Photosensitization in Horses

Photosensitization in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Poor Appetite / Weight Loss


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Most common symptoms

Poor Appetite / Weight Loss

Photosensitization in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

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What is Photosensitization?

Your horse’s skin may become very sensitive to ultraviolet light because of a photodynamic agent present on his skin. Photodynamic agents are activated by sunlight and can cause great harm to the skin. If you notice that your horse is having an adverse reaction to the sunlight, it is important to keep him away from ultra violet light.  He also should be seen by a veterinarian, who will diagnose the cause of the sensitivity.

Photosensitization is the body’s inability to react normally to sunlight; the skin and eyes become overly sensitive.  The horse appears to have a sunburn, even after a small amount of sun exposure.

Symptoms of Photosensitization in Horses

Symptoms may include:

  • Skin lesions
  • Eyes tearing
  • Red skin
  • Lacerations
  • Blisters
  • Open blisters with pus
  • Crusty skin
  • Reluctance to be saddled
  • Skin necrosis 
  • Mild colic
  • Weight loss


  • Primary Photosensitization

    occurs when a photosensitizing agent is consumed, injected or absorbed through the skin

  • Secondary Photosensitization

    occurs as a result of liver dysfunction

Causes of Photosensitization in Horses

Photosensitizing agents may include the following plants and medications:

Primary Photosensitization

Plants that have been linked to primary photosensitizing:

  • Buckwheat
  • St. John’s Wort 
  • Medicago polymorpha (burr trefoil)
  • Rye grass
  • Smartweed
  • Bishop’s weed
  • Spring Parsley

Medications that have been linked to primary photosensitizing are:

  • Tetracycline
  • Phenothiazine
  • Thiazides
  • Methylene blue
  • Sulfonamides

Secondary Photosensitization

Secondary photosensitization is the most common cause of photosensitization in horses.  It may be caused by:

  • Liver dysfunction - due to bacterial, viral or fungal infections

  • Bile duct lesions
  • Excessive ingestion of clover and alfalfa may cause liver damage, which then may lead to photosensitivity
  • Certain medications may cause liver damage, which then may cause photosensitivity

Diagnosis of Photosensitization in Horses

The veterinarian will go over the medical history of your horse. He may ask to see vaccination, dental and deworming records so be sure to have them on hand.  Let the veterinarian know if your horse is currently on any medications. Your horse’s diet will be discussed. The veterinarian may also want to see the pasture where your horse forages. He will be checking for clover, alfalfa, or other plants associated with photosensitization. 

The veterinarian will then perform a full physical exam. This may include an examination of the skin to evaluate whether medication is needed for burning, lesions, or irritation. Diagnostic tests may include:

  • Serum chemistry panel - This blood test evaluates organ function (kidneys, liver, blood protein and electrolytes)

  • Urinalysis - To verify kidney function, crystals, bacteria and blood in the urine
  • Skin biopsy - To  rule out  bacterial infection

  • Ultrasound - In order to check liver, kidneys and renal function

Treatment of Photosensitization in Horses

Regardless of what type of photosensitization your horse is diagnosed with, it will be very important to keep him away from sunlight. He may be allowed to graze after sundown. 

To provide some immediate relief for the inflammation and swelling, the veterinarian may suggest cooling the horse off with water. Corticosteroids, topical antiseptics and antibiotic creams may be prescribed for the skin lesions. Skin lesions must be kept clean and free from horseflies, which can lay eggs in the lesions (maggots).  

If your horse was diagnosed with liver dysfunction, the treatment plan may include:

  • Antibiotics and anti-inflammatory medications
  • No clover or alfalfa
  • Reduced protein in the horse’s diet
  • Folic acid and vitamins supplements
  • Diluted vinegar added to the horse’s water may help reduce the production of ammonia in his body

Recovery of Photosensitization in Horses

The horse will need follow-up visits by the veterinarian in order to monitor his progress.  The veterinarian may want to take a complete blood count to ensure there are no bacterial infections.  He will also want to make sure the lesions are healing well.  Horses recovering from primary photosensitization have a very good prognosis of making a full recovery.

Patients with liver dysfunction may need to be on a lifelong treatment plan. The veterinarian will want to take blood work, abdominal x-rays, and ultrasounds every few months. These diagnostic tests will help monitor the liver’s function and size.  If the liver condition was diagnosed and treated in the early stages, prognosis is good.  It is important to follow the treatment plan provided for your horse to ensure continued improvement.  If you notice new symptoms in your horse’s condition or any reactions to the medications prescribed, call your veterinarian.

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