A particularly unpleasant consequence for horses left out in the wet is a condition commonly called Rain Scald (or Rain Rot). This is an acute bacterial inflammatory skin disease caused by a bug called Dermatophilus congolensis, the very same nasty that causes Mud Fever in wet conditions.
The bacterium invades the skin of the horse, lying dormant until it is activated by the skin being compromised in some way, as in prolonged wetness or being attacked by insects.
While Mud Fever attacks the horse's lower legs, Rain Scald causes crusty, pus-filled scabs to form on the parts of the body most exposed to rain - the top of the head, along the neck and across the back. The scabs peel off along with hair to leave bare spots of red, sore skin. The scabby tufts are known as "paintbrush lesions". They are uncomfortable, and when the horse scratches the affected area, the bacteria spreads. The lesions may be dotted around the horse's rain exposed dorsal surfaces, or may join together to cover a large area. Whichever is the case, when you detect Rain Scald you should act quickly to remove the cause: first get him out of the rain!
Most horses recover from Rain Scald without any treatment, as long as you make sure he has access to shelter in wet weather, or cover him with a waterproof turnout rug. Avoid riding him until he heals if he has lesions in the saddle area (the saddle will make him sweat and make the lesions more painful). Make sure he isn't troubled by insects, and that all his tack is squeaky clean. It's not a good idea to put heavy blankets on a horse with Rain Scald because they trap moisture underneath, which will only exacerbate the condition.
It's best to take action against Rain Scald as soon as you spot the scabs. Bathe the horse with an antimicrobial shampoo - like Shapley's MediCare shampoo - and remove the scabs gently, curry the horse and make sure he is completely dry.
It's easy to mistake Rain Scald for a fungal infection such as ringworm. If you're not sure, get a definite diagnosis from a veterinarian who will probably take a skin scraping for microscopic examination to identify the bacteria.
If the horse is left out in the rain with no protection, the condition will persist and spread, and could lead to a secondary bacterial infection. The infection can spread to other horses, so it's important to keep him away from his companions while healing, and keep his grooming tools and tack disinfected and separate from theirs.
Recovery from rain scald can take several weeks, and your horse does not become immune once he has suffered from it. It could well recur every winter. Prevention is better than cure, and the way to keep your horse Rain Scald free is good management and good hygiene.