These shrubs aren't particularly prone to any disease, although honey fungus kills them quite quickly. How long have you had it, first, second, where's it growing, ground or pot, sun or shade? And have you actually seen anything present on the bush?

No 27365 Berberis thunbergii thank you Bamboo for your response .I've had it since 93 we have it grown into a hedge 20 yards x 7ft, it's being nibbled along the lower branches, last year it was stripped bare after a number of weeks they grew back again .I have not seen anything on the hedge so it's a mystery has to what is doing it .It is situated along west side of my front garden.

Do rabbits eat the foliage, or muntjac deer?

Berberis thunbergii:-No it's not any of those animals, I suspect it's some sort of insect ,they must be nibbling through the night I'm not very good with techno so I can't manage a photo of the damage.Thanks again Bamboo.