Vet Vibes: Rain Scald

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VET VIBES

If weather predictions are to be believed this winter is likely to be wet, creating the perfect conditions for a case of Rain Scald. Veterinarian DR. DOUG ENGLISH takes a closer look.

Askin infection resulting in the formation of hair encrusted scabs, Rain Scald is also known as Greasy Heel, Rain Rot, Mud Fever and, more officially, Streptothricosis.

In mild cases, you might have discovered a few scabs matted with hair. When you remove the scab, the underside is moist and can sometimes leave raw skin behind. But if the case is severe, the coat over your horse’s back and hindquarters will feel hard, with numerous scabs in close proximity. Rain Scald on the lower limbs is known as Mud Fever or Greasy Heel, with similar symptoms presenting. Although uncommon, Rain Scald may also affect the face.

Rain scald is caused by a dermatophyte named Dermatophilus congolensis which is a Gram-positive filamentous actinomycete, a unique organism that has properties of both fungi and bacteria but is not classified as either. They can produce serious skin disease in a wide range of domestic and wild animals throughout the world, especially in the humid tropics and subtropics. The skin lesions are characterised by patches of raised, crusty, matted tufts of hair, and pustular crusts that pull off leaving a wet or pussy skin surface. Tightly adhered crusts will cause some pain to remove. The condition affects all mammals – sheep, cattle and goats are commonly affected, and unfortunately, so are horses.

This organism has an unusual life cycle involving two forms: a long, branching, filament-like structure, and mobile spores that can reproduce. The organism can be found on the skin of healthy horses, who although are symptom-free, carry it and can infect other horses.

On the plus side, the organism only attacks the epidermis and mostly does not go deeper to infect the skin's keratin in stratum corneum or dermis proper. On the down side it's mostly found in the external environment. It can survive for even longer periods of time (up to seven months) on gear like boots, rugs, and girths. Many times, I’ve seen it spread in stables from trackwork riders’ boots where they’ve come into contact with an infected horse while working. Bedding is also a factor, and a significant increase in airborne microbial contamination has been found in stalls with straw bedding when compared to stalls with wood shavings and paper‐based bedding. Both Biting Stable Fly and the common House Fly have been demonstrated to carry and spread the organism.

Lesions may be in one area or more widely spread, and their distribution gives a clue as to the underlying cause. Conditions which can predispose a horse to infection include areas where excessive moisture and/or biting insects are present, if the horse has wet hair, or where skin trauma is common, under the saddle or girth for example. In this latter case, the organism can be spread by contaminated straps and a four per cent chlorhexidine solution will kill off the organism after soaking for an hour. Crusts from infected horses pose the highest risk because of their large infectious load.

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Apart from skin infections, in rare instances the organism can apparently cause infections in the lymph nodes, but I have never seen this in my 46 years of practice.

A common site of infection is the horse’s pastern because it is an area often wet from dew, or from walking in water or mud. But infection in this area shouldn’t be confused with Chorioptic mange, the most common form of mange in horses, which is usually associated with pastern dermatitis in feathered horses, draft horses for example

You should also be aware that, although uncommon, Dermatophilus congolensis can affect humans, causing infections in the skin, fingernails and toenails. Interestingly, diagnosis of human infection is underestimated in people in contact with animals or contaminated soil.

Conditions that predispose horses to infection include: biting insects; overcrowding; ticks and other parasites; an illness e.g. Cushing’s disease; an inadequate diet; stress such as cold weather; age; wet humid conditions; and a long, water retaining coat. However, the organism itself is infectious and does not need any predisposing conditions, but the lesions are made worse if those conditions exist.

The scabs protect the organism and removing scabs by hand is painful for your horse. Applying coconut oil, almond oil, raw honey, albarol oil, or baby oil to the scabs helps to soften them so that, after an hour or so, they are then easier to wash off. Adding tea tree oil in a ratio of one part per 100 in any of the oils mentioned above is proven to kill the organism.

Alternatively, apply a disinfecting shampoo such as Aloveen, Hossgloss, Malaseb, Topizole, or Austrazole with a sponge and leave on for approximately 30 minutes before rinsing off with a bucket of water containing a cup of vinegar to help re-establish the protective acid skin mantle.

Injections of antibiotics like penicillin or oxytetracycline are reasonably effective but do not penetrate sufficiently to affect the thread-like hyphae in the far outer skin. Injections are generally unnecessary except in severe conditions where there is deeper penetration by the organism, and should always be used together with topical applications, which remain the first choice for treatment.

Protecting your horse by stabling and rugging is recommended, but if rugged, remove the rug daily to inspect and treat the lesions. The tea tree/carrier oil blend is also effective if applied to the lesions daily (after washing and removing scabs). This mixture can also be rubbed into areas that look a bit suspicious and have emerging bubbles and crusts. You can also spot sponge wash suspected areas with a disinfectant shampoo before applying the tea tree oil blend. Alcohol sanitisers can also be wiped over potential problem areas.

Hygiene around infected horses is particularly important because the organism will spread if you don’t take appropriate precautions. Wash your hands or use alcohol sanitisers when handling infected horses, disinfect gear, and wear overalls which must be removed when you’ve finished the treatment. Blankets, wraps, and similar items should be washed and hot-air dried.

Finally, learning to differentiate Dermatophilus congolensis from ringworm infections is most important. Ringworm is an outwardly growing hairless lesions with minimal scabbing, is highly contagious, and requires a very different approach to treatment.