The pathology of the early stages of the crustacean parasite, Lernaeocera branchialis (L.), on Atlantic cod, Gadus morhua L.

Lernaeocera branchialis is a penellid copepod, the adults of which are commonly found on gadoids in the North Atlantic and North Sea. L. branchialis has a severe negative impact on wild gadoids and is a potentially serious pathogen of farmed cod, Gadus morhua. This paper describes an experimental infection by cohabiting the intermediate host, the flounder, Platichthys flesus, heavily infected with juvenile stages of L. branchialis, with hatchery-reared cod in order to study the pathology of the early stages of infection of the final host. Early stages of L. branchialis induce extensive local gill hyperplasia, large intravascular thrombus formation and a moderate cellular immune response in the cardiac and branchial tissues of G. morhua. The parasite appears to migrate within the lumen of the afferent branchial artery, ventral aorta and in turn, that of the bulbus arteriosus. These findings are consistent with the presence of a haematophagous, voluminous and rapidly invasive parasite.