Photosensitization in the Horse


Photosensitization is a serious skin condition characterized by "sunburned," crusty skin that dies and sloughs away. It is usually caused by a reaction to something the horse has eaten, but the skin problem does not appear until the animal is exposed to sunlight. Christine Rees, DVM, Dipl. ACVD (dermatology), assistant professor of dermatology in the college of veterinary medicine at Texas A&M University, says three factors contribute to the development of photosensitization: Presence of a photoactivating substance in the skin, exposure to ultraviolet light, and lack of skin pigment (which enables more UV light to penetrate the skin).

"There are three classifications of photosensitization: Primary photosensitization, hepatogenous photosensitization (due to liver impairment), and photosensitization due to abnormal pigment production," says Rees. "The latter, called porphyria, is genetic, very rare, and seen more often in cattle than in horses."

Primary Photosensitization

The photosensitizing agent (usually a plant or a drug) is eaten or injected and travels to the skin or contacts the skin, explains Rees. "Examples of plants that cause primary photosensitization include St. John's Wort, buckwheat, burr trefoil, smartweed, and perennial rye grass," she says. "Examples of drugs that cause photosensitization include phenothiazine, thiazides, rose bengal, acriflavines, methylene blue, sulfonamides (trimethoprim sulfa), and tetracycline."

At the University of Idaho's Analytical Sciences Laboratory and the Washington Animal Disease Diagnostic Laboratory, toxicologist Patricia Talcott, DVM, PhD, Dipl. ABVT, sometimes sees cases of photosensitization in horses. She also teaches at Washington State University's College of Veterinary Medicine.

"One form (of skin photosensitization) is caused by aberrant pigment metabolism, which arises from either inherited or acquired defects of enzymes involved in synthesis of porphyrins," she says. "This form is very rare. For practical purposes, I usually look at photosensitivity in just two categories. Primary photosensitization is when the animal ingests something (usually a plant) that contains a photodynamic pigment. For example, St. John's Wort contains a primary photodynamic pigment that, when ingested, is absorbed from the gastrointestinal tract and circulates in the bloodstream, and ends up in the skin--in the cutaneous circulation. It there reacts with light, primarily in the unpigmented areas of skin or thinly haired areas that have little protection from the sun's rays." This reaction produces classic skin lesions.

Stan W. Casteel, DVM, PhD, Dipl. ABVT, associate professor in the department of veterinary pathobiology at the University of Missouri's College of Veterinary Medicine, says photosensitization is usually due to horses eating something they normally would not ingest. "To prevent photosensitization, you need to prevent access to the plants containing these compounds," he says. "St. John's Wort, for instance, is also used as an herbal supplement for humans, and people should not take too much of it or they could become photosensitized.

"We don't see this type of photosensitization very often in horses, since most of the primary photosensitizers, particularly here in the Northwest, are rarely grazed by horses," adds Casteel. "Sometimes alfalfa has been associated with primary photosensitization, but nobody really knows why."

Secondary Photosensitization

Secondary photosensitization is a result of liver impairment and is the most frequent form seen in horses, according to Talcott. "Forage plants contain a lot of chlorophyll (the part that makes plants green), which is converted to a porphyrin (a photodynamic agent) called phylloerythrin, which in turn is excreted from the body through the bile ducts of the liver," says Talcott. "In an animal with a liver problem that disrupts biliary excretion, phylloerythrin is not properly excreted, leading to its buildup in the general circulation system."

Certain plants can cause liver damage, impairing that organ's ability to excrete phylloerythrin. In horses, some of the plants that can affect the liver include alsike clover (Trifolium hybridum) and red clover (Trifolium pratense). These are associated with hepatic disease and secondary photosensitivity. All the plants that contain pyrrolizidine alkaloids that can affect cattle also affect horses--plants like Senecio species (ragwort, groundsel), Cynoglossum (hound's tongue), and Amsinckia (fiddleneck). Other hepatotoxic (affecting the liver) plants that can be associated with secondary photosensitization include Kochia scoparia (Mexican fireweed), Tetradymia (rabbit brush), Xanthium (cocklebur), Crotalaria, rape seed, kale, heliotrope, tarweed, comfrey, rattleweed, paintbrush, horsebrush, bermudagrass, mushrooms, buttercup, and lantana. There are many others.

"Alsike clover and red clover are unique in our area as one of the more common causes of hepatic disease in horses," says Talcott. "If a horse pasture has a lot of clover, it could be a problem, depending on the amount eaten and various environmental conditions. Since we don't know what the toxic principle is, we can't tell owners how to manage it or how much the horse has to ingest to receive a toxic amount. I always tell clients to be very careful with alsike clover and red clover. There have been cases of poisonings in horses at levels of less than 20% of the diet. Since we don't know if it's something produced by the plant, or a mold that grows on the plant, it's hard to make concrete recommendations."

She says that the mold commonly found on alsike clover is also found on all other clovers; the problem might be due to a product produced by the plant in response to the mold.

"These two plants (alsike and red clover) cause liver problems unique to the horse; the majority of the rest of the offending plants will likely affect horses and cattle alike," says Talcott. "The only major difference is that horses are often in a situation where they are less apt to be exposed to some of those plants. Most owners have a different management system for horses (than cattle) and different quality or types of feeds."

Liver damage in a horse might not be noticed until the animal develops photosensitization. "There might be mild colic or weight loss, but this might go unrecognized as a liver problem," says Talcott. "Usually the photosensitivity lesions are so striking that this is the first clue there might be impaired liver function. The tricky part is if you are looking at horses that don't have any unpigmented areas, you'll never see that--and the animal may just go into a potentially very severe episode of liver disease, which may be fatal.

"The important thing in understanding photosensitivity is realizing there are two types--the primary versus the secondary," says Talcott. "When you see it (in the non-pigmented areas of skin), you need to have your veterinarian perform a thorough work-up, which includes blood testing to assess whether there is liver impairment. That will markedly direct the work-up thereafter and make a tremendous difference in prognosis."

Liver damage can be drug-related, or due to an abscess--anything that impairs the liver's ability to filter blood. Liver-caused photosensitization could happen in winter (since there can be enough chlorophyll in hay or hay pellets to cause a problem), depending on where you live, how much sunlight the horse is exposed to, and how much non-pigmented skin he has.

Rees explains that infections of the liver can be due to bacteria, fungi, Anacystis (blue-green algae) in water, or Phomopsis leptostromiformis (a fungus that grows on lupine plants).

"Cancer of the liver such as lymphosarcoma or hepatic carcinoma can also result in development of photosensitization," explains Rees. "Bloodwork, radiographs, or ultrasound are useful if an infection or cancer is suspected. Chemicals can also cause liver damage and subsequent photosensitization. Examples of chemicals that can affect the liver include copper, phosphorus, carbon tetrachloride, phenanthridium, and serum/antiserum."

Talcott says certain infectious diseases that don't affect ruminants will target the liver in horses. "Sometimes a horse will develop a hepatitis of unknown cause," says Talcott. "The liver is an organ that can be affected every now and then and you never figure out why. Ascending bacterial infections are also not uncommon in horses, coming from the gut and going up the bile duct. You may not be aware that the horse has an infection until you draw a blood sample."

Signs of Photosensitization

Skin lesions are the usual sign, although the cornea of the eye can be affected as well. "In severe cases, people might think the lesion is a laceration or injury," says Talcott. "The eye may look sunburned or may water. If the horse has white socks or a white nose, the owner might think the animal got into barbed wire because the lesion is so deep and bleeding."

The skin is red, swollen, and blistered. When the blisters break, the raw areas can ooze and become infected secondarily, creating pus. The area is often swollen, crusty, and scaling, and it will ultimately slough and peel away.

"The lesions may not be confined to the face and muzzle; a light-skinned horse may get it under the tail as well," says Casteel.

The problem is seen most often in summer when sunlight is intense (more ultraviolet light penetrating the atmosphere, with stronger UV rays), and most commonly in horses at pasture eating green plants. The crusting lesions are found on lightly haired and/or non-pigmented areas of the body, says Rees.

"Usually if a toxic plant is involved, multiple animals will be affected and all of them will be in the same pasture," explains Rees. "A definitive diagnosis can be based on skin biopsy; the skin biopsies for horses with sunburn look different on histology than photosensitization."


Whether it's primary or secondary photosensitization, it's important to get the animal out of the sun. "The length of time necessary will depend on whether it's a primary or secondary problem," explains Talcott. "The animal needs basic supportive care for the lesions. This typically consists of some type of bathing to clean the lesions or use of a topical ointment to ease the pain and inflammation, depending on the location and severity."

Cold water (hydrotherapy) can help reduce swelling on the lower legs. Steroids can help relieve the inflammation.

"Topical antiseptics or antibiotics can help if a secondary infection is present," says Rees. "If infection is severe, systemic antibiotics may be necessary."

Casteel says that it is important to prevent secondary bacterial infections because the skin is raw and open. "Antibiotic creams are used, but to get adequate protection, you also need an injectable antibiotic, especially if the horse has an infection," he says. "The wounds should be protected from flies; flies are attracted to open, oozing areas to lay their eggs and may produce maggot infection.

"Once you get the animal out of direct sunlight, there is some immediate relief from pain," continues Casteel. "Also keep in mind that the onset of the chemical reaction that produces the skin problem is instant. If you put an affected animal back out into the sunlight, it will begin to squirm and be uncomfortable within five minutes.

An affected animal can be kept indoors during daylight hours and grazed at night. The barn doesn't have to be dark; the animal just needs to be protected from direct sunlight.

"The healing process can be prolonged," says Casteel. "The horse may shed large chunks of skin, just like leather. Recovery may take months, because once the skin is gone, it must re-grow."

Talcott adds that if there is a liver problem, "it may require additional diagnostics to determine the cause of the hepatic disease. This is what you really have to treat; the photosensitivity lesions are the least of that horse's problems.

"The treatment, in addition to wound therapy, will be based on whatever is the inciting agent for the liver problem," and should be aggressive, continues Talcott. "The cause may vary tremendously--from a mild bout of hepatitis to a fibrotic liver problem from which the horse cannot recover. Usually, by the time the liver is not functioning enough to metabolize phylloerythrin, there is a significant part destroyed. Some animals do recover, but it isn't something to be taken lightly."

If a horse develops photosensitization more than once, it might be due to certain plants in the pasture, or a liver problem (that might possibly be due to recurring infection). "You'd want to look at the liver to see if there is impairment, or if the animal is getting repeated exposure to a primary photosensitizer."

Take-Home Message

Many horses with lightly pigmented skin can get sunburn, but some horses will suffer from a severe skin reaction to the sun when they have ingested certain plants or been given specific drugs, including some antibiotics. Liver problems also can make horses more sensitive to sunlight. If your horse develops skin problems in lightly pigmented areas, seek advice from your veterinarian in order to determine its cause.

HOW DOES PHOTOSENSITIVITY WORK? Pathogenesis of Photosensitization

In order for skin lesions to occur, a photodynamic agent in the skin must absorb UV light. This produces oxygen radicals, which in turn damage the skin. "The word photosensitization simply means hypersensitivity to sunlight. A certain band of the sun's rays is what causes it," says Stan W. Casteel, DVM, PhD, Dipl. ABVT, associate professor in the Department of Veterinary Pathobiology at the University of Missouri's College of Veterinary Medicine.

When a photodynamic agent eaten by the horse is absorbed from the gut and into the bloodstream, it travels to the skin. When the specific ultraviolet rays from sunlight contact the compound, they activate it. "The sun's rays put it into an excited state and it reacts with adjacent bio-molecules (proteins in the capillaries or skin cells, saturated and unsaturated fatty acids, or the nucleic acid in DNA or RNA)," explains Casteel. "If these rogue activated molecules interact with life-sustaining bio-molecules to a significant extent, they destroy the cells." If enough cells are damaged, the skin dies and sloughs away.--Heather Smith Thomas

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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