Photosensitization (also known as photodermatitis) is light-induced inflammation of the skin (dermatitis) caused by a heightened sensitivity of the skin to sunlight. This condition is non-contagious and often looks like sunburnt skin on our horses and ponies, and can often be difficult to diagnose or distinguish from sunburn.
It occurs in susceptible horses and ponies when phototoxic or photoactive substances accumulate in the skin and interact with sunlight. This can result in severe itchy, crusty and often painful dermatitis. It generally occurs in non-pigmented (i.e. white) skin and skin with little hair cover (such as the muzzle, eyelids, ears and coronary bands).
Equine photosensitivity may be caused by the ingestion of plant or fungal products, chlorophyll metabolites, or other chemicals. It can also occur in conjunction with the use of some medications.
Effects of ingestion of the offending substance may be cumulative and a horse may not develop signs of photodermatitis immediately.
Using lucerne/alfalfa or clover as an example, horses may not develop any clinical signs of photosensitisation until weeks after initially consuming the hay, chaff or pasture which is causing the reaction. We can be feeding our horses this feedstuff for quite some time without any issues, before seemingly all of a sudden, photodermatitis occurs. This can be difficult and confusing, as we often expect clinical signs to appear immediately or very soon after consumption of the offending feed. This can lead to constantly changing our horse’s feed, in attempt to identify the culprit. As a result, this may significantly increase a horse’s risk of developing colic or laminitis.
Other recognised potential causes of photosensitisation include gluten sensitivity; reactions to drugs such as tetracycline antibiotics and phenothiazine tranquillisers; exposure to certain chemicals; and consumption of pastures rich in phosphorus fertilisers.
These are commonly overlooked, but in the author’s experience, generally contribute to photosensitivity reactions more commonly than consumption of lucerne/alfalfa and clover alone, which are often thought to be culprits.
Photosensitisation may also occur as a result of impaired liver function in a horse (known as secondary photosensitivity/photosensitisation).
For these reasons, the effective prevention and treatment of photosensitisation should involve thorough analysis of diet, management, pasture, supplement and medication use in susceptible horses. In many cases, there may be more than one factor contributing to photosensitisation.
The complexity of the condition can also help to explain why simply applying topical creams or lotions and/or simply removing lucerne/alfalfa from the diet or adding a mycotoxin binder to our horse or pony’s feed often doesn’t provide a long-term or effective solution. Removing certain feeds from a diet can also contribute to nutritional insufficiencies or deficiencies, which can sometimes further compound the problem.
It is critically important to address the underlying cause(s) of the photosensitivity if we want treatment to be effective and to prevent recurrence.
Camilla Whishaw is a highly regarded, experienced horsewoman and naturopath, helping to holistically treat and manage a broad range of equine health conditions and injuries, with a passion for mare and stallion fertility.
As a world-renowned practitioner, presenter, author, and consultant in the field of Equine Naturopathy, Camilla shares her knowledge through keynote presentations, interviews, lectures, panel sessions, and workshop training.
