The larval stages of fluke (sporocysts, rediae) also survive in those snails for long periods, and resume development when climatic conditions improve.
The egg production of adult flukes is responsible for the degree of pasture contamination. Fluke survive for many years in the liver of infected sheep; the adult fluke lays between 20,000 and 50,000 eggs a day, and over a long period. In cattle, the egg production declines as the animal develops a natural resistance to chronic infections.
The epidemiology of the disease is influenced by the grazing habits of animals. Cattle often graze in the wet marshy areas favoured by the fluke snail, so the eggs are deposited in a suitable environment. If food is available elsewhere, sheep and goats prefer to graze away from marshy pastures. Long wet seasons are usually associated with a higher infection rate but sheep are more likely to ingest large numbers of cysts during dry periods after a wet season, when the animals are forced to graze in swampy areas, resulting in heavy infection