Ligulosis In Two Cyprinid Hosts: Rutilus Rutilus And Gobio Gobio

The pathology of Ligula-infections in Rutilus rutilus and Gobia gobio differs in several respects. In natural infections of R. rutilus, there is a pronounced host cellular response which can be demonstrated in fry shortly after infection and which becomes more pronounced with fish age. In contrast, no cellular response is ever found in natural infections of G. gobio. It has been proposed that parasites from R. rutilus and G. gobio differ in their ability to stimulate a host response. Transplantation experiments lend support to this view, although it cannot be excluded that host factors may also play a role. There are qualitative and quantitative differences in splenic and pronephric leucocyte counts, and in spleen weights, between the two hosts.In R. rutilus, infection is invariably accompanied by cytological changes in the gonadotrophs of the pituitary gland, and an inhibition of gonadal development. Thus, the gonads fail to undergo seasonal cycles of development and regression, and they resemble those of immature fish, irrespective of host age. Detailed studies have not been undertaken with G. gobio. However, such cytological evidence as is available indicates that, although pituitary gonadotrophs are reduced in apparent number in infected fish, gametogenesis proceeds further in parasitised G. gobio than in R. rutilus.It is suggested that these differences might indicate that Ligula from R. rutilus and G. gobio are different strains or species. Although comparison of iso-electric focusing gels of parasite extracts supports this view, with seven of 43 identifiable bands differing in worms from the two hosts, evidence from enzyme analysis does not.