Higher attack rate of fish-borne trematodes (Heterophyidae) in common carp fingerlings (Cyprinus carpio) at lower fish weight.

Fish-borne zoonotic trematodes (FZTs) can cause pathology in humans. Fish weight was reported as important risk factor for transmission from snail to fish. However, in fingerlings, the relation between fish weight and infection is unknown. Aim was quantifying the effect of fish weight on infection probability, attack rate, and metacercariae burden of FZTs in common carps (Cyprinus carpio) between 1 and 20 g. Fish were either used as controls (n = 66) or exposed to 250 parapleurolophocercous cercariae (n = 254). Fish weight was analysed as continuous explanatory variable or classified in four categories with average weights of 0.7 g (n = 116), 4.0 g (n = 58), 8.2 g (n = 57) and 14.2 g (n = 23). The inverse relation between percentage of fish with metacercariae and fish weight is reflected in lower percentages of infected fish at higher weights [%infected = 100/(1 + e([-2.02+0.15 fish weight (g)])); p < 0.01], i.e. 89 %, 85 %, 63 % and 61 %, respectively, in the four groups. Control fish did not get infected. Attack rates were 0.0087, 0.0073, 0.0040 and 0.0033 fish infected per cercariae, respectively; the first two attack rates being significantly higher than the latter two. Mean number of metacercariae per weight group was 5, 5, 2 and 1, respectively, (p < 0.05), with an inverse relation using weight as continuous explanatory variable [p < 0.01; number metacercariae = e(1.76-0.13fish weight(g))]. Concluding, an inverse relation exists between fish weight and probability of infection, attack rate and parasite burden in common carp fingerlings. Reducing transmission to fingerlings might be an effective intervention method to improve food safety, reduce the absolute amount of FZTs in the environment and eventually reduce incidence in humans.