TREMATODE INFECTIONS IN A GASTROPOD HOST MISREPRESENTED BY OBSERVING SHED CERCARIAE

Cercarial emergence is often used to determine trematode infections in gastropods. Specimens of Ilyanassa obsoleta (Say) (n = 206) were examined for trematode infections by two methods, cercarial emergence and dissection. Dissection revealed five trematode species in 18 infection combinations. Cercarial emergence failed to reveal many infections and thereby severely underestimated parasitism. Percent failed determinations for each trematode species found were: Himasthla quissetensis (Miller and Northup)= 10.8; Lepocreadium setiferoides (Miller and Northup)= 30.6; Zoogonus rubellus (Olsson)= 43.8; Gynaecotyla adunca (Linton)= 80.2; and Austrobilharzia variglandis (Miller and Northup)= 64.7. Infections in snails harboring two and three trematode species were misrepresented in 80 and 93% of cases, respectively. Overall, cercarial release wrongly determined parasitism in 59.1% of infected snails examined. Thus, as a method, cercarial emergence severely underestimates parasite prevalence and overlooks many species that are present. © 1990.