The stronger light and longer days of summer mean that sunburn and diseases associated with sunshine and UV rays are once again becoming relevant.
Horses are affected by the sun in much the same way that we are. To understand the risks posed, it’s worth considering what exactly it is in sunlight that causes the problem.
The sun emits rays in the form of electromagnetic radiation, comprising a continuous spectrum of wavelengths. The ultraviolet (UV) spectrum — UVA, UVB and UVC — is the most relevant for skin conditions.
UVA radiation penetrates deeper into the skin than UVB and is the spectrum associated with photosensitivity reactions.
UVB is often referred to as the sunburn spectrum and is about 1,000 times more “reddening” than UVA. This type of light damages the skin cell DNA. The body recognises this damage and triggers defence mechanisms, including DNA repair, to revert the damage. Irreparably damaged skin cells will peel off and increased melanin production in the form of tanning will help prevent future skin damage.
Horses cannot tan, however, like humans, so the equine body employs other strategies to protect itself against the sun. One such defence mechanism is a protective covering of hair or the presence of pigment in the skin.
Hairless areas, particularly the muzzle and around the eye, are therefore more vulnerable to sun damage. While darker-coloured horses have a fair degree of protection, light-skinned animals such as skewbalds, piebalds, paints and Appaloosas are particularly susceptible. Any areas of white hair (and underlying skin), including socks, are also at risk.
While UVC light is also damaging to cells, this type of light typically does not reach the earth’s surface because of the ozone layer.
Feeling the burn
Sunburn is called phototoxicity and is a dose-related response to light exposure. The more sun, the greater the potential damage to skin.
The problem is common in horses, especially in areas of white, damaged or depigmented skin or scarring. Affected areas become warm and tender to the touch, developing an erythematous (reddened) and scaly appearance. Severe sunburn can eventually result in necrosis (death) of the skin, which may ooze fluid or serum.
The areas that are fully face-on to sunlight are typically the most affected, for example the top of the muzzle and blaze, the back and the rump.
If the skin is blistering and/or oozing, it’s important to seek veterinary attention. Your vet may recommend antibacterial bathing to remove crusts and emollient creams to moisturise the skin, along with a prescription for antibiotics if there is any evidence of secondary bacterial infection.
Anti-inflammatories may also be required, depending on the severity of the sunburn. These may be applied directly onto the area, given orally or injected into the bloodstream, sometimes in combination.
Excessive UV radiation, usually in conjunction with sunburn, is thought to incite squamous cell carcinoma (SCC). This skin cancer can cause local and sometimes aggressive cancer, which often takes a cauliflower appearance.
White or unpigmented areas such as the muzzle, around the eyes, the eye itself, the tips of the ears and occasionally white socks are most susceptible to SCC.
Another condition triggered by excess UV light is leukocytoclastic vasculitis, which typically affects white-socked lower limbs. The skin breaks out in crusting or ulcerative lesions, which may develop secondary bacterial infection.
Leukocytoclastic vasculitis can look very much like mud fever, so a biopsy of the skin may be required to make a diagnosis. If you notice that your horse still suffers when the weather is dry or sunny — or only when it is dry and sunny — then it is likely that mud fever is not the cause of this soreness and crusting of the skin.
Avoidance of UV light, which may mean stabling your horse during the daytime, should help. Corticosteroids may be required in severe cases, administered topically or by mouth or injection.
Sunshine can cause further complications.
Photoallergy also affects sun-exposed areas of the body but occurs due to direct contact with a photosensitising agent. The resulting allergic reaction is most often seen on white muzzles and the extremities of horses grazing on pasture containing clover, particularly a pale pink or whitish-flowered clover called alsike. The best prevention is avoidance of the allergen.
A potentially serious problem is photosensitisation, which can be either primary or secondary to liver disease. Photosensitisation causes increased vulnerability of the skin to the damaging effects of UV light because of the production, ingestion or contact with a photodynamic agent. The main signs are reddened, inflamed and tender skin, often with the presence of oozing blisters that form a yellowish crust. Skin can become itchy and raw, and may bleed.
Examples of plants known to cause primary photosensitisation in the UK include St John’s wort, ragwort and hogweed.
Secondary photosensitisation requires liver failure to allow potent photodynamic agents such as phylloerythrin to pass into the blood unchanged, without being detoxified within the liver. The causes are many and include plants such as ragwort, mycotoxins (natural toxins produced by fungi) and toxins, plus other liver diseases and obstruction of bile ducts with gall stones.
If you notice sunburn on your horse in places not regularly exposed to sun, or in conditions that are not that sunny, then further veterinary investigation may be warranted to check liver function and to identify damage. Blood tests can be helpful initially, but these may indicate a need to biopsy the liver to fully evaluate the cause.
Treatment of the underlying liver disease will be crucial to controlling the photosensitisation and these patients often have to be stabled during daylight hours to stop any further damage to the skin.
The prognosis is usually good for primary photosensitisation, but more guarded for secondary. Much will depend upon the level of liver compromise.
Prevention is better than cure when it comes to summer sun problems, so don’t wait for a pink and peeling nose before taking action. Therefore…
- Provide your horse with field shade where possible, in the form of a shelter, thick hedging or trees.
- Reduce sun exposure with night-time turn-out, or avoid peak UV times (10am-3pm). Be vigilant, too, on cloudy days, as UV levels may still be high.
- Invest in a UV face mask which covers the ears and, ideally, the muzzle — especially if your horse has light or non-pigmented skin in these areas. Use a UV-resistant fly sheet, too, or UV-resistant boots for those susceptible to problems.
- Apply sun block on small areas, such as the flesh mark between the nostrils. Dr Ivens recommends zinc oxide cream licensed for horses, such as FiltaBac, which also has antibacterial properties. Alternatives are available, but always try products not made specifically for horses on a small area to check for any reaction before more widespread use.
Sunscreens with a high sun-protection factor (SPF) of 30-55 can be used, but avoid products containing PABA (para-aminobenzoic acid) — some horses are sensitive to this chemical and might develop a rash. Hypoallergenic sunscreens and those marketed for children tend to be high SPF and formulated for more sensitive skin.
- Reapply sun block throughout the day or use products that are not water-soluble, as sun protection can wash away when horses drink from a trough or graze on dew-drenched grass.
Ref: Horse & Hound; 22 June 2017