Employing visual inspection and Magnetic Resonance Imaging to investigate Anisakis simplex s.l. infection in herring viscera

Anisakis spp. and Pseudoterranova spp. are anisakid nematodes with complex life cycles that utilise crustacean, fish and, finally, marine mammal hosts. Many species of marine fish are used by these larval anisakids as paratenic or intermediate hosts, and these are capable of infecting human consumers, causing public health, economic and social concerns worldwide. They are responsible for a fish-borne zoonosis called anisakidosis and associated allergic problems in humans, monetary loss to the fishing industry and the avoidance of fish products by consumers. An understanding of the parasite/fish relationship and the development of improved methods and tools for parasite detection are important to provide better insights to their biology, as well as technology to control the quality and safety of fishery products. In the present study, Anisakis simplex s.l. infection levels and location within the viscera of Atlantic herring (Clupea harengus) were visually investigated to provide a better understanding of settlement behaviour within the host fish, and to test if Magnetic Resonance Imaging (MRI) is a useful technology to detect anisakids in the viscera of whole herring. MRI potential to detect anisakids in fish muscle was also tested. Visual inspection determined an ascaridoid prevalence of 76% (>90% A. simplex s.l. (likely A. simplex s.s.) in herring viscera, with 69% of the parasites located outside the hind stomach and intestine areas. MRI demonstrated a capacity to detect A. simplex s.l. accumulations in the viscera of whole herring, A. simplex s.l. larvae and their movements within fish muscle, and Pseudoterranova sp. larva in fish muscle. Visual inspection of herring showed that A. simplex s.l. larvae frequently accumulate in the posterior end of the terminal blind sac of the stomach, around the ductus pneumaticus, as had previously been reported in herring and other clupeids. It has been hypothesized that the food digestion process, visceral organ topography and physicochemical conditions of fish tissues may play an important role in determining the encapsulation site and in vivo migration behaviours of A. simplex s.l. larvae within fish hosts. MRI showed potential to investigate the occurrence of anisakids and possibly other macroparasites of fish, and to detect anisakids in fishery products in situ, in a 3D environment and in a non-invasive and non-destructive way. Further investigation is required to determine if MRI can be used as a routine anisakid inspection tool by the fishing industry.