Liver flukes can be found anywhere in areas with high rainfall and standing water. The ideal regions of the US are the Southeast and Northwest. The life cycle depends on an aquatic environment as that is where the snails (intermediate host) are active. There are many snail species, but only one species can act as the intermediate host, therefore limiting potential transmission. The snails come out of aestivation (dormancy) when the temperature and rainfall increase in the spring and go back into aestivation during early summer especially when conditions are hot and drier. Asexual reproduction in the snail results in a great many infective forms being available attached to forage that is now above the water level through the summer and into the fall.
The Moredun Research Institute (of Scotland) has prepared a video (6 min 53 sec) on the life cycle of the liver fluke (Fasciola hepatica).
All age groups of animals are susceptible to infection. The important things to remember are that both the flukes and the right snails have to be present. Without one there is no infection potential. As an example, for years the right snails were always present in the standing water areas on the Louisiana State University sheep farm, but the animals didn’t have any fluke infection. One year, six replacement ewes were brought in from a fluke area and infection became an issue. Infection was controlled by defluking with Valbazen® each fall (after any new infection has matured) for several years as Valbazen® is only effective against mature flukes.