Viability of Cryptocotyle lingua metacercariae from Atlantic cod (Gadus morhua) after exposure to freezing and heating in the temperature range from −80 °C to 100 °C

The presence of parasites in fish products is a problem that concerns consumers and authorities due to the potential hazards it may cause. The majority of studies on the viability of parasites in marine fish products currently focus on nematodes of the family Anisakidae whereas only few are concerned with trematodes. In this study on the heterophyid trematode Cryptocotyle lingua (identified by morphometric and molecular techniques) we isolated metacercariae from Atlantic cod (Gadus morhua) and incubated the parasites in cod muscle tissue at different temperatures ranging from −80 °C to 100 °C and subsequently tested their viability. SEM images were made to assess the physical damage caused to parasites exposed to different temperatures. Temperatures between 50 °C and 100 °C and between −80 °C and −20 °C killed the metacercariae present in fish flesh in less than 2 h. Controls kept at 5 °C survived for nine days. Extreme freezing temperatures caused minimal visual physical damage to cysts, but the tegument of metacercariae was severely affected at all temperatures when incubated for long periods.