A study was made on the host-finding capacity of Fasciola hepatica miracidia in relation to time, number of miracidia per snail, and several physico-chemical environmental factors. Specimens of Lymnaea truncatula were exposed to radiolabelled miracidia and the subsequent snail-bound radioactivity was used to measure the host-finding capacity of the larvae.
Maximum snail-bound radioactivity was achieved after 45–60 minutes exposure to miracidia in a volume of 80 ml water. The efficiency of the snail-location was unaltered up to a volume of 4.5 liters. A linear proportionality was demonstrated between the number of miracidia and the amount of radioactivity obtained in exposed snails. After 4 hours exposure snails placed in darkness took up the same amount of radioactivity as snails placed in bright illumination. Additional experiments showed that miracidia preferentially scan the lighted zone of a given environment, but if no snails are available in this zone, snails in shade are also effectively located.
The host-finding capacity was unaltered up to a salinity level of 3.79%. At 4.74% and 5.68% the snail-location proceeded at a slower rate, but the final levels reached after 3 hours were comparable to that obtained at low salinity. The over-all host-finding capacity was reduced at a salinity level of 7.58% and at 9.47% the capacity had ceased. The host-finding capacity was unaltered in the pH range 5.4 to 8.4, but it was reduced at pH 8.9. Further studies showed that the host-finding capacity was clearly inhibited in water with a high turbidity level.