Rain rot doesn't need rain despite its name. Dampness from sweating under a rug or from work will also do it sometimes. If the skin is weakened from lack of Vit A, any dampness can make it vulnerable to the bugs that cause this.
(BTW these bugs are everywhere and in the ground. Rain rot is not contagious even tho it sometimes appears to be. It appears to be sometimes because whole herds will be deficient from similar feeding routines that allow a deficiency this time of year)