The output of cercariae of Fasciola hepatica by Lymnaea truncatula and the distribution of metacercariae on grass.

In many infection experiments attention has been focused mainly on the rate of the larval development of F. hepatica and the number of rediae and mature cercariae produced by infected L. truncatula (Kendall, 1949; Roberts, 1950; Boray, 1969). Very little consideration has been given to the actual numbers of cercariae which emerge from infected snails. Knowledge of the output of cercariae from infected snails is of importance in the design of control measures because the level of infection of domestic animals will be related to the number of freed cercariae which emerged from infected snails. When cercariae emerge from the body of the snail they encyst on herbage. The actual emergence and the behaviour of the newly emerged cercariae have been described in detail by Kendall & McCullough (1951). The survival of the metacercariae serves to maintain the life-cycle of F. hepatica during periods which are unsuitable for the production of cercariae. Shirai (1927), Ross & McKay (1929) and Boray & Enigk (1964) have all demonstrated that the survival of metacercariae depends on the environmental conditions, especially on the relative humidity and the temperature. Similar effects of temperature and relative humidity on the metacercariae of .F. gigantica have been reported by Alicata (1938). There seems, however, to be no information on the actual distribution of metacercariae on the herbage. The aim of the following experiments is twofold: (a) to determine the output of cercariae of F. hepatica from infected L. truncatula and (b) to investigate the distribution of the metacercariae (cercarial cysts) on grass and the possibility of vertical migration of the cercariae before cyst formation.