Photosensitization in Horses

Kaela Schraer, DVM
By Kaela Schraer, DVM on Oct. 10, 2023
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What Is Photosensitization in Horses?

Photosensitization is a condition in which the light-haired areas of a horse become very sensitive to UV light.

This can result in injury to the skin and pain for the horse. There are two main pathways that lead to this injury: primary photosensitization and secondary or hepatogenous photosensitization. Both types are fairly uncommon.

There is no breed predisposition to this condition, however since it primarily affects areas with light colored fur, paint horses and other breeds with light colored or white fur may show more obvious symptoms.

Types of Equine Photosensitization

  • Primary Photosensitization (Type I): caused by the production of substances, often referred to as photodynamic compounds or agents, in the bloodstream that interact with UV light when they reach the skin surface.

  • Secondary (Hepatogenous) Photosensitization (Type II): Liver injury through toxins, infections, and neoplasia can cause the production of photodynamic agents (any substance that when exposed to ultraviolet light, creates a reaction that can ultimately lead to changes and damage within surrounding tissues).

Symptoms of Photosensitization in Horses

  • Itching/pain

  • Photophobia (avoidance of light)

  • Erythema (redness) of light pigmented areas

  • Blistering, scaling, and peeling of affected areas

  • Hair loss

It’s important to differentiate between photosensitization and sunburn. Sunburn is typically restricted to the light-colored skin around the eyes and muzzle because there is no fur to protect these areas. Photosensitization will also affect areas of skin that are covered by white or light-colored hair.

Causes of Photosensitization in Horse

In a normal horse, the skin and fur act as a protective barrier against the elements. In cases of photosensitization, whether the cause is primary or secondary to liver damage, UV light reacts with factors in the horse’s bloodstream when they reach the skin that cause inflammation and damage. Ingestion or exposure to toxic plants, side effects from certain medications (sulfa drugs and tetracyclines), and liver damage are some of the causes of photosensitization in horses.

Causative Toxic Plants

Primary photosensitization due to toxic plants may include:

  1. Perennial ryegrass

  2. St. John’s Wort

  3. Buckwheat

Secondary photosensitization due to toxic plants may include:

  1. Clover

  2. Alfalfa

  3. Blue green algae

  4. Pyrrolizidine alkaloid containing plants such as ragwort

  5. Moldy bermudagrass

In most of these cases, the toxic plant must be ingested. In some instances, as with certain clovers, contact with the plan during grazing is all that is needed for photosensitization to occur.

How Veterinarians Diagnose Photosensitization in Horses

Your veterinarian will start with a full physical and dermatologic (skin) exam. They will look for the classic symptoms seen with photosensitization such as redness, hair loss, blistering and peeling. They may take samples (biopsies) of the skin to analyze for other possible causes of the skin damage.

Blood work evaluating organ function (biochemistry panel) can be useful for determining if liver damage has played a role. It may also be useful to do a survey of pasture and other forage sources to identify toxic plants.

Treatment of Photosensitization in Horses

The primary treatment for photosensitization is addressing the underlying cause. This can involve removal of toxic plants in the pasture/forage or treatment of any type of liver infection.

It is not always possible to resolve the underlying condition, for example in cases of liver cancer.  In these situations, supportive care and treatment of the symptoms can help improve the horse’s comfort and quality of life. This can include steroid therapy to help slow the reaction and wound care for the blisters and inflammation. These are often palliative measures.

Recovery and Management of Photosensitization in Horses

Recovery is dependent on the underlying cause and severity of the case. In all cases, increased sun protection is recommended. This can be accomplished by changing turnout routines to primarily occur overnight, using full face fly masks and fly sheets, and in some cases even zinc oxide or sunscreen.

Photosensitization can predispose horses to wounds, eye issues, and even some cancers such as squamous cell carcinoma.

Photosensitization in Horses FAQs

What plants cause photosensitivity in horses?

Many plants can cause photosensitization, the most common includde: St. John’s wort, perennial ryegrass, and buckwheat.

How do you reverse photosensitivity?

Removal of the underlying cause of photosensitization is the main way to stop the process from continuing. That may involve removing plants from the environment or addressing liver damage when possible.

Can you fix photosensitivity?

If there is a known cause of the photosensitization that can be removed, it can be fixed. In many cases where liver damage is involved, this will likely be a lifelong issue with a guarded prognosis.

Featured Image: Getty/ProjectB


Judd, Bob. Sunburn and Photosensitization in Horses. VIN, Veterinary Partner. 2017.

Photosensitization. Clinical Veterinary Advisor. 2012.

Puschner, B et al. “Alfalfa hay induced primary photosensitization in horses.” Veterinary journal (London, England : 1997) vol. 211. 2016.

Rashmir-Raven, Ann, and Rebecca S. McConnico. Photosensitization. Robinson’s Current Therapy in Equine Medicine, 2015.

Slater, Kristen. Sunburn & Photosensitivity. AAEP, The Horse.

Stegelmeier, Bryan L. Equine photosensitization. Clinical Techniques in Equine Practice. 2002.


Kaela Schraer, DVM


Kaela Schraer, DVM


Dr. Kaela Schraer graduated from Tufts Cummings School of Veterinary Medicine in 2017 with her doctorate in veterinary medicine. After...

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