The RWAF does not advocate regular use of worming products for rabbits, ie every quarter, as is recommend for cats and dogs for example. However their are times when the use of 9 day courses might be helpful:
1 to reduce the risk of infection at that specific time (as in the study) eg around introduction for short periods of time eg mating, as per the Suter paper.
2 To suppress chronic infections and reduce the signs associated with them.
This is not backed up by any scientific studies, but it is often reported by rabbit owners that improvement occurs within a few days of regular periodic treatments. This could be coincidental, or due to other factors, but requires more work to investigate. 28 day courses are generally advised for animals with clinical signs of infection, as this is the only regime investigated for treatment.
It is important to combine this with regular effective cleaning of the environment, especially towards the end of the treatment course, and concurrent treatment of all in-contact rabbits, to avoid re-infection with spores. Anecdotally, some people have found that 42 day courses have been more effective than 28 day courses in clearing EC. This may be due to longer persistence of the organisms in the animal, or the environment, or inadequate hygiene measures. In any case, it is vital to combine treatment with daily cleaning of the environment, in particular, prevention of urine and faeces contamination of food and water, to prevent re-infection by other rabbits or themselves.
Longer than 28 day, or repeat courses should only be carried out under the direction of your veterinary surgeon and should not be recommended as standard as they have not been proven to be safe at this point. The RWAF stresses that it is more important to avoid re-infection during treatment than to give a course longer than 28 days.