Actually G adn K horses that never see rain or mud get rain rot and mud fever not as often but they do :)
it is a organism that is alive on all horses skin ... just takes two things to trigger it weak immune system and opening of some type in skin :)
I'm aware of that, but the two things you have listed are not the main reasons.
This is directly from the Mereck Vetinary Manual
The natural habitat of D congolensis is unknown. Attempts to isolate it from soil have been unsuccessful, although it is probably a saprophyte in the soil. It has been isolated only from the integument of various animals and is restricted to the living layers of the epidermis. Asymptomatic chronically infected animals are considered the primary reservoir.Factors such as prolonged wetting by rain, high humidity, high temperature, and various ectoparasites that reduce or permeate the natural barriers of the integument influence the development
, prevalence, seasonal incidence, and transmission of dermatophilosis. The organism can exist in a quiescent form within the epidermis until infection is exacerbated by climatic conditions. Clinical Findings:Dermatophilosis is seen in animals at all ages but is most prevalent in the young, animals chronically exposed to moisture, and immunosuppressed hosts
. Lesions on a host can vary from acute to chronic. Age, sex, and breed do not seem to affect host susceptibility. Pruritus is variable. Most affected animals recover spontaneously within 3 wk of the initial infection ( provided that chronic maceration of the skin does not occur). In general, the onset of dry weather speeds healing. Uncomplicated skin lesions heal without scar formation. These infections usually have little effect on general health.
There's lots more if you like.