A formula for the mean infection success per miracidium and a method of proving the homogeneous susceptibility of snail populations to trematode infection

A formula for computing the mean infection success per miracidium (“ infection index”) is given for every permutation of the numbers of snails and miracidia in miracidial exposure experiments. This “ infection index” allows the comparison of the results from quantitatively different exposure methods and may be important for characterizing special trematode- intermediate host systems. By means of the “ infection index” it is shown that a laboratory population of Lymnaea stagnalis is homogeneous with regard to its susceptibility to Isthmiophora melis miracidia, i.e. that there is no appreciable rate of resistant individuals. Some experimental results which have been published by other authors and have been interpreted by them as indicating the resistance of a portion of snail populations can be numerically explained by means of the formula of the “ infection index” and without necessarily assuming resistance.