Does my horse have Mud Fever/Greasy Heel or Photosensitivity – or is it sunburn?

Mud Fever (Greasy Heel), Photosensitivity and sunburn can present with similar symptoms in horses, despite being different issues caused by different factors, that also require different treatment plans. The key difference lies in what’s driving the skin damage – bacterial infection, internal photosensitivity or external UV exposure.

Key Highlights (Quick Read)

• Mud Fever (Greasy Heel), Photosensitivity and sunburn can look similar but have different underlying causes

• Photosensitivity is triggered internally by fluorescing compounds in the bloodstream, while Mud Fever is caused by bacterial infection

• Sunburn and photosensitivity most often affect low-pigment or lightly haired skin

• Damage from sunburn or photosensitivity can compromise the skin barrier and allow mud fever bacteria to enter

• Lush green grass, algae-contaminated water and impaired liver function can contribute to photosensitivity

• Changeable conditions - strong sun followed by wet or muddy weather - increase the risk of overlapping skin issues

• Correctly identifying the primary driver is essential to stop symptoms from recurring

Mud Fever, Photosensitivity and Sunburn: Why They’re Often Confused

Mud Fever (also known as Greasy Heel), Photosensitivity, and sunburn tend to affect the same areas on a horse’s body, and present with similar symptoms such as broken skin on the stockings and in areas with low pigmentation. In some cases, sunburn or photosensitivity damages the skin barrier first, creating an entry point for bacteria associated with mud fever. Treating for mud fever or sunburn alone may provide short-term improvement, but symptoms are likely to return if photosensitivity is the underlying driver.

How to tell the difference at a glance with our comparison table for mud fever vs. sunburn vs photosensitivity

How to Tell What You’re Dealing With

We’ve put together some useful tips to help horse owners focus on treating the true cause of their horse’s condition, and not just the symptoms they are presenting with.

Are mud fever symptoms recurring?

If symptoms resembling Mud Fever/Greasy Heel or sunburn seem to appear overnight or keep recurring despite careful management, this can indicate photosensitivity rather than a simple bacterial infection.

Is there a lot of lush green grass around?

The time of year is also a clue. Photosensitivity often occurs in Autumn and Spring when there is lots of lush green grass that contains fluorescing chemicals such as chlorophyll. When this grass is digested, these compounds pass into the bloodstream where they circulate around the body. The sun's ultraviolet rays can pass easily through pink skin and areas without much hair and into the blood vessels, causing the compounds to fluoresce – and this causes a type of 'sunburn' that occurs from the inside out.

Photosensitivity commonly occurs in areas with less hair such as stockings and muzzles, and in light-coloured horses that have less pigment, however horses with dark coats can be affected too.

Is there algae in the water trough?

It's not just green grass that can cause a photosensitive reaction. Those blue-green algae in the water trough and other toxic elements in the grazing can impair the ability of the liver to get rid of chlorophyll from the system, causing it to build up over time.

Where Does Sunburn Fit In?

Sunburn is caused by direct ultraviolet exposure and, like photosensitivity, most commonly affects areas of skin that lack pigment.

In mild cases, sunburn may resolve once UV exposure is reduced. However, sun-damaged skin can still become fragile and inflamed, increasing susceptibility to secondary infections - particularly when followed by wet or muddy conditions.

Does your horse have broken skin?

Both photosensitivity and sunburn present with broken skin which provides an entry point for the Dermatophilus congolensis bacteria that cause Mud Fever / Greasy Heel. If you treat the Mud Fever / Greasy Heel or sunburn symptoms but don't treat the underlying Photosensitivity, the bacteria keep getting in and infection will keep recurring.

Which remedies should you use?

For Photosensitivity

Our Photosensitivity Support remedy promotes a natural immune response to both primary and secondary photosensitivity which can cause skin damage, ulceration and the build-up of fluid. For more all-round support, we offer three remedies in a Photosensitivity Kit at a discounted price – a 1L jerry can of Photosensitivity Support, 1L of Liver Plus Equine to help rid the bloodstream of fluorescing compounds, and a 90ml Puffiness Plus for minor swelling and oedema during an acute flare-up.

For sunburn

Sol Plus uses a similar concept to some oral medications - like tetracycline antibiotics and acne drugs - that make humans more sensitive to sunburn. Our formula for horses works a bit like this but in reverse, to support natural defences against UV rays at a cellular level. Sol Plus can also be used following sun damage as dosing supports a normal immune response to the skin irritation and discomfort caused by sunburn.

For mud fever / greasy heel, rain scald and mud rash 

Our Mud & Rain remedy is given orally and is professionally formulated to support a normal immune response to the most common pathogens associated with Mud Fever / Greasy Heel, Rain Scald and Mud Rash. It supports healing from the inside out and is part of our mission to provide no sting, no mess and no stress solutions to everyday health conditions.

This remedy can be dosed straight into the mouth or on a treat using our pump bottle, or use our jerry can formulation to dose onto feed or into the water trough to provide cost effective cover for multiple animals. Read our guide to find out more about prevention and management of Mud Fever / Greasy Heel.

Why should you not remove the scabs when using our Mud & Rain remedy?

Scabs are unsightly, but if you are using our Mud & Rain remedy, we recommend leaving them in place - they'll dry up and fall away once the skin underneath has healed. In our experienced observing the thousands of cases we've dealt with over the years, this should take 2-3 weeks on fresh areas and a little longer on old scabs.

So how do oral remedies work?

We get asked that a lot, so we’ve put together this quick comparison of treatment methods, and written answers to some of the most commonly asked questions in this blog.

Final Thoughts

Mud fever, sunburn and photosensitivity are often part of a bigger picture, particularly during seasons of strong sun and changeable weather. Sunburn can add another layer of complexity, especially on low-pigment skin.

Looking beyond what’s happening on the surface and identifying whether skin damage is being driven externally, internally, or by infection is what allows the skin to recover properly and helps prevent the cycle from repeating.