Photosensitization in Horses: What Needs To Know As Owner

Photosensitization in horses is when the skin becomes overly sensitive to sunlight. It can result in severe sunburn, blistering, and, in some cases, even tissue damage. This sensitivity occurs due to the interaction of specific compounds within the body, typically in the bloodstream, with the sun’s ultraviolet (UV) light.

Types of Photosensitization in Horses

There are two main types of photosensitization in horses:

1. Primary Photosensitization: Primary photosensitivity occurs when a horse ingests or absorbs certain substances that sensitize the skin to sunlight. These substances, called photosensitizing agents, can be found in plants, drugs, or chemicals. When these agents are absorbed into the bloodstream and then exposed to sunlight, they react with UV light, leading to skin damage. Examples of plants that contain photosensitizing agents include St. John’s Wort, buckwheat, and certain types of clover.
2. Secondary Photosensitization: In this type, the liver cannot correctly process certain compounds, leading to their accumulation in the bloodstream. These compounds can then interact with sunlight, causing skin damage. Liver damage or dysfunction is often the underlying cause of secondary photosensitization. Various factors, including toxic substances, infections, or certain metabolic disorders, can cause liver disease in horses.

What Causes Photosensitivity in Horses

Various factors can cause photosensitivity in horses, but it primarily involves the interaction of certain compounds in the body with ultraviolet (UV) light from the sun. Here are the leading causes of photosensitivity in horses:

1. Ingestion of Photosensitizing Plants: Some plants contain compounds that can make horses sensitive to sunlight when ingested. These compounds, known as photosensitizers, include photodynamic pigments or alkaloids. When the horse consumes these plants, the compounds are absorbed into the bloodstream and accumulate in the skin. These compounds react with UV light upon exposure to sunlight, leading to skin damage. Examples of plants that can cause photosensitivity in horses include St. John’s Wort, buckwheat, certain types of clover, and members of the genus Hypericum.
2. Certain Medications: Certain drugs or medications can make horses more sensitive to sunlight. These medications may contain photosensitizing agents or alter the horse’s metabolism in a way that increases susceptibility to photosensitivity. Common medications associated with photosensitivity in horses include some antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and tranquilizers. It’s necessary to follow dosage instructions carefully and monitor horses for any symptoms of adverse reactions, including photosensitivity, when administering medications.
3. Liver Dysfunction: The liver plays a vital role in detoxifying the body and processing various compounds. Liver dysfunction or damage can lead to the accumulation of certain substances in the bloodstream, making the skin more sensitive to sunlight. Conditions such as liver disease or liver damage caused by toxins or infections can result in secondary photosensitivity in horses. Addressing the underlying liver issue is essential for managing photosensitivity in these cases.
4. Genetic Factors: While less common, certain genetic factors may predispose some horses to photosensitivity. These genetic predispositions can affect how the horse’s body processes certain compounds or responds to sunlight. Breed, coat color, and individual variations in metabolism may all play a role in determining a horse’s susceptibility to photosensitivity.
5. Chemical Exposure: Certain chemicals, such as those found in topical treatments, shampoos, or insect repellents, can also cause photosensitivity in horses. Some chemicals may sensitize the skin to sunlight, leading to sunburn or other skin reactions when the horse is exposed to UV light.

What plants cause Photosensitivity in Horses?

Several plants contain compounds that can cause photosensitivity in horses when ingested. These plants contain substances known as photosensitizers, which can make the skin more sensitive to sunlight. When horses consume these plants and are subsequently exposed to sunlight, the interaction between the photosensitizing compounds and ultraviolet (UV) light can lead to skin damage. Here are some common plants that can cause photosensitivity in horses:

1. St. John’s Wort (Hypericum perforatum): This perennial herb contains hypericin and hyperforin, photodynamic pigments known to cause photosensitivity in horses and other animals.
2. Alsike Clover (Trifolium hybridum): Alsike clover is a legume commonly found in pastures. It contains compounds called pyrrolizidine alkaloids, which can cause liver damage and photosensitivity when ingested by horses.
3. Red Clover (Trifolium pratense): Red clover is another common pasture legume. Like Alsike clover, it contains pyrrolizidine alkaloids that can cause liver damage and photosensitivity in horses.
4. White Clover (Trifolium repens): While white clover is generally safe for horses to consume in moderation, certain varieties may contain pyrrolizidine alkaloids, which can pose a risk of photosensitivity if ingested in large quantities.
5. Buckwheat (Fagopyrum esculentum): Buckwheat is a grain crop that can be found in pastures or as a contaminant in horse feed. It contains fagopyrin, a photosensitizing compound that can cause horse skin reactions.
6. Tansy Ragwort (Senecio jacobaea): Tansy ragwort is a common weed that contains pyrrolizidine alkaloids. Ingestion of tansy ragwort can cause liver damage and photosensitivity in horses.
7. Common Ragwort (Senecio vulgaris): Similar to tansy ragwort, common ragwort contains pyrrolizidine alkaloids and can cause photosensitivity and liver damage in horses.
8. Perennial Ryegrass (Lolium perenne): While perennial ryegrass itself does not contain photosensitizing compounds, it can become infected with the fungus Rhizoctonia leguminicola, which produces alkaloids that cause liver damage and photosensitivity in horses when ingested.

Clinical Signs of Photosensitization in Horses

The clinical signs of photosensitization in horses can vary depending on the underlying cause and the severity of the disease. However, some common signs and symptoms to watch for include:

1. Sunburn-like Skin Lesions: Affected areas of the skin may appear reddened, inflamed, and sunburned. The skin may feel hot to the touch, and there may be areas of swelling or blistering.
2. Crust Formation: In more severe cases, crusts or scabs may form on the affected areas of the skin. These crusts may be thick and tacky and can lead to hair loss or sloughing of the skin if they are removed.
3. Pain and Discomfort: Horses with photosensitization may exhibit discomfort, such as reluctance to move, stiffness, or sensitivity when touched, especially in the affected areas.
4. Itching and Rubbing: Horses may exhibit itching or discomfort by rubbing the affected areas against objects such as fences, posts, or stall walls. This behavior can exacerbate skin damage and lead to further irritation.
5. Swelling: Swelling of the affected areas of the skin may occur, particularly in areas such as the muzzle, ears, or areas with white markings, which are more prone to sunburn.
6. Blistering and Ulceration: In severe cases of photosensitization, blistering, and ulceration of the skin may occur. These lesions can be painful and may predispose the horse to secondary bacterial infections.
7. Secondary Infections: Skin damage caused by photosensitization can increase the risk of secondary bacterial or fungal infections. Signs of infection may show increased redness, swelling, discharge, and foul odor from the affected areas.
8. Behavioral Changes: Horses with photosensitization may exhibit changes in behavior, such as lethargy, depression, or decreased appetite, especially if they are experiencing pain or discomfort.

Diagnosis of Photosensitivity in Horses

Diagnosing photosensitivity in horses typically involves a combination of clinical examination, history taking, and sometimes laboratory tests. Here’s how veterinarians typically diagnose photosensitivity in horses:

1. Clinical Examination: The veterinarian will perform a thorough physical examination of the horse, paying close attention to any areas of the skin that appear inflamed, reddened, swollen, blistered, or ulcerated. They will also assess the distribution of lesions and inquire about the horse’s recent exposure to sunlight and any changes in diet or medication.
2. History Taking: Obtaining a detailed history from the horse owner or caretaker is crucial for understanding potential causes of photosensitivity. The veterinarian will inquire about the horse’s diet, including recent changes or introductions of new feeds or supplements, as well as any recent medications the horse may have received. They will also ask about the horse’s turnout schedule, access to pasture, and any known exposure to potentially photosensitizing plants or chemicals.
3. Differential Diagnosis: Photosensitivity can resemble other skin conditions so that the veterinarian will consider differential diagnoses, including sunburn, allergic reactions, dermatitis, insect bites, fungal or bacterial infections, and autoimmune skin diseases. Differentiating between these conditions may require additional diagnostic tests.
4. Laboratory Tests: If the cause of photosensitivity is not immediately apparent or if liver dysfunction is suspected, the veterinarian may recommend laboratory tests. Blood tests, including liver function tests, may help assess liver health and identify any abnormalities. A liver biopsy may be performed to evaluate liver tissue more closely for signs of damage or disease.
5. Skin Biopsy: In some cases, a skin biopsy may be recommended to obtain a tissue sample from affected areas of the skin. A skin biopsy can help confirm the diagnosis of photosensitivity and rule out other skin conditions.
6. Elimination Diet: If the horse’s diet is suspected to be a contributing factor, the veterinarian may recommend an elimination diet to identify and remove potential triggers. This involves gradually removing certain feed ingredients or supplements from the horse’s diet while closely monitoring for improvements in clinical signs.
7. Response to Treatment: Response to treatment can also be an important diagnostic indicator. If the horse’s clinical signs improve with appropriate management, including sun protection, dietary changes, and supportive care, it can provide further evidence of photosensitivity as the underlying cause.

Treatment of Photosensitization in Horses

Treating photosensitization in horses typically involves a combination of supportive care, addressing the underlying cause, and managing secondary complications. Here are the main components of treatment:

1. Sun Protection: Protecting the affected horse from further sun exposure is essential to prevent the worsening of skin damage. This may involve keeping the horse in a shaded area during peak sunlight hours, using lightweight turnout sheets or fly masks with UV protection, and providing access to shelter or stabling during sunny periods.
2. Topical Treatments: Topical treatments may soothe and protect the affected skin. This can include applying emollient creams or ointments to moisturize the skin and provide a barrier against further irritation. Calamine lotion or aloe vera gel may also be used to alleviate discomfort and promote the healing of sunburned skin.
3. Pain Management: Horses with photosensitization may experience pain and discomfort, especially if they have developed sunburn or blistering. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone or flunixin meglumine may be prescribed to relieve pain and reduce inflammation.
4. Wound Care: If the horse has developed blisters, ulcers, or open sores on the skin, proper wound care is essential to prevent infection and promote healing. This may involve gently cleaning the affected areas with a mild antiseptic solution and applying topical antibiotics or wound dressings as needed.
5. Dietary Management: Dietary management is essential if photosensitization is caused by ingesting photosensitizing plants or liver dysfunction. This may involve removing the horse from the pasture or providing access to a dry lot with hay or other forage free from photosensitizing plants. Additionally, avoiding feeds or supplements that may contain photosensitizing compounds is essential.
6. Liver Support: In cases where photosensitivity is caused by liver dysfunction, supportive treatment to improve liver function may be necessary. This can include dietary modifications to support liver health and the administration of hepatoprotective supplements or medications under the guidance of a veterinarian.
7. Identification and Removal of Triggers: It is important to identify and remove potential triggers of photosensitivity, such as photosensitizing plants or medications, to prevent the recurrence of the condition. Horse owners should be educated about the types of plants that can cause photosensitivity and take measures to remove them from pastures or restrict access to grazing areas.
8. Monitoring and Follow-Up: Horses with photosensitization should be closely monitored for signs of improvement or complications. Regular veterinary follow-up appointments may be necessary to assess progress, adjust treatment as needed, and monitor liver function if indicated.

Prevention of Photosensitivity in Horses

Preventing photosensitivity in horses involves several strategies to minimize exposure to photosensitizing agents and protect the skin from sunlight. Here are some key preventive measures:

1. Pasture Management: Maintain pastures free of known photosensitizing plants such as St. John’s Wort, Alsike Clover, Tansy Ragwort, and Buckwheat. Regularly inspect pastures for toxic plants and promptly remove them to prevent horse ingestion. Consider rotating pastures or using herbicides to control weed growth.
2. Feed Management: Ensure hay and other forage sources are protected from photosensitizing plants. Monitor the quality and composition of commercial horse feeds and supplements to avoid those containing potential photosensitizing agents.
3. Sun Protection: Provide access to shaded areas, such as run-in sheds, barns, or trees, during peak sunlight hours, typically between 10 a.m. and 4 p.m. Additionally, consider using lightweight turnout sheets, fly masks with UV protection, or sunscreens specifically formulated for horses to shield the skin from direct sunlight.
4. Dietary Supplements: Certain dietary supplements, such as those containing antioxidants like vitamin E and selenium, may support skin health and reduce the risk of photosensitivity. Consult a veterinarian or equine nutritionist to determine appropriate supplementation for individual horses.
5. Water Quality: Ensure that horses have access to clean, fresh water. Poor water quality or contamination with algae or other toxins can contribute to liver damage and photosensitivity.
6. Medication Management: Use caution when administering medications to horses, especially those known to cause photosensitivity. Follow dosage instructions carefully and monitor for adverse reactions, including photosensitivity, when introducing new medications or supplements.
7. Regular Veterinary Care: Schedule routine veterinary examinations to monitor the overall health of horses and detect any signs of liver dysfunction or photosensitivity early on. Discuss preventive measures with your veterinarian and seek guidance on appropriate management practices.
8. Educational Outreach: Educate horse owners, caretakers, and stable staff about the signs and dangers of photosensitivity and preventive measures to minimize the risk. Provide resources and training on pasture management, plant identification, and safe medication practices.

Conclusion on Photosensitivity in Horses

In conclusion, photosensitivity in horses is a condition characterized by heightened skin sensitivity to sunlight, leading to sunburn, inflammation, and potential tissue damage. Various factors, including ingestion of photosensitizing plants, certain medications, liver dysfunction, genetic predispositions, and chemical exposure can cause it. Common clinical signs of photosensitivity include sunburn-like lesions, swelling, blistering, pain, and discomfort.

Diagnosis of photosensitivity involves:

• A thorough clinical examination.
• History taking.
• Sometimes, laboratory tests identify the underlying cause and rule out other skin conditions.

Treatment typically involves sun protection, topical treatments, pain management, wound care, dietary management, liver support, and removal of potential triggers. Prevention strategies include pasture and feed management, sun protection measures, dietary supplementation, medication management, regular veterinary care, and educational outreach.

By implementing preventive measures and promptly addressing any signs of photosensitivity, horse owners and caretakers can minimize the risk of skin damage and promote their horses’ overall health and well-being. Collaboration between veterinarians, equine nutritionists, and horse owners is essential for effectively managing and preventing photosensitivity in horses. With proper care and attention, horses can lead comfortable, healthy lives free from the adverse effects of photosensitivity.