The relationship between miracidial dose, production of cercariae, and reproductive activity of the host in the combination Trichobilharzia ocellata and Lymnaea stagnalis.

The effects of exposure of juvenile Lymnaea stagnalis to one, two or, four miracidia of Trichobilharzia ocellata on the following parameters were studied: infection rate, length of prepatent period, production of cercariae, growth and ovipository activity of the snails, and the weights of their accessory sex organs. An infection rate of 100% was established with all miracidial doses. Morality of the snails was low in all experimental groups. In cercarial production high-(HP) and low-productive (LP) infections could be distinguished. An increase in miracidial dose at exposure results in an increase in the number of snails with HP-infections and decreases the length of the prepatent period of these infections. Snails with HP-infections grow faster than controls after day 14 post-exposure and show giant growth, while the accessory sex organs remain very small. The snails show hardly any ovipository activity. In all snails with HP-infections these effects are observed independent of the miracidial dose at exposure. Snails with LP-infections demonstrate no signs of gigantism, their accessory sex organs are almost identical to those of controls, and the snails show ovipository activity. It is concluded that the occurrence of HP- and LP-infections indicates differences in reproductive capacity of the parasite. HP-infections can only develop when the parasites can adjust the host to their requirements at an early stage of the infection.