Why are we still talking about mud fever?

This bears testament to two facts: one, we are again experiencing an exceedingly wet winter and, two, that there is no universal remedy for this common and frustrating condition.

Despite the best efforts of scientists there is still no vaccine for mud fever and its control is based on understanding factors responsible for infection. So, what are they and how can we control them?

Mud fever, along with rain scald, is caused by the bacterium Dermatophilus congolensis. It also affects cattle and sheep, producing conditions called lumpy wool and strawberry foot rot, and these diseases are collectively referred to as dermatophilosis. Goats, pigs, dogs, cats and humans are less commonly affected, but dermatophilosis is the commonest skin disease of crocodiles in Australia.

Dermatophilus congolensis is naturally found on the skin, with asymptomatic carriers and animals that are clinically affected acting as a reservoir for spread of infection. Bacteria are also thought to reside in soil and mud.

But wet conditions and mud are not the primary cause of mud fever or rain scald. Rather high moisture levels weaken the skin's normal defences to infection, enabling any dermatophilus bacteria to penetrate and establish infection.

Clients often express horror when horses wearing rugs develop rain scald, especially if the rug had cost a week's wages. But this can happen if horses become too hot under a rug, with sweat producing moist conditions that encourage Dermatophilus infection.

Other factors that contribute to dermatophilosis include malnutrition, concurrent diseases, mechanical trauma or irritation of the skin. Genetic factors may also be important, as some horses appear to never get mud fever even when turned out in the wettest conditions while others are perennially affected.

Mites, especially of the chorioptic type that affect horses with heavily feathered legs, are an important trigger factor because they disrupt the skin surface and, by causing itching, produce further damage. Some arena surfaces weaken the skin's defences, either because they are abrasive or irritate the lower leg.

Fungal infections and a disorder of the immune system called leukocytoclastic vasculitis can also cause inflammation and trigger dermatophilosis. The latter condition affects unpigmented or pink skin on the back of the pastern and is aggravated by sunlight, often causing confusion as horses seem to be affected by mud fever in sunny and dry weather.

Given what we know about mud fever, treatment should be straight forward. Surely, it's simply a matter of eliminating infection and preventing further disease by reducing the numbers of dermatophilus bacteria on the skin and ensuring that trigger factors are avoided?

Infection is active under the scabs that are characteristic of both mud fever and rain scald. Removing these scabs and washing with an antiseptic scrub or shampoo is an effective way of eliminating dermatophilosis. Horses may resent the picking off of large or multiple scabs, but softening by repeated washing with warm water or application of emollients like udder or nappy cream will help.

In horses where the disease is widespread and topical treatment resented, or proving ineffective, it is tempting to resort to a course of antibiotics. Dermatophilus congolensis has traditionally been sensitive to the broad spectrum antibiotic granules that are commonly given orally to horses.

But these trimethoprim sulphonamide antibiotics are not always effective in cases of mud fever. This may be because the bacteria have become resistant. However, in some cases, it is because other non-bacterial conditions – such as fungal infection, mites or leukocytoclastic vasculitis – are responsible for the disease.

It is therefore advisable to establish the cause of the mud fever, particularly if an initial course of topic treatment or antibiotics does not cure the condition. Careful clinical examination together with sampling of skin for bacteria, fungi and mites, or even a biopsy, will enable treatment to be correctly tailored.

Clients sometimes moan that vets won't prescribe antibiotics for common conditions like mud fever without seeing the horse. But this is precisely why: we need to see the patient to ensure antibiotic treatment is indicated and that the right dose is prescribed. Instead of antibiotics it might be that an antifungal wash, an injection for mites or a steroid cream for leukocytoclastic vasculitis is required.

It is when thoughts turn to preventing mud fever that opinions really differ. The obvious solution is to avoid wet and muddy fields but it is not always wise to stable horses for extended periods and then expect to ride them! Turnout in arenas or yards with a dry surface will help overcome this concern.

For those that are turned out, liberal application of barrier creams and oils, such as pig oil, goose crease or petroleum jelly, will protect the legs against erosion from wet conditions. Waterproof boots or chaps may also help. Fencing off muddy gateways and moving hay feeders to drier ground will further reduce exposure.

But what do you do with the mud on legs when your horse comes in? There is an argument that washing mud off simply makes the legs wetter and that leaving it in place will further protect the skin from moisture. This approach is supported by the observation that some out-wintered horses do not develop mud fever despite having legs caked in mud.

I tend to disagree with this approach, taking the view that washing mud off and drying the lower limb reduces the numbers of Dermatophilus bacteria on the skin and limits potential for mud to irritate the skin. Combined with stabling on a deep dry bed, and once or twice weekly washing of the legs with an antibacterial shampoo, this represents the best chance of avoiding mud fever.

What do you do with horses with heavily feathered legs? I prefer not to clip the hair that offers natural protection, but appreciate that it may be difficult to wash all the mud out following a day at grass. Here, there is a case for simply drying the legs and not washing the mud off.

There is a wide variety of lotions and potions marketed for mud fever, but these are not necessarily a 'magic bullet'. Rather, they should be used as part of a range of measures that limit exposure to wet conditions, reduce bacterial numbers and protect the skin's natural defences.

Finally, it is perhaps a measure of just how wet the recent year has been that discussions of mud fever have raged in English-based equestrian press. Welcome to our world!