Seasonal transmission of Fasciola hepatica in Montana, USA, with detection of infected intermediate hosts using a DNA-based assay.

A cashmere goat ranch in the Beaverhead Valley, southwest Montana, USA reported acute fasciolosis in March 1992. The ranch was used as a study site to gather seasonal transmission data for the liver fluke, Fasciola hepatica. Testing of snails for infection with a nucleic acid-based assay, and use of tracer sheep at the site has shown the seasonal transmission of the parasite is only in late autumn. The snail responsible for transmission was identified as Lymnaea modicella although another known intermediate host species, Lymnaea bulimoides, was found in one collection at the study site. Tracer sheep used over a 12-month period became infected with F. hepatica while on pasture during a period between September 10 and November 12, 1993. During the 28 months of study, 3072 individual lymnaeid snails were tested for infection. One sample of L. modicella containing 25 snails, collected in August 1994 contained liver fluke ribosomal RNA.