histopathological changes in maRINE FISH WITH NATURALLY OCCURING VIRAL INFECTION  

Aquaculture industry in Malaysia is expanding rapidly, producing approximately 526,507 tonnes with estimated wholesale value of RM 3 billion (DoF, 2011). Of this, about 25,374 tonnes of fish were from marine aquaculture cages, with estimated wholesale value of RM 558 million. However, intensification and commercialization of aquaculture industry lead to disease problems caused by bacteria, viruses, parasites and fungi (Bondad-Reantaso et al., 2005). This report describes the histopathological lesions associated with naturally occuring viral infection in fish kept in open-sea cage culture.

A marine fish farm practicing open-sea cage culture system with history of high rate of mortality was selected. A one-year epidemiology study was conducted between February 2014 and January 2015 to determine the prevalence of both bacterial and viral infections involving 3 main cultured species; tiger grouper (Epinephelus fuscoguttatus), Crimson snapper (Lutjanus sp.) and sea bass (Lates calcarifer). Sick fish were collected at monthly intervals for virology and histopathology examinations. Major clinical signs included exophthalmia and abnormal swimming pattern. Internal organs showed enlarged and congested kidney and spleen, and soft and watery brain. Laboratory results revealed either betanodavirus or iridovirus infection. Histological examinations revealed haemorrhagic lesions in the spleen, particularly in the red pulp (Fig.1) and necrosis of the kidneys due to iridovirus infection. Focal encephalitis involving mononuclear cells was often observed following betanodavirus infection (Fig. 2). In conclusion, iridovirus infection involved spleen and kidneys while betanodavirus involved brain tissue.