Mud Fever…….dispelling the myths!

Roger Dixon BVM&S Cert AVP (ESM) MRCVS
Ashbrook Equine Hospital, Cheshire

Being a horse owner in the UK means dealing with rain and wet horses on a regular basis! The wet weather we experience in winter, turns many fields into a quagmire, especially near gateways and fences. Horses often spend long periods of time turned out up to their fetlocks in mud. It is at this time of year that we usually see a large number of cases of mud fever.

What Is ‘Mud Fever’?

‘Mud fever’ is a skin infection of the lower limbs caused by the bacterium Dermatophilus congolensis. This bacterium survives in the environment and in scabs as spores that become activated in wet conditions. The spores germinate to produce hyphae; threadlike tentacles that penetrate living skin and cause an inflammatory reaction. Other names commonly used for mud fever include ‘Greasy Heels’ and ‘Cracked Heels’. The same bacterium can also affect the back and rump, when it is termed ‘Rain Scald’. The principles of treatment and prevention are the same for both conditions.

What Predisposes to Mud Fever?

Although we use the term ‘mud fever’ it is actually moisture and wet, rather than mud itself, which is necessary for the infection to develop. When skin is in contact with moisture for prolonged periods of time with no opportunity to effectively dry out, the outer layer of the skin weakens and becomes more susceptible to trauma. The bacterium then gains entry through breaks in the skin to cause infection. Horses that spend long periods turned out in wet muddy fields, and horses that are stabled when wet without their legs being thoroughly dried are therefore predisposed. Fine-skinned breeds appear more susceptible to mud fever, with white legs often developing a more severe infection than dark legs.

Clinical Signs of Mud Fever

The first sign of mud fever is usually one or more crusty scabs over the back of the lower limbs (most commonly the pastern or fetlock regions). Very often more than one limb is affected. The scabs form from serum oozing through small fissures in the skin. This also causes hair to become matted together in clumps. If the scabs are removed, the skin surface underneath is usually ulcerated, painful and covered in white/yellow/green pus which contains the causal bacterium. Frequently, the overlying hair comes off with the scab and the hair roots protrude through the scab (the so-called ‘paintbrush’ lesions). This pattern of infection is also seen over the back and rump in cases of rain scald.

In many cases the area may be hot, swollen and very painful to touch. The horse may become lame and in severe cases the region may swell as secondary infections develop. Occasionally, acute severe cellulitis may develop which causes the whole leg to swell and the horse rapidly becomes non weight-bearing.

Differential Diagnoses

There are several other conditions which may be mistaken for mud fever. It should also be remembered that more than one condition may be present at any one time:

• Ringworm
  – This is a fungal infection of the hair follicles which commonly affects the head, body and limbs, depending which species of ringworm is present. It typically presents as areas of patchy hair loss with flaky skin. If scabs are lifted off they tend to reveal more dry flaky skin beneath rather than ulcerated skin and pus as in mud fever.
• Chorioptic ‘feather’ mites:
  – These typically affect horses with long feathers and are usually itchy rather than painful. Affected horses commonly stamp their feet. The skin under the feathers may be grossly thickened in long-standing cases.
• Equine Cushing’s Disease:
  – This causes suppression of the immune system and predisposes to infection. Therefore, in cases of mud fever which do not improve as expected with treatment, your vet may advise blood testing for Cushing’s.
• Photosensitization:
  – If the lesions affect only white legs, then your vet may advise a blood sample to test for liver disease because this may cause photosensitization.

Treatment of Mud Fever

• Move the horse to a clean dry area and prevent prolonged contact with moisture. Treatment will fail if the horse remains in a muddy field. Box rest in a well-drained stable. Turn out in a ménage or dry field is appropriate.
• Clip the hair. This may require sedation by your vet and is important to:
  – allow circulation of air
  – enable thorough drying of the skin after treatment with medicated washes 
  – accurately assess the extent of the infection
• Wash the leg in warm water to remove mud and loose debris.
• Remove the scabs from the leg. The causal organism survives in the scabs and in the pus underneath so scab removal is crucial to successful treatment. It is best to soak the scabs in warm water and a medicated wash such as pevidine or chlorhexidine (‘Hibiscrub’). Warm water helps to open the pores in the skin and allow greater penetration of the antiseptic wash.
  It is important to avoid excessive scrubbing during scab removal e.g. with a brush or fingernails, otherwise further trauma and inflammation will develop. 
• Dry the area thoroughly by patting with clean towels (dirty towels may reinfect the leg) or hairdryer.
• Remove the scabs from the environment:
  – Remove all hair clippings and scabs from the stable and disinfect the stable with a disinfectant such as Virkon to reduce the chance of reinfection.

This regime may need to be repeated daily for several days to gradually remove all of the scabs.

• Apply topical creams, if appropriate:
  – Many different creams, lotions and potions have been used to treat mud fever in t he past. They include emollients to maintain hydration of the skin, barrier creams to prevent moisture penetration of the outer skin layers, and anti-inflammatory and antibiotic creams.
  – Your vet will advise you on the appropriate cream to use in your horse’s case. Your vet may prescribe an antibiotic cream such as Flamazine.
  – In recent years, a Lime Sulphur wash (‘Lime Plus Dip’) has been shown to be very effective at treating mud fever. Following removal of debris with a medicated wash or shampoo, the lime sulphur is sprayed or ‘painted’ on to the skin. It is very good at drying out the skin and has the added advantage of being an excellent treatment for Chorioptic ‘feather’ mites and ringworm. The disadvantages are it’s pungent smell, which resembles rotten eggs, and it’s tendency to stain gold jewellery black on a permanent basis!! Please consult your vet for more details.
• The limb is usually left unbandaged to allow air to circulate. Bandaging tends to trap moisture next to the skin and prevents the skin from completely drying. This can delay healing and lead to reinfection.
• Severe cases may require either oral or injectable antibiotics. These may need to be given for a long period e.g. 10 days after signs of mud fever have disappeared. If in doubt, always consult your vet for advice.
• Painkillers/anti-inflammatories may be needed to relieve your horse’s discomfort.
• Ensure your horse has adequate tetanus cover.

Prevention of Mud Fever

Some horses do seem to be more susceptible to mud fever than others and it is a condition which can be very difficult to prevent altogether. However there are management practices which will reduce the chances of mud fever developing:Avoid grazing in very wet conditions, particularly on poorly drained soil which is known to become boggy.

• Avoid grazing in very wet conditions, particularly on poorly drained soil which is known to become boggy.
• Use electric fence to fence off badly poached areas of the fields e.g. next to gateways.
• Rotate fields to avoid poaching.
• Where grazing in wet conditions is unavoidable, apply barrier creams to the legs before turnout. There are many on the market; contact your vet for specific advice.
• Dry the legs thoroughly when stabling your horse at night or after exercise in wet conditions. If the legs are very muddy, only hose/wash them if you can be sure of completely drying them afterwards. In some cases it may be better to wait until the mud has dried, then brush off the mud.
• Avoid over-washing and over-vigorous grooming of the limbs; even minor damage to the skin can pave the way for the introduction of infection.
• Avoid applying bandages onto wet legs

Summary

Mud fever is a very common condition in horses, but can also be one of the most frustrating. Some horses will continue to be susceptible despite the owner doing everything ‘by the book’ and taking all the recommended steps, whilst other horses seem to spend months on end knee-deep in mud with no apparent adverse affects (although we would not recommend this!). Some horses will have one or two small scabs for a long time which may suddenly progress to acute severe cellulitis overnight with no obvious precipitating factor.

If your horse has suffered from mud fever in the past it is important to keep the horse as dry as possible and remain vigilant. Early treatment is often necessary to prevent an innocuous-looking scab from progressing into a more serious condition that may require weeks of intensive management before a full recovery is achieved. 

There are many different treatments used on horses in the UK to treat mud fever, some of which are steeped in ancient horse folklore with no scientific basis! Apart from being a waste of money, some old-fashioned treatments may actually do more harm than good, whilst others have since been proven by science to be beneficial and have been adapted into commercially available preparations. In all but the mildest cases it is advisable to seek advice from your vet to aid a speedy recovery.