Consider getting a UVAire system which kills the micro-spores:
Field Controls | Air Cleaners | Power Venting | Combustion Air | Draft Control | UV-Aire | Ventilation | Water Conditioners | Healthy Home System | IAQ
You can get inexpensive rotary brushes/flex line to use while the fan is running:
Dryer Duct Cleaning Tool - Help Keep Your Energy Bills Down - InfoBarrel.
Good filters changed frequently is a must.
(From a quick search, the pro equipment is super pricey)