Mud fever is a catchall phrase for a variety of skin issues that arise in damp, muddy conditions. And rain rot shares the same characteristics, just typically on the back, withers or neck. The same bacterium, dermatophilus congolensis, causes both conditions, along with many others.Here is the interesting thing about this bacteria, it’s in a family called actinomycetes. Actinomycetes behave like both bacteria and fungi. This is why the debate on how to treat mud fever lasted so long. Because it displays symptoms of both. Horses naturally have these organisms on their skin; they lay dormant until muddy, damp conditions cause them to flare-up.