Photosensitization, or light-induced dermatitis (photodermatitis), is a noncontagious condition in horses where the skin becomes extremely sensitive to sunlight. This condition often mimics a sunburn, but it is much more serious and painful. 
Photosensitization is most commonly caused by ingesting toxic plants containing pigments, which are absorbed from the gastrointestinal tract and transported to the skin. When exposed to ultraviolet (UV) rays from sunlight, the pigments cause a complex photosensitivity reaction in the horse’s skin. 
Non-pigmented (i.e. white) skin is especially sensitive to reactive compounds, as is skin with little hair cover (i.e. muzzle, eyelids, ears). 
Successful treatment of photosensitization requires addressing the cause of the condition and administering supportive care. It is important to alleviate irritation and potential infection in the damaged skin to allow for healing.
Photosensitization is difficult to diagnose and distinguish from sunburn.  If you think your horse could be affected by photosensitization, contact your veterinarian immediately for an examination.
Photosensitization in Horses
Horses with photosensitization experience a serious adverse reaction to sunlight due to the presence of photodynamic agents in their skin. Photodynamic compounds can include plant pigments, fungal toxins, bacteria, and chemicals. 
Ultraviolet light from the sun activates these compounds, resulting in chemical reactions that damage skin cells and cause significant irritation for the horse. 
The face and body can develop red rashes (erythema), lesions, leaking wounds and scabs. In severe cases, necrosis (tissue death) can occur.  Horses may begin rubbing or scratching the affected area, resulting in further tissue damage.
Photosensitization can affect both pigmented and non-pigmented areas of the body.  However, tissues from lightly pigmented areas of the body or areas with less hair are often severely affected since they have less protection from the sun.
This includes, but is not limited to, areas of the face (muzzle, ears, eyelids, lips) and body (tail, coronary bands, vulva).  Horses with light skin and hair are commonly affected all over the body.
Clinical signs of photosensitization tend to develop within hours of sunlight exposure.  However, some signs of photosensitivity do not present for several weeks following sun exposure.
The following symptoms may indicate photosensitivity in your horse:. 
- Hair loss
- Photophobia (eye discomfort in light)
- Scratching and rubbing ears, eyelids and muzzle
- Redness and swelling of the skin
- Skin lesions, hives and scales
- Edematous swelling
- Blisters with pus
- Scab formation
- Oral lesions
- Diarrhea or mild colic
Horses kept indoors during the day or with minimum exposure to sunlight may display subtle signs of photosensitivity, with slight edema of the skin being the most prominent symptom. 
Causes of Photosensitivity
Photosensitivity in horses is categorized as either primary or secondary, depending on the causative agent triggering the skin reaction:
- Primary photosensitivity: Occurs following ingestion or topical exposure to a phototoxic agent, such as toxic plants or chemicals.
- Secondary photosensitivity: Occurs due to liver dysfunction in the horse, resulting in improper clearance of phototoxic agents.
Some forms of photosensitivity have an uncertain cause but may be related to the consumption of forages, such as lush white clover. 
In all cases, UV light exposure activates phototoxins, resulting in skin sensitivity and tissue damage.
Phototoxins are compounds that cause a toxic response in the skin when activated by the presence of UV light. These toxins can have identifiable or unknown origins, often originating from within plants, forages and weeds. 
Plant pigments ingested by horses can be highly phototoxic after coming into contact with UV light. These phototoxins are absorbed through the gastrointestinal tract and are transported via the circulatory system to the soft tissues of the skin, exposed mucous membranes and eyes. Phototoxins may also be absorbed directly through the skin following topical exposure.
UV light transforms the toxic compounds into an activated state, triggering a destructive cascade of reactions within skin tissue cells.
Primary (Type 1 or systemic) photosensitization occurs when skin cell membranes get damaged by a phototoxic agent that enters the circulation system through ingestion, absorption or injection.
Following exposure of the phototoxin to UV light, a toxic reaction occurs, resulting in severe skin inflammation. Primary photosensitivity is often acute with a very rapid onset.
This type of photosensitivity is rare in horses since exposure to causal agents at pasture is highly unlikely.  Below are some of the toxic plants and other chemicals that can cause primary photosensitivity.
St. John’s Wort
St. John’s Wort (Hypericum perforatum) is a medicinal herb that is native to Europe. It can be identified by its height, dark green leaves, tiny spots and yellow flowers.
The plant contains several photodynamic compounds that can cause primary photosensitization in horses and other livestock.
When the horse ingests St. John’s Wort, a highly fluorescent pigment called hypericin enters the bloodstream and circulates in the body.  Hypericin becomes energized under the skin when it comes in contact with sunlight, causing extensive damage to surrounding cell membranes. 
Horses are highly susceptible to hypericin toxicity due to a lack of liver enzymes to break down this toxin, allowing hypericin to enter the bloodstream rapidly. 
Illness in livestock due to St John’s Wort ingestion has been reported in Europe, America, Australia, New Zealand and other surrounding islands.
Other plants that can induce primary photosensitivity in horses include: 
- Buckwheat (Fagopyrum esculentum)
- Bishop’s weed (Ammi majus)
- Spring parsley (Cymopterus spp.)
- Some varieties of clover (Medicago spp.)
Other Photodynamic Agents
The following photodynamic agents have been known to induce primary photosensitivity in horses: 
- Chemicals (fly sprays, coat products)
- Phosphorus fertilizer
- Pesticides (pentachloro-phenols)
- Coal tar pitch
- Wood preservatives
- Medication (tetracycline, phenothiazine tranquilizers)
- Moldy feed (aflatoxin B)
Researchers have also found a potential link between photosensitization and gluten allergy in horses. 
Secondary photosensitivity develops due to pre-existing impaired liver function in the horse. The liver plays an important role in detoxifying and breaking down toxic substances in the horse’s body.
Horses with a chronically or acutely diseased liver cannot properly metabolize and excrete phylloerythrin. This is a chlorophyll by-product made by microorganisms in the gut following the ingestion of plants.
Phylloerythrin is a potent photodynamic compound that is very sensitive to light.  In horses with normal liver function, phylloerythrin is excreted into the horse’s bile to be removed from the body.
However, in horses with liver disease or dysfunction, phylloerythrin re-enters circulation and causes a phototoxic reaction when it travels through and under the skin.
These chlorophyll metabolites become energized when exposed to UV light, injuring surrounding blood vessels and cell membranes and causing sudden necrosis of the skin. 
Many primary photosensitizing agents can induce liver damage in horses, indirectly causing secondary photosensitization. This makes it difficult to distinguish between the two types of photosensitization. 
Hepatopathy, or liver disease, is fairly common in horses and often occurs in outbreaks. These outbreaks can be caused by plant toxicosis or infectious pathogens, with new research suggesting viral hepatitides as a causal agent.  However, most diagnoses of liver disease lack a clear cause or etiology.
The clinical signs of liver disease in the horse are: 
- Weight loss and poor appetite
- Depression and lethargy
- Jaundice (yellow skin)
- Behavioural changes
If you think your horse may have liver disease, contact a veterinarian immediately for an examination.
Pyrrolizidine alkaloids (PA)
Plants containing pyrrolizidine alkaloids (PA) are common causal agents of secondary photosensitivity. These secondary metabolites found in toxic plants can cause severe damage to the liver and nervous system of animals, leading to an accumulation of phylloerythrin in the skin and blood.
More than 350 PAs have been identified, with approximately 3% of all flowering plant species containing one or more of the phytotoxins.  Many of these plants are invasive species that contaminate pastures, fields and even dry hay. 
The plant families most commonly associated with hepatotoxicity include:
- Boraginaceae (forget-me-not)
- Asteraceae (daisy)
- Fabaceae (legumes)
Horses are highly susceptible to acute or chronic liver damage following PA toxicosis, especially if pyrrolizidine alkaloids are consumed in large quantities over long periods of time. 
Fortunately, recent evidence suggests that PA toxicosis is uncommon in horses.  Horses will eat toxic plants in areas of drought or if other, more palatable forage is unavailable. However, they will rarely eat toxic plants under normal conditions.
Plants can become infected with mold or fungi when rotting or subjected to wet, damp weather. These fungi produce phytoalexins, which may be toxic when ingested. Research shows these antimicrobial compounds can cause photosensitization when ingested by livestock. 
The horse is a monogastric species, making it more susceptible to mycotoxins than ruminants. Outbreaks of mycotoxin-associated hepatopathy in horses are often resolved once the horse is removed from the source of the fungi. 
Alsike clover (Trifolium hybridum) is a common forage crop for livestock in northern Canada because it is adapted to cool climates and poorly drained soils.  However, horses that ingest this clover can develop serious illnesses, including: 
- Primary photosensitivity
- Secondary photosensitivity
- Alsike clover poisoning
- Enlarged liver
Researchers believe that mycotoxins grown on clover in wet conditions may contribute to phototoxic reactions. 
If a grazing area is heavily infested with alsike clover, it is recommended that horses are removed and a complete pasture renovation takes place. Photosensitivity is reversible once the affected horse is removed from the feed source or pasture, so long as the horse hasn’t developed liver disease. 
The following neurological signs may indicate clover-associated liver damage:
- Lack of motivation
- Incoordination or ataxia
- Poor appetite and body condition
Early detection is critical to successfully treating and minimizing damage due to photosensitivity. Contact your veterinarian to schedule an examination at the first signs of skin irritation in your horse.
To diagnose photosensitivity, your veterinarian will conduct a complete physical exam of your horse, inspect their environment, assess pasture and hay for contaminants and obtain a full history, including asking about any medications your horse is on.
If you have recently applied any topical products to your horse’s skin or coat, make sure to inform your veterinarian about the ingredients in those products.
Photosensitivity is most common in the spring and summer when sunlight is intense. A high ultraviolet index (UV index) can make it easier for your veterinarian to diagnose this condition.
After visually inspecting your horse’s skin, your veterinarian can rule out other conditions and assess organ function using tests such as: 
- Skin biopsy: Used to rule out bacterial infections and make a definitive diagnosis
- Blood chemistry panel: Used to assess liver and kidney function
- Liver biopsy: May be indicated to confirm liver disease
Several skin conditions have symptoms that mimic photosensitivity in horses, which can make diagnosis difficult.
To confirm a diagnosis, your veterinarian will rule out other common conditions that can induce hair loss and skin irritation, such as: 
- Contact dermatitis
- Infectious conditions (i.e. mud fever, dermatophilosis)
Treatment of photosensitization in horses involves removing exposure to the causal photoreactive agent, minimizing exposure to UV light, and soothing your horse’s symptoms.
Your horse may require long-term treatment to prevent flare-ups. Horses with secondary photosensitivity must have a lifelong treatment plan to address liver disease or dysfunction.
Remove affected horses from sunlight for immediate relief. Keep horses indoors during the day and provide pasture turnout at night.  Feed dry, non-legume hay that does not contain any trace of toxic plants or contaminants. 
Keep skin wounds clean to promote healing and prevent infection.  Clean affected areas with antiseptic soap daily and apply topical antibiotics, corticosteroids, or creams to soothe dry, cracked skin. If an infection is present, a systemic antibiotic may be required. Cool the horse with clean, fresh water to reduce pain and swelling from lesions.
Necrotic skin tissue attracts flies and insects, which can lay eggs in the skin (myiasis) or cause infection. Implementing effective fly control measures is essential to supporting healing.
The key to preventing photosensitivity in horses is to avoid exposure to phototoxic agents and to support your horse’s liver health.
If your horse is exposed to phototoxins or suffers from liver dysfunction, you can prevent photosensitivity flare-ups by blocking UV rays from reaching non-pigmented, sensitive skin.
Keep affected horses indoors during the day or outfit them with sun visors, UV sheets, and UV socks to block sensitive skin from exposure to sun rays.
Supplementing your horse’s diet with Milk Thistle extract can help support liver health and protect against free radical damage. 
Milk thistle contains silymarin, an antioxidant compound that helps to support the liver’s normal defences against toxic compounds. 
The prognosis for horses with photosensitization depends on the location and severity of skin lesions and the presence of liver damage.
Horses with primary photosensitization generally have a good prognosis and often make a full recovery, whereas horses with secondary photosensitization can have a poor prognosis.
It can take several weeks for lesions to heal. Fortunately, even in cases of severe necrosis, most skin lesions heal well if supportive care is administered. 
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Alsike Clover Poisoning, Photosensitization or Photodermatitis in Horses. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). 2022.
- Barrington, G. M. Photosensitization in Horses. Merck Veterinary Manual. 2019.
- Bourke, C. A. Effects of Hypericum Perforatum (St. John’s wort) on animal health and production. Plant Production Quarterly. 1997.
- Collet, M. G. Photosensitisation diseases of animals: Classification and a weight of evidence approach to primary causes. Toxicon X. 2019.
- Durham, A. E. Association between forage mycotoxins and liver disease in horses. J Vet Intern Med. 2022.
- Foreman, J. H. Disorders of the Liver in Horses. Merck VeterinaryManual. 2019.
- Gupta, R. C. Veterinary Toxicology: Basic and Clinical Principles. Veterinary Toxicology, 2nd edition. 2011.
- Kentucky Equine Research Staff. Sunburn and Photosensitivity in Horses. Kentucky Equine Research (KER). 2004.
- Mulrow, C. et al. Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects: Summary. AHRQ Evidence Report Summaries. 2000.
- Nation, P. N. Alsike clover poisoning: A review. Can Vet J. 1989.
- Puschner, B. Alfalfa hay induced primary photosensitization in horses. The Vet J. 2016.
- Quinn, J. C. et al. Secondary Plant Products Causing Photosensitization in Grazing Herbivores: Their Structure, Activity and Regulation. Int J Mol Sci. 2014.
- Rashmir-Raven, A. Photosensitization. Robinson’s Current Therapy in Equine Medicine (Seventh Edition). 2015.
- Slater, K. Skin Problems. American Association of Equine Practitioners (AAEP). 2012.
- Sunburn, Photosensitivity or Contact Dermatitis in Horses. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). 2022.
- Stegelmeier, B. L. Equine photosensitization. Clin Tech Equine Pract. 2002.
- Stegelmeier, B. L. et al. Plant induced photosensitivity and dermatitis in livestock. Vet Clin North Am Food Anim Pract. 2020.
- Stegelmeier, B. L. Pyrrolizidine Alkaloid-Containing Toxic Plants (Senecio, Crotalaria, Cynoglossum, Amsinckia, Heliotropium, and Echium spp.). Publications from USDA-ARS/UNL Faculty. 2011.
- Yeruham, I. et al. An apparently gluten-induced photosensitivity in horses. Vet. hum. toxicol. 1999.
- Hackett, Eileen S. et al. Evaluation of antioxidant capacity and inflammatory cytokine gene expression in horses fed silibinin complexed with phospholipid. Am J Vet Res. 2013.
- Wright RG, Ireland MJ. Case report: alsike clover poisoning, an old but should not be forgotten problem. Proceedings of the Equine Nutrition and Physiology Society 2003: 236-237.
- Brendemuehl, J. Photosensitization in Horses. Equine Extension Veterinarian, Univ. of Illinois College of Veterinary Medicine. 2005.
Leave A Comment