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. [14]

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. [7]

Non-pigmented (i.e. white) skin is especially sensitive to reactive compounds, as is skin with little hair cover (i.e. muzzle, eyelids, ears). [2]

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. [2] 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. [2]

Ultraviolet light from the sun activates these compounds, resulting in chemical reactions that damage skin cells and cause significant irritation for the horse. [2]

The face and body can develop red rashes (erythema), lesions, leaking wounds and scabs. In severe cases, necrosis (tissue death) can occur. [16] 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. [2] 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). [16] Horses with light skin and hair are commonly affected all over the body.

Clinical Signs

Clinical signs of photosensitization tend to develop within hours of sunlight exposure. [16] However, some signs of photosensitivity do not present for several weeks following sun exposure.

The following symptoms may indicate photosensitivity in your horse:. [8][14]

  • 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. [1]

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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. [21]

In all cases, UV light exposure activates phototoxins, resulting in skin sensitivity and tissue damage.

Phototoxins

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. [4]

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 Photosensitivity

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. [11] 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. [4] Hypericin becomes energized under the skin when it comes in contact with sunlight, causing extensive damage to surrounding cell membranes. [3]

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. [3]

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

Other plants that can induce primary photosensitivity in horses include: [11]

  • 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: [15]

  • 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. [19]

Secondary Photosensitivity

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. [7][11] In horses with normal liver function, phylloerythrin is excreted into the horse’s bile to be removed from the body.

However, in hor