The onset of Autumn normally coincides with an increase in rainfall, humidity and the presence of ideal conditions for the proliferation of bacterial and fungal diseases.
Skin conditions including mudfever and rain scald are common at this time of year and we frequently field phone calls asking for advice on how to treat mud fever and rain scald.
Mud fever has a number of synonyms including greasy heel, cracked heels, pastern dermatitis, grapes or scratches. Pastern dermatitis is probably the most correct term given for the condition that result in scab formation and the presence of oozing skin around the pasterns and heel bulbs. It may be seen in horses of any type, age or gender but is common in draft and feathered breeds, arabs and any horses with white socks. In Southland it is not uncommon to see pastern dermatitis in Standardbred and Thoroughbred racehorses that are worked on gritted tracks.
What is mud fever caused by?
In general, mud fever is caused by a fungus like bacterial organism called Dermatophilus congolensis which lives naturally on the horse’s skin. Under normal conditions, this bacterial doesn’t cause any problems, however when favourable conditions arise (such as higher moisture levels, heavy dews, heavy rainfall and mud), Dermatophilus congolensis bacteria replicate and produce “zoospores’, which then penetrate the skin causing inflammation. It is not uncommon for other secondary conditions to develop in combination with Dermatophilus congolensis bacteria causing pre-existing damage to the skin or irritation, which then allow bacteria to invade. These may include one or more of the following; "
- Continual wetting and drying in the Winter by rain and mud, causing the skin to chap and crack. Wounds, grazes or irritation of the skin by sand and grit (such as from race tracks) can also damage the skin.
- Rough vegetation and stubble fields can damage the skin on the horse’s heels, predisposing them to infection. Alkaline soils (eg. clay) are also potentially irritating to the skin.
- Secondary bacterial infection is present in a number of cases. Once D. congolensis lesions have developed, it becomes easier for other bacteria to also become involved. Staphylococus and Pseudomonas organisms are commonly implicated in secondary infections and cause even more inflammation. In some cases, there may also be a fungal infection (such as ringworm and yeast infections) present.
- Insect damage; biting flies can cause damage to the skin allowing bacteria to penetrate. The biting stable fly ‘Stomoxys calcitrans’ can trigger irritation by biting the horses limbs and can also act as an agent, spreading bacteria from one horse to another. These flies are commonly around manure heaps and although common on dairy farms in more Northern parts of New Zealand, warmer conditions in Southland can result in these fly numbers increasing.
- Chemical dermatitis caused by over vigorous washing of the lower limbs with irritant soaps can result in disruption of the normal skin bacterial flora and stripping the protective oils from the skin, causing cracking and irritation.
- Chorioptic mange in heavy, feathered horses (clydesdales, gypsy cobs, shires etc). This hypersensitivity reaction can cause intense irritation, stomping and biting of the limbs, resulting in trauma that allows bacteria to proliferate. Control of the bacteria alone is only a temporary measure without also treating for mange.
- Photosensitization or sunburn where heightened skin reaction can occur with direct exposure to sunlight helps to explain why the lesions commonly occur on white (unpigmented) areas of the leg. Anecdotally, photosensitization seems to be worse if horses are grazing on clover or alfalfa. Horses with liver disease can also develop photosensitisation on unpigmented areas of the limbs or face.
- Immune mediated disease of the blood vessels (leucocytoclastic vasculitis).
- Some horses seem to be more sensitive to developing pastern dermatitis and have a more severe reaction. While researchers believe there is a genetic component, they have not identified a specific gene.
What are the clinical signs of mud fever?
Affected horses have characteristic scabbing lesions on the back side of the pastern and fetlock, heels or coronet, with white areas much more commonly affected. When the scab is picked off, it pulls away from the skin with a tuft of hair and pus is present underneath it. The surrounding skin is often reddened, itchy, swollen and sensitive. In severe cases, swelling may extend up the limb resulting in cellulitis (a bacterial infection of the skin and underlying tissues), causing severe lameness. Further irritation can result in thickening and scaling of the skin or the development of nodules (“grapes” ) of inflamed tissue in particularly severe and chronic cases.
How do we diagnose and treat mud fever?
Early diagnosis of the causal factors is helpful. While the classic, clinical signs of scabbing and crusting lesions on the pastern allow a diagnosis to be made in the majority of cases, recognition of other irritant factors is also important to obtain full resolution. Once the lesions become chronic (longstanding) it is frequently impossible to determine the exact diagnosis. Diagnostic methods may include hair plucks and skin scrapes to look for parasites, swabs to culture for bacteria or fungi, biopsies to look for characteristic pathological changes (e.g. immune mediated disease) and blood samples to look for underlying disease.
How is mud fever treated?
As any horse owner who has dealt with this problem knows, mud fever can be particularly frustrating to treat and it is important to manage all other predisposing factors. While it may seem to be an easy disease to identify and treat, seeking early Veterinary advice is important. With improper treatment, a horse may have frequent flare-ups and the problem can persist for months. If lesions are present on the horse’s pastern, the area should be clipped to prevent matting of the hair with serum and pus. This also prevents serum scald, which occurs when there is sufficient skin damage for serum to ooze out onto the skin – scalding it in effect. Clipping the hair improves air circulation to the skin, helping keep the skin dry and healthy. After clipping, be sure to clean and disinfect the clippers well since pastern dermatitis can be transferred between horses on clipper blades. Following washing of the affected areas with an antibacterial soap such as chlorhexidine (Hibitane®) or iodine (Betadine®) scrub, ensure that the limb is thoroughly dried with clean towels. The scabs should be gently picked off the skin, but this can be a very painful process for the horse and your horse may require sedation to allow this to be done. Oily emollients (emu oil etc) or zinc oxide cream, betadine ointment etc to soften the scabs makes removal easier and less painful . ‘Stop Greasy’ is a product that VetSouth stock, that can have beneficial protective effects of preventing the skin becoming cracked and macerated. In most cases, topical antibiotics applied directly to the lesions are effective, however if the leg is hot and swollen or the horse is lame, systemic antibiotics may be prescribed.
Systemic antiinflammatories (e.g. phenylbutazone) may also be used in these cases. Topical corticosteroids which helps alleviate pain and swelling can also be used to treat severe lesions. Remember, if your horse is competing, take note of any performance withholding periods that the drugs used may contain. It is recommended to keep horses stabled for a period of the day to keep the legs dry and keep them out of direct sunlight until the lesions have resolved. In some cases, wrapping the legs to keep the swelling down and keep the sunlight off is recommended, but should be removed if the bandaging gets wet. Removing horses from morning pastures with dew can also be very beneficial. It can take weeks of diligent, daily therapy for lesions to completely heal. Even with aggressive therapy, recurrence in some horses occurs, especially if management changes are not made to prevent further skin damage. In the Winter months, the major difficulty for most owners comes in avoiding muddy and wet conditions and recurrence of the condition is therefore common.
How can mud fever be prevented?
For horses that are predisposed to mud fever, horse owners can make several management changes to reduce the frequency and severity of mud fever flare-ups: reducing moisture in your horse’s environment is key to controlling it. Avoiding muddy areas in Winter is not easy in Southland and if stables aren’t available, a well drained, raised Winter stand off area made of bark may suffice. Excessive bathing or washing the legs can predispose a horse to developing pastern dermatitis. If frequent washing can’t be avoided, try applying a barrier cream, such as Vaseline®, Vitamin A cream etc. to protect the skin. Always dry your horse’s legs well after bathing.
What is rain scald?
Rain scald is a bacterial infection of the skin that results in the formation of matted scabs usually affecting the back and rump but occasionally the lower limbs.
What causes rain scald?
The same bacteria that causes mud fever, Dermatophilus congolensis also causes rain scald. Like mud fever, prolonged wetting and cracking of the skin surface, as occurs in wet weather, allows the bacteria to invade the surface layer of the skin and establish an infection.
What does rain scald look like?
In mild cases, the horse may just have a few flat scabs containing mats of hair. When the scabs are removed, the underneath surface is slightly moist and sometimes raw. In severe cases, the coat over the horse’s back and rump will feel hard and painful and will consist of many scabs lying next to each other. If the scabs are gently removed, the horse may be left with a large area of bare, raw, skin Skin conditions caused by fungi (e.g. ringworm) or other bacteria (Staphyloccus spps.) can cause similar symptoms. These conditions can be differentiated by taking a skin scraping or hair pluck and looking for the causal fungi or bacteria under a microscope, in the laboratory.
How is rain scald treated?
Affected areas should be gently washed with a mild disinfectant shampoo or solution e.g. chlorhexidine or povidone iodine, and as many of the scabs as possible removed without causing excessive discomfort to the horse. The skin should be dried or allowed to dry completely and a broad-spectrum antibiotic powder, spray or ointment applied. Gradually removing the scabs over several days, applying an antibiotic preparation and keeping the horse dry will result in resolution of the condition. If the horse is to go outside, covering the horse with a waterproof, breathable rug is recommended. Occassionally systemic antibiotics are required in severe cases. Supplementation of the horse’s diet with flaxseed oil high in omega 3, 6 and 9 is recommended as these have natural anti-inflammatory properties which help to promote skin healing.
Can rain scald be prevented?
As the bacteria multiplies best in warm, wet conditions, keeping the horse stabled, sheltered, or rugged with a waterproof rug during wet weather, protects the skin from prolonged wetting and helps to prevent infection. Some horses appear to be more susceptible to rain scald and it is not uncommon for a dam and her foal to succumb to the condition together after heavy rain.
- Heather Busby