I ALWAYS brush out the excess dirt out of my brushes when I am done using each one... it takes less than a second... Also How will you get a dirty horse clean with dirty brushes!!?
once a month I throw all
my grooming stuff into a big bucket throw some bleach in and fill it with water and let them soak for about 1/2 an hour. Then shak em out and put them in direct sunlight to dry... (and the UV rays will kill any remaining bacteria) Plus if you consistently clean your grooming supplies your horse will be less likey to get skin problem or contract them for other horses whos owners have barrowed your grooming stuff. Keeping your stuff clean will also make it last longer and work better! I groom multiple times a day and over time the dirt builds up at the base of the bristles.