Discussion of the relationships betweenSchistosoma and their intermediate hosts, assessment of the degree of host-parasite compatibility and evaluation of schistosome taxonomy

Abstract

Studies of the spectrum of the intermediate hosts of schistosomes and the relationships between parasites and intermediate host snails have been carried out with each researcher using different methods. A comparison between compatibility studies has therefore been impossible. The following three basic problems in these kind of studies are discussed in this paper: (1) the standardization of experimental materials such as schistosomes, intermediate and final hosts, (2) the methods involved in the experiments, and (3) the sort of data which must be collected. Standardized methods and materials are described in this paper. The relationships between the genusSchistosoma and the intermediate host snails belonging to the two generaBiomphalaria andBulinus, and the mechanisms behind the process resulting in various degrees of compatibility between the two involved organisms are reviewed and discussed. Why certain combinations of schistosomes and snails result in the production of cercariae and others do not is still unknown, but there is now some indication of a camouflage behaviour during the intramolluscan stages, e.g., the larvae in the snails cover themselves with snail material and are not recognized as foreign objects. The same mechanism has already been found in the schistosomules in the final host. Much of the confusion in the discussion of the relationships between schistosomes and intermediate hosts is the result of a lack of an objective way to describe and assess the compatibility between the two. Based on a series of experiments withS. haematobium, S. intercalatum, S. bovis, S. mansoni, and several species of possible intermediate hosts, the following index for estimating the degree of compatibility is proposed: the total cercarial production from 100 exposed snails (TCP/100 exposed snails). Seven classes have been suggested: the first one (Class 0) with a TCP/100 exposed snails of zero, is called ‘refractory’; the next one (Class I) called ‘not very compatible’ has a TCP/100 exposed snails between 1 and 10,000; the last group (Class IV) called ‘extremely compatible’ produced more than 500,001 per 100 exposed snails during the entire lifespan. The ability to use a spectrum of intermediate hosts and the compatibility in the taxonomy of schistosomes is discussed. The results for the different species of schistosomes indicated that each species consists of strains with ‘hybrid populations’ in between.