Scientists have found that there is a relationship between the abundance of some species of molluscs

Scientists have found that there is a relationship between the abundance of some species of molluscs in water bodies near pastures and helminthic diseases of cattle grazing there. Explain this dependency.

Many worms and other parasites develop to such a large size that they cannot exist in the body of small creatures. But at the same time, small living organisms are their carriers and intermediate hosts. So, if an egg of a bovine tapeworm enters the body of a mollusk, it will not develop there. Eggs begin to develop only after penetration into cattle, since they are the main hosts of bovine tapeworm.