Identification, Location, and Emergence of the Cercaria of Leuceruthrus micropteri (Trematoda: Azygiidae) Recovered from the Snail Pleurocera semicarinata (Gastropoda: Pleuroceridae) at North Elkhorn Creek, Kentucky

Abstract

We have encountered azygiid cercariae shed from the snail, Pleurocera semicarinata, at North Elkhorn Creek, Scott County, Kentucky, since 2004. Samples of these cercariae were frozen, and their DNA analyzed. The resulting alignment total length of 394 base pairs from partial 5.8S and 28S genes and the complete ribosomal internal transcribed spacer 2 (ITS2) confirmed our identification of the North Elkhorn Creek cercaria as Leuceruthrus micropteri. There was a 100% match to adult L. micropteri obtained from largemouth bass at Wheeler Reservoir, Alabama. Prevalence of infections by L. micropteri in snails was low at North Elkhorn Creek during 2004 (1.8% [7/400]) and 2017 (0.9% [4/432]). Quantitative data were obtained regarding cercarial emergence (daily and long-term patterns), site of infection of rediae, and functional morphology of the distome and tail stem. Most (96.1%) cercariae were released during the 12-hr light phase of a 12-hr light:12-hr dark cycle, and the average number of cercariae released per 7 snails per day over 21 d ranged between 0 and 2.3. Individual snails most frequently shed between 0 and 2 (range 0–6) cercariae per day. Snails shed cercariae for an average of 12.6 ± 3.1 d of the 21-d sampling period. Histology revealed rediae in the bottom whorl of the snail within the perintestinal sinus separated from the mantle cavity and gills by a thin mantle membrane. Envelopment of the distome body by the tail stem in vitro required less than 2 min. The lip of the tail chamber moved forward over the distome body until the latter was completely enclosed.