The experimental histopathology of Lernaea polymorpha Yu, 1938 infection in naive Aristichthys nobilis (Richardson) and a comparison with the lesion in naturally infected clinically resistant fish

Abstract. Bighead carp, Aristichthys nobilis (Richardson), fingerlings which had not been previously exposed to nauplii of Lernaea polymorpha were infected and sacrificed at various time intervals up to 21 days. Clinical observations showed that punctate haemorrhages developed following skin penetration at 13 days post-exposure. The adult female parasite was observed embedded in the skin and histopathology showed that the parasite penetrated the host tissue at an angle, sliding between the overlapping scales. It caused extensive tissue disruption, necrosis and haemorrhage along its path of entry, which was followed by an acute inflammatory response succeeded by a highly vascular chronic granulomatous fibrosis, whereby collagen fibres encapsulated the horns of the parasite. This contrasted with the predominantly haemorrhagic ulcerative lesions examined on immune fish subjected to natural infection, which were extensive but rarely accompanied by penetration through the dermis. There was a very extensive infiltrate of eosinophilic granular cells, club cells and lymphocytes within the spongiotic epidermis and the extensive haemorrhage was largely subepidermal.