I believe that 9 days can be useful in lowering the spore count in a carrier thus keeping at a manageable level that the rabbits own antibodies can control and prevent from becoming an active symptomatic infestation.
I also believe that treating too frequently with a longer course can be detrimental in a rabbit that isn't fighting an active infection because it can lower the spore count and thus the rabbits antibody response will decline. Should the antibody count decline then the rabbit will have no defence should the course finish and the spore count begin to increase. There is also the risk that overtreating may cause the protooa to mutate and develop an immunity to Fenbendaole, as has been seen in some horse wormers.
In my suspected carriers they have a 9 day course of Panacur twice a year and at times of risk. Any I consider to have an active infection will have a 6 week course as will their cohabiting partners to protect them during the cyclical shedding cycles.
According to the binary sheets for Panacur rabbits who were treated with Panacur whilst being exposed to EC spores still tested negative for exposure after the experiment (
http://forums.rabbitrehome.org.uk/showthread.php?t=110797&highlight=panacur+binary). However timing Panacuring to coincide with the release of spores in a domestic environment would be near on impossible to determine.
My vets agrees with what I have written above as I have discussed it with her, however ask any other vet and I guarantee they will have a different opinion. Apparently even if you pin down FHB you won't get the same response everytime. :lol: