Rain scald, or Dermatophilosis, is a skin condition in horses that most owners will be familiar with. It’s caused by a bacteria — Dermatophilus congolensis — which lives on the skin of ‘carrier’ horses. A carrier horse will be prone to developing rain scald, but may not always show clinical signs. They are however the source of infection for other horses.
Young horses, and senior horses with poor skin immunity, or animals introduced from a property with low contamination are more likely to develop acute or chronic rain scald.
For a horse to be infected there must not only be a carrier animal close by, moisture and skin trauma must also be present. When a carrier horse’s skin is soaked, such as occurs during rain, the bacteria release zoospores, the infective stage of the bacteria. Flies carry these zoospores from the carrier to a non-infected horse. Brushes and rugs may also be responsible for transmitting the zoospores.
If the skin of a non-infected horse is healthy and intact the bacteria cannot invade and cause rain scald. If the skin is damaged or broken, by fly bites, other skin diseases, or trauma, the bacteria are able to invade the skin and infect the animal.
The most commonly affected areas are the croup, loins, saddle area, neck and face, the areas of the body that take the brunt of rain. In severe cases the disease may become generalised over the entire body and limbs.
The visible signs of infected skin is a discharge which dries and causes the hair to become matted and crusted. The skin underneath these crusts is often pink, moist, and occasionally bleeding. A greenish-yellow pus will often accumulate on the under surface of the crust. Affected areas can be sore to touch.
Rain scald can also affect the limbs, with white, non-pigmented skin being more susceptible. When the legs are affected, they can become painful and swollen, causing lameness.
Severely affected horses may also develop a fever, be lethargic/depressed, have poor appetite, and swollen lymph nodes. For a horse to become this severely affected there are generally other contributing factors, such as an underlying disease that’s causing suppression of the immune system.
How is rain scald diagnosed?
The characteristic appearance of the condition makes diagnosis by visual examination quite straightforward. It can be confirmed by taking smears of the discharge from the lesions and testing them to establish the bacteria present. Acute lesions can be sampled and cultured.
Treatment is straightforward, involving the application of antibacterial agents to the skin, usually in the form of washes. Chlorhexidine and povidone-iodine scrub are effective. Chlorhexidine is the active ingredient in Malaseb, a dog shampoo which is also very effective against rain scald in horses.
Wet the affected areas and apply the antibacterial scrub, gently massaging the affected areas to remove the scabs. Leave the wash on for ten minutes before thoroughly rinsing off. Repeat this every day for 5-7 days, and then twice a week until all the lesions have healed. Severely affected horses may benefit from systemic antibiotics, but the majority of cases do not need this treatment.
Although relatively easy to treat, unless the underlying cause of the skin trauma that allowed the infection in the first place is addressed, the condition may not resolve fully. It is important that, after treatment, any affected horse has a waterproof rug to prevent maceration of the skin and a recurrence of the disease. Affected horses should have their own rugs, saddle cloths, brushes, etc., which should not be shared with other horses.
Horses that have not previously had rain scald can get some protection by being rugged, although, if there is damage to the skin, they can still contract the disease. It is difficult, if not impossible, to prevent the flies that transmit the disease from landing on and biting the horse, allowing the bacteria to enter the skin. Rugging offers some protection, as does the regular application of fly repellents also helps, but it is virtually impossible to completely prevent flies from landing on your horse. Owners should be vigilant in checking for the signs of rain scald and treating it early.