Photosensitization is an uncommon yet potentially serious cause of equine dermatological conditions. Cases range from simple nuisance from pasture plant contact to life-threatening liver disease. The condition occurs when a photodynamic agent sensitizes the skin to UV sunlight rays. Clinical signs typically appear a few hours after intense sunlight exposure and include skin reddening, edema (fluid swelling), oozing, scab formation, and necrosis. The condition differs from sunburn or photodermatitis (sun allergy), which occur without a photodynamic agent. Recognizing and treating it early can improve horses’ comfort and, in cases of hepatogenous (liver) origin, chances of survival.

Cutaneous photodermatitis causes mild to severe dermatitis with swelling and inflammation of light/white haired areas in horses that have been exposed to photodynamic agents. The condition can affect horses of any age, breed, or sex; however, older animals might be predisposed due to preexisting liver disease. Clinical signs include reddened, swollen, and possibly crusting and oozing of white- or nonhaired or unpigmented skin with hair loss. Often, secondary bacterial infections occur requiring treatment using appropriate antimicrobials.

Horses with concurrent liver disease might show signs of lethargy, jaundice, and mild to moderate colic (abdominal pain) as well as weight loss and ill thrift. In some cases signs of hepatoencephalopathy (abnormal brain function due to liver disease) occur. Abdominal ultrasound examination might reveal abnormalities. If one horse in a herd has photosensitization, be sure to evaluate herdmates, which might also be affected.

The many causes can be categorized as:

1. Plant Ingestion of plants with photodynamic properties.
2. Fungal Ingestion of liver-toxic plants and substances.
3. Chemical Exposure to chemicals in some fly sprays and antibiotics.
4. Hepatic Primary liver disease such as cholecystitis (inflammation of bile duct passages and liver) or Theiler’s Disease.

Risk factors for photosensitization can be challenging to identify and might include exposure to the photosensitizing agents listed above, having white- or light-pigmented haired skin, preexisting liver disease, or exposure to diseases causing liver ­insufficiency.

For a successful outcome, it’s important to recognize and begin treating affected horses early and to continue appropriate maintenance. Remove the inciting cause to prevent further damage, and place the patient in a darkened stall away from direct sunlight to minimize tissue damage. Thoroughly clean affected areas with mild shampoo such as Ivory, baby shampoo, or blue Dawn dishwashing liquid. Disinfect skin surfaces with mild antiseptic agents to minimize secondary invasion of microbes such as bacteria or fungal agents. Apply protective ointments (e.g., silversulfadiazine, furazolidone) to minimize fly strike (maggot infestation under the skin after flies lay eggs) and soothe skin and exposed subdermal tissue. Cold hydrotherapy can help rid skin surfaces of debris, reduce inflammation, and prevent fly strike. In some cases light bandaging with veterinary guidance might be helpful. Maintaining a clean environment is critical for continued healing to minimize fly strike and secondary topical infections. Remove manure and soiled bedding as frequently as possible and provide multimodal fly control.

Controlled exercise is another important aspect of care, as long as movement is comfortable for the patient and does not exacerbate lesions. Create a plan that includes stall rest out of sunlight, no forced exercise, and hand-walking as the patient will tolerate. As part of supportive care, maintain the horse on a good plane of nutrition (high-quality forage and concentrate), provide a low-protein diet if liver disease is involved, and offer easily accessible fresh, clean water. Also ensure the horse is eating, drinking, defecating, and urinating appropriately.

In some serious cases of photodermatitis, the veterinarian might recommend surgical debridement of affected dead skin/tissue to improve recovery. Follow-up bloodwork is helpful for determining prognosis, especially if liver disease is involved. Thus, regular veterinary physical evaluations are important.

Possible complications of photosensitization can include systemic infections, liver failure, cellulitis, progressive lameness, joint involvement, and laminitis. However, if you recognize it early and remove the inciting cause, the horse should have a fair to good prognosis for recovery with proper therapy. Preventing photosensitization is critical. Owners should always provide enough good-quality forage and supplemental concentrate so horses do not ingest weeds or other toxic plants. Including your county or parish agriculture agent or other local plant specialist in pasture surveillance can help you understand what’s in your pastures and manage grazing to prevent ingestion of potentially toxic plants.