Mud fever in horses: clarify the causes, assess feeding honestly

What is mud fever – and why is it not a single diagnosis?

Mud fever, technically equine pastern dermatitis, is a non-specific skin reaction pattern on the lower limbs, usually in the pastern. The specialist review speaks of a multifactorial syndrome for which there are still few original studies on many aspects ¹. An older review puts it similarly: mud fever is a reaction pattern of the skin and should be regarded as a syndrome rather than a diagnosis ³.

According to the available data, age and breed influence how often and how severely mud fever occurs; heavy horses with feathers and a large cannon bone circumference are particularly affected, as are hind legs and unpigmented limbs. Associations with Chorioptes mites and an altered skin microbiome have been shown repeatedly, but not with fungal infections. It has not yet been shown that a single pathogen is the cause ¹.

Why does mud fever need to be investigated by a vet?

Which treatment is suitable depends on the cause. If contributing and perpetuating factors are not identified, failures are more likely ³. In a Swiss owner survey, 69% of treatment decisions were made without veterinary advice; this made it harder to identify the cause and often made treatment longer and more involved ⁷.

Depending on the picture, investigation can include skin samples, such as cytology of fresh crusts ⁵. If light sensitivity is suspected, liver function should be checked ⁶.

• How long have the changes been present, and on which legs?
• Are the affected areas white or pigmented?
• What are the turnout area, bedding and pasture like?
• What has already been applied or given?
• What changes to feeding were made in the same period?

What can you influence in management and care?

For bacterial dermatophilosis, the link with wet conditions is well described: persistently wet pasterns in wet turnout areas, stables or on pasture lead to infections of the lower limbs, and dry weather speeds up healing ⁵. For mud fever as a whole, however, the evidence on rain and humidity is inconsistent ¹. Dry turnout areas therefore make sense but are no guarantee.

For heavy horses with lymphoedema, the review recommends clipping the feathers for cleaning and treatment and drying the legs thoroughly ⁴. In a study of Rhenish German draught horses, horses kept on pasture or pasture with paddock had a lower risk of higher lymphoedema scores, and certain straw bedding a higher one ⁹. These are observed associations, not proven causes.

• Reduce muddy, permanently wet areas in the turnout area.
• Dry legs thoroughly after washing or after getting wet.
• Agree on care and treatment products with your vet instead of trying out several products one after another.
• Document changes with date and photo.

What role do zinc and copper really play?

Zinc is often named as a skin nutrient. What is documented: zinc deficiency causes slowed growth, loss of appetite, skin changes on the lower limbs and hair loss in foals. According to the same specialist overview, the consequences of zinc deficiency or excess in adult horses are not documented ². The frequently cited link between zinc deficiency and outbreaks of dermatophilosis refers in the specialist information to cattle feed, not horses ⁵.

More is not better: too much zinc can impair copper absorption; a zinc-to-copper ratio of about 3:1 to 4:1 in the ration is recommended ². For the total ration, legal maximum levels of 120 mg zinc and 25 mg copper per kg complete feed apply across all sources ¹⁰¹¹.

The sensible question is therefore not which zinc product to buy, but: is the zinc and copper supply from hay, pasture, concentrates and all supplements together in line with requirements ¹²? A calculated gap can be closed in a targeted way. An additional skin product on top of a ration that is already covered only increases the total amount.

• Zinc and copper in horses
• When trace elements become too much

What is known about special feeds for mud fever?

The specialist review describes treatment of mud fever as largely empirical; the main subjects of study were antibacterial agents, fatty acids, aromatic oils, moisturisers and approaches against Chorioptes mites ¹. We have not found a controlled study showing that a particular feed or complementary feed cures or prevents mud fever.

There is a genuine link to feeding in lymphoedema of heavy horses: in the study of Rhenish German draught horses, hay with straw in winter was associated with a lower risk of higher scores, and haylage and concentrates in winter with a higher one ⁹. The review summarises these findings in the same way ⁴. These too are associations from observational data, not proof. So discuss any change to the ration for affected heavy horses with your vet.

• Forage as the basis of the ration
• Concentrates only when needed

Can pasture plants make the skin sensitive to light?

Yes, and this is the most important link to feeding for skin damage on white legs. In photosensitisation, photoactive substances in the skin react with UV light and damage it. Primary photosensitisation can be triggered by plants such as St John's wort and buckwheat. In the liver-related form, a breakdown product of chlorophyll accumulates because the liver is damaged; triggers named include alsike clover, red clover and ragworts (Senecio species) ⁶.

In horses, the liver-related form is more common. Lightly pigmented or sparsely haired areas of skin are affected ⁸. As unpigmented limbs are also affected more often in mud fever ¹, it is worth looking at the pasture when changes appear on white markings.

• Accounting for pasture grass in the ration

What belongs in your feeding record?

With these records, the conversation with your vet can separate what the ration, management and skin findings each contribute. A ration calculation shows gaps and excesses; it does not replace a skin diagnosis ¹².

• Hay and haylage with daily amount and, if available, analysis.
• Grazing time, pasture area and conspicuous plants on the pasture and in the hay.
• Concentrates and every supplement with declaration and actual daily amount ¹³.
• Feeding changes with dates, kept separate from observations of the skin.

• Understanding hay analysis
• Sweet itch: feeding and its limits

Key takeaways

• Mud fever is a reaction pattern with several possible causes; causes and contributing factors are clarified by your vet before treatment ³¹.
• Zinc belongs in the total ration in line with requirements: too much interferes with copper absorption, and levels are legally limited ²¹⁰.
• Inflamed white markings after grazing may indicate light sensitivity caused by plants or liver damage ⁶.

Frequently asked questions