Transmission and control of Fish-borne Zoonotic Trematodes in aquaculture

Summary

Fish-borne Zoonotic Trematodes (FZTs) affect the health of millions of humans worldwide. For persistence, the life cycle of FZTs depends on aquatic snails, fish, and definitive hosts like humans, pigs or chickens. Definitive hosts can become infected by eating raw or undercooked fish. Integrated Agriculture Aquaculture (IAA) systems improve the livelihood of small scale farmers, but may enhance transmission of FZTs because all types of hosts and all transmission routes can be present on a single farm. This thesiscombines experiments, statistical analyses and mathematical modelling to gain insight into transmission mechanisms of FZTs to fish in aquaculture and to use this insight to compare and discuss control measures against FZTs. Currently, medication of humans is the main strategy to control FZTs. Modelling indicated that this does not lead to elimination of FZTs because both humans and definitive hosts other than humans will maintain the life cycle of FZTs independently. Treatment of (a part of) these host types may eliminate FZTs, e.g. treating all humans and 54% of definitive hosts other than humans. Aquaculture may provide opportunities for control of FZTs by adapting management measures. Experiments showed that smaller fish get more often and more heavily infected with FZTs than larger fish; common carp (Cyprinus carpio) of more than 50 g rarely acquire new infections. Once carp are infected, FZTs persist for at least 27 weeks, implying that harvestable fish still contain FZTs and, therefore, are a risk to human health. In most IAA systems, fish are kept FZT free until 0.5 g before being stocked into a fish pond where they are very likely to be exposed to FZTs. Stocking fish at more than 25 g, or at more than 14 g in combination with treating all humans with anthelmintics, may lead to elimination of FZTs. Also, control of snails by either decreasing density or increasing mortality of snails may lead to elimination of FZTs in aquaculture. Farmers and policy makers should evaluate which combination of control measures is attractive to them.