phylloerythrin is not properly excreted, leading to its buildup in the skin and general circulation system.

The result is lesions on the skin--especially around the muzzle, nostrils, and eyelids--that can progress to necrosis and skin sloughing, Rashmir-Raven said.

Diagnosis isn't as straightforward as vesicular stomatitis and pythiosis. Because there's more than one type of equine photosensitization--hepatogenous and primary (see sidebar at left)--determining which kind the horse has is a first priority. "To determine the type of photosensitization being encountered, it's important to perform liver function tests (via serum chemistries)," Rashmir-Raven said. Also, she noted, it's important to review a detailed history of any drugs the horse has consumed and to evaluate the pasture or hay he eats for the presence of potentially dangerous plants.

"The primary goal of treatment in any photosensitization is to prevent further damage," Rashmir-Raven said. She suggested:

Keeping the horse stalled during daylight hours and turning him out at night to avoid further sun damage;

Cleaning lesions gently as needed;

Feeding grass hay that is free of any "offending" weeds or plants and has a moderate protein content.

Of course, the liver damage must be treated as well; however, Rashmir-Raven said treatment depends on the extent of damage and should be decided by the attending veterinarian. Horses can suffer liver disease from a variety of causes and each needs specific treatment.


Finally, Rashmir-Raven closed by discussing anhidrosis--the inability to sweat. "The result is exercise intolerance, and in extreme cases, collapse and death can occur," she explained. Affected horses might also experience tachypnea (increased respiratory rate), respiratory distress, and elevated rectal temperature, she added.

While anhidrosis doesn't have a breed, gender, age, or coat color predisposition, Rashmir-Raven said it's most commonly noted in hot and humid climates and tends to be more prevalent among horses in training.

A veterinarian can make a presumptive diagnosis if the horse doesn't sweat; however, he or she can make a definitive diagnosis via an epinephrine or terbutaline injection test, in which different concentrations of the drug are injected to prompt sweating. During this test normal horses respond to all concentrations administered while horses with anhidrosis will only respond to the highest concentration.

"The most successful remedy for anhidrosis is to move the horse to a more temperate climate," Rashmir-Raven said. Fully understanding that's not always an option, she also suggested some other management techniques:

Keep the horse stalled and use barn fans to keep the area cool;

Work the horse during the coolest part of the day and, if needed, adjust exercise intensity accordingly; and

Try feeding a nutritional supplement designed to help anhidrotic horses.

Medical management of anhidrosis can also include clenbuterol administration prior to work, vitamin E supplements, iodinated casein (a protein found in milk that is used as a binding agent) administration, thyroid supplementation, and others. Rashmir-Raven stressed it's important to discuss these options with a veterinarian before implementing them.

Take-Home Message

While not all equine dermatology problems are difficult to manage, it's important to have an understanding of how to diagnose and treat the ones that are. Work closely with a veterinarian to manage difficult skin conditions.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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