Effect of fish size on transmission of fish-borne trematodes (Heterophyidae) to common carps (Cyprinus carpio) and implications for intervention

Fish-borne trematodes are reported to affect the health of more than 40 million people worldwide. Few experimental studies are available on fish size dependent gain (attack rates of cercariae) or loss (mortality of metacercariae) of fish-borne trematodes. Aim was to quantify the relation between fish size and attack rates of fish-borne trematodes in common carps (Cyprinus carpio). Effect of fish size and cercariae dose were tested in a 3 × 4 factorial design with 5 fish per combination of treatments (n = 60). Individually kept small (1 g), medium (25 g) and large (45 g) carps were exposed to 0, 10, 50 or 250 parapleurolophocercous-cercariae (Heterophyidae) for 48 h. Fish were digested 21 days post exposure to count metacercariae. Percentages of fish containing metacercariae, and attack rates of cercariae to fish were higher (63%, 0.08 fish infected per cercariae) for small common carps than for medium (20%, 0.004 fish infected per cercariae) and large common carps (5%, 0.0007 fish infected per cercariae), but never zero. It was concluded that exposure of small fish is an important risk factor for transmission of fish-borne trematodes. The results suggest that control measurements aimed at reducing transmission to small fish may considerably reduce the absolute amount of fish-borne trematodes.