Seasonal occurrence of Cyathocotyle bushiensis Khan, 1962 (Digenea: Cyathocotylidae) metacercariae in the intermediate host Bithynia tentaculata L. (Gastropoda: Prosobranchia)

Abstract

Cyathocotyle bushiensis Khan, 1962 (Digenea) has been associated with late-summer duck mortality in southern Quebec. In an attempt to relate these die-offs with seasonal availability of the infective metacercariae to the ducks, two intermediate host (Bithynia tentaculata L.) populations were regularly sampled in 1984 and 1985. Prevalence varied between 5.3 and 57.3% at the South River, and between 9.1 and 49.7% at Lake St. Francis over the period of the study. Maximum abundances at the two localities were 1.38 and 1.66 metacercariae per snail, respectively. No close link between availability of metacercariae and periods of duck mortality was found, as the total prevalence and abundance of the cysts decreased through the summer, probably as a consequence of snail recruitment and low cercarial transmission rates, and increased in the autumn. Prevalence and abundance of metacercariae increased with snail age, estimated by both size and annual growth lines, and the metacercariae were found to be aggregated in the snail population. Comparison of infection levels reached at the end of the two sampling seasons by two year classes of B. tentaculata suggested that transmission of cercariae was higher in 1984 than in 1985. This was supported by the results of exposure of sentinel snails in an infected area during the 2 years of the study: transmission was detected in 1984 but not in 1985. In addition to C. bushiensis metacercariae, the sentinel snails acquired Sphaeridiotrema globulus (Rudolphi, 1814), another potential duck pathogen, in both years. The peak rate of transmission of S. globulus metacercariae occurred in August.