Kia Benson, DVM
Associate Veterinarian, Clinical Toxicology
As I discussed in my previous Sunburn blog, sunburn can be a big medical problem for animals as well as humans. Last time we looked at cats and dogs; today we’re going to discuss sunburn in horses and pet pot-bellied pigs.
To recap, sunburn is an inflammatory skin disease caused by prolonged exposure to ultraviolet (UV) radiation. UV radiation comprises about 9% of the solar spectrum. UVA radiation causes skin ageing and is also likely to cause skin cancer. UVB causes redness and sunburn and is a major risk factor for all types of skin cancer. Prolonged and repeated exposure to UV radiation can worsen the degree of cell damage, which can alter the skin’s immunological system. This may result in skin cancer. Melanin in pigmented skin absorbs UV rays and can help prevent sunburn.
Sun-related inflammation and lesions in horses are a little more complicated than those in cats and dogs. While horses can develop sunburn/solar dermatitis from over-exposure to UV light, they can also develop something called photosensitization.
Just like cats and dogs, horses with areas of white hair/pink skin (especially on their face like a blaze, a stripe, etc.), light-colored or grey horses, or areas with sparse hair like a horse’s muzzle are at risk of sunburn. The most common areas for sun-related damage in horses are on the head – especially the muzzle, the eyelids, and around the eye. Horses with large areas of white on the legs may also develop sunburn.
Photosensitization is different from sunburn in that it tends to cause more severe lesions. Sunburn causes redness, swelling, blistering, and peeling skin. Photosensitization causes red, open sores that turn into crusty and scabbing lesions. The affected skin cells die and are then sloughed or shed. Both sunburn and photosensitization involve the white/pink skin or thin haired areas. However, photosensitization often develops in places other than on the head or legs of the horse.
Photosensitization is divided into two types – primary photosensitization or secondary photosensitization. Primary photosensitization occurs when a horse ingests a plant containing a photodynamic compound that can react to sunlight. When a horse eats such a plant, the compound enters the blood stream from the GI tract and then reacts to the sun’s UV light as the blood moves through blood vessels in the skin. On horses with areas of white in their haircoats a chemical reaction develops that damages skin cells, causing them to die. Plants that are known to cause primary photosensitization include St. John’s wort, perennial Rye grass, wild Buckwheat, and possibly Alsike and White Clovers. Horses on poor, weedy pasture may be more at risk.
Although it does not contain a photodynamic compound, Alfalfa hay can also sometimes trigger primary photosensitization for reasons that are not yet known. Certain drugs can cause photosensitization as well, including trimethoprim sulfa and tetracycline.
Secondary photosensitization occurs after a horse develops liver disease. Whatever the cause of the liver disease, when a horse has pre-existing liver issues the bile ducts in the liver cannot excrete a chlorophyll (chlorophyll makes plants green) byproduct. Instead the byproduct builds up in the horse’s circulatory system and reacts to UV light. The reaction causes photosensitization lesions in white coated areas on the horse.
Horses with suspected photosensitization reactions need to be removed from their pasture and checked by a veterinarian for possible liver disease. A biopsy may be needed to definitively diagnose liver disease.
Once the triggering plant or drug has been removed from the environment a horse with primary photosensitization would be expected to completely recover. Stabling indoors with dry hay and avoiding exposure to UV light will be necessary to facilitate recovery, especially if an obvious plant or drug cannot be identified as the culprit.
Horses with secondary photosensitization will require long term management of their liver damage via supplements and diet, along with inside stabling for a while to avoid exposure to UV light. Time to resolution of skin lesions will depend on their severity – some horses may need to be euthanized in the face of severe skin sloughing.
Chronic exposure of white/light-colored skin to UV radiation can be a factor in causing Squamous Cell Carcinoma skin cancer in horses. The eyelid is the most common site in horses, followed by the genitalia. Physical examination and veterinary consultation will be needed to diagnosis sunburn vs. photosensitization vs. skin cancer.
Pet Pot-Bellied Pigs
Pigs can easily become sunburned anywhere they have a white coat/pink skin or a thin coat. The areas which are most commonly affected are the back, the ear tips, the nose, and the mammary glands/teats.
Clinical signs of sunburn in pigs can include reddened, thickened and peeling skin that feels hot and is sensitive to the touch. Blisters can form and the pigs show obvious discomfort. Pain from sunburned backs can cause pigs to drop or “dip” their abdomen to the ground/floor as they try to move away from the pain. Or, the pigs might be restless and agitated from the pain.
Mild acute cases of sunburn can be treated with a topical steroid cream. Severe cases may require pain medications and systemic steroids. Sunburned pet pot-bellied pigs may need to be kept inside or in shaded stalls while they heal.
Fortunately, skin cancer from UV exposure does not seem to develop in pet pot-bellied pigs. The most common type of skin cancer in pigs is melanoma (cancer in pigmented skin) and seems to have a hereditary or genetic basis.
The key to preventing sunburn in horses and pet pot-bellied pigs is avoidance of too much sun and its damaging UV rays. The key to avoidance of photosensitization hazards for horses is regular pasture checks to remove suspected plants or possible application of an appropriate herbicide if necessary.
Cold weather blankets, summer fly-sheets and fly masks and other forms of protection from the elements have long been designed for horses. These days fly sheets will typically be made of a reflective material that blocks UV light and keeps the sun’s rays as well as flies away from a horse’s skin. Horses also require protection while being ridden. Visors and even sunglasses that can attach to a bridle’s browband are available.
Like their larger cousins, pet pot-bellied pigs can use wallowing in the mud to protect themselves from harmful UV radiation. However, today’s more stylish pig may want to check into some trendy fashion forward clothing available on the market.
Besides using clothing protective against UV rays, you can prevent sunburn in pet pot-bellied pigs and horse by restricting an animal’s exposure to the sun, keeping them indoors during the time of the most intense UV radiation (typically between 9 AM to 3 PM). This may mean turning out horses and pet pot-bellied pigs in paddocks or pastures at night instead of the day. Shelter is another key component of prevention. Buildings like run-in sheds or trees in a pasture that allow animals to get into shade and out of the sun are important.
Sunscreen products will protect horses and pet pot-bellied pigs just like they protect other animals. There are horse and pig-specific sunscreen products out on the market. Just be sure they protect against both UVA and UVB radiation. As with human products, they need to be applied 15 minutes or so before exposure and should be re-applied frequently during exposure.
If you are not certain whether or not a sunscreen is safe to use on your horse or pet pot-bellied pig, please consult a veterinarian or Pet Poison Helpline® before trying it out. Note that the information in this blog does not cover pigs for food production – just pet pot-bellied pigs. Restrictions apply concerning what medications and other products can and cannot be used with regard to animals destined for human consumption.
This blog is also not a substitute for a physical examination and consultation with a veterinarian. If you are concerned about sunburn or photosensitization in your animal companion, please consult your veterinarian. Consultation with a veterinary oncologist is recommended if your pet has been diagnosed with any form of skin cancer.