Epidemiology
The two primary requirements for the establishment of liver fluke are a suitable snail (the intermediate host) and an environment that suits the fluke eggs, the snails and the larval fluke – such as springs, slow-moving streams with marshy banks, irrigation channels and seepages.
In Australia, the most important intermediate host is the indigenous freshwater snail, Lymnaea tomentosa. An introduced North American snail (L. columella) and an introduced snail from the Pacific area (L.viridis), found in defined locations of the NSW coast, have also been identified as additional intermediate hosts.
The fluke eggs are passed in the faeces into wet areas. Here they hatch, when mean temperatures increase to above 10°C (mostly from mid-September to May). In summer, the eggs take approximately 21 days to develop into miracidia; in the spring and autumn, hatching can take up to 90 days.
The larva (miracidium) invades the snail, where it develops and multiplies. One single miracidium hatching from a fluke egg can produce up to 4000 infective cysts (metacercariae). Actively swimming cercariae released from the snail attach to substrates, especially vegetation. The tail is shed and the cercaria forms a resistant cyst stage (metacercaria). In the presence of sufficient moisture the metacercariae will remain alive for many weeks, depending on the temperature. They survive longer at below 20°C; higher temperatures and desiccation will destroy the metacercariae in a short time.