Particle selection by snail hosts of human schistosomes in relation to their survival strategies and possible control by 'environmental antibodies'.
Experiments were undertaken with Biomphalaria glabrata to elucidate the mechanisms used for selecting, regulating and utilizing sand particles and evaluate particulate controlled-release formulations (CRF), or 'environmental antibodies', to control target snails. The snails selected sand particles by size, mass and density. The numbers and mass of sand particles in the guts increased with snail size, and fed snails had significantly more sand particles in their guts than deprived snails. Snails benefited from ingesting sand particles. Increasing the weights of 63-125-µm sand particles from 8.5 mg to 150-170 mg resulted in progressive, significant, increases in the numbers of sand particles in the various gut compartments and also enhanced snail growth. Snails did not discriminate between sand and glass particles of the same size. When offered unwashed acidic sand there was a lag of 5-6 h before ingestion, but this could be reduced by preconditioning in media rich in base cations or chemical factors of snail origin. Under equilibrium conditions ingested sand constituted approximately 2.5% of the total snail weight but this amount was exceeded when snails were presented with large quantities of sand. The parts of the gut involved in sorting and regulating the turnover of sand particles and food were described. The gizzard, caecal and digestive gland strings traversed the gut as far as the rectum in approximately 2.8 h, 6-8 h and 8 h, respectively. Coprophagy by the snails facilitated the recycling of sand and its retention in the gut. Optimally sized, 63-250-µm, protein microcapsules containing butanoate and maltose were ingested by snails in significantly greater amounts than untreated control microcapsules. The addition of copper caused the capsules to become repellent. These results will advance the design of effective, ecologically acceptable, environmental antibodies in the form of controlled-release microcapsules to control schistosome snails. This work illustrates how a combination of detailed study of snail anatomy, feeding experiments and ecological understanding can contribute to the control of the intermediate hosts of a major human parasite.