Proterometra macrostoma (Digenea: Azygiidae): variations in cercarial morphology and physiology.

Snails, Elimia semicarinata, infected with Proterometra macrostoma were collected monthly in 1990 and 1991 from North Elkhorn Creek near Lexington, Kentucky, and kept on a 12:12 h light-dark cycle for 2 weeks. Cercariae emerging from snails were classified into 8 strains (I-VIII) based on differences in number and distribution of spined and spineless papillae on the tail. Cercariae also had unique patterns of emergence, swimming behaviour and infectivity in 4 species of sunfish. Of 513 infected snails collected in May, 339 had pure infections with the strain frequencies (% of 339): I, 46.6; II, 7.7; III, 12.1; IV, 8.8; V, 0.6; VI, 2.7; VII, 11.8; VIII, 9.7. In the multiple infections, 159 snails shed 2 strains, 14 shed 3, and 1 snail shed 4 strains simultaneously. A comparison of sunfish and parasite populations in Kentucky, Ohio and Michigan indicated that strain frequency in P. macrostoma is regulated by the species composition of the sunfish population.