Heterophyid metacercariae in free living and farmed fish of El-Max Bay, West of Alexandria, Egypt

Abstract

BackgroundThe role of fish living freely in their natural habitats in the transmission of fish-borne trematodes is well recognized. Moreover, the role played by aquaculture fish has also gained great attention in the last few years.ObjectivesTo investigate the rate, density, distribution of infection, and infectivity of heterophyid metacercariae in free living and farmed fish collected from El-Max Bay, a Mediterranean coastal bay in Egypt. The influence of freezing duration on the infectivity of the detected metacercariae was also evaluated.Materials and methodsTilapia nilotica and Mugil cephalus from both habitats were examined for encysted heterophyid metacercariae using a compression method. The density of infection was estimated by the number of metacercariae per gram of trunk tissue following artificial digestion. The distribution of infection was studied in snips taken from the head, gills, trunk, viscera, and tail. Infectivity of the collected metacercariae was tested in rats. The susceptibility of metacercariae to freezing was evaluated by assessment of their infectivity to rats after they were kept frozen at −15°C for 4, 7, and 14 days.ResultsRates of infection with heterophyid metacercariae ranged from 11 to 23% in the different groups of fish. Free living fish had a significantly higher rate of infection and/or density as well as higher infectivity of metacercariae than farmed fish. Higher metacercarial density was observed in the trunk and viscera of the studied fish compared with the head, tail, and gills. Infectivity of the detected metacercariae decreased gradually with increasing duration of freezing.ConclusionBoth free living and farmed fish can transmit Heterophyes parasites, the former being somewhat more important. The potential risk of human infection is considered to be high. Freezing for 2 weeks is an effective means of inactivating the parasite. Our results underscore the need to raise awareness among public health agencies, consumers, and aquaculture managers of the measures needed to reduce transmission of this intestinal fluke.