Nodavirus Infection in Freshwater Ornamental Fishes in India: Diagnostic Histopathology and Nested RT-PCR#

Abstract

Betanodaviruses are the causative agents of the disease known as viral nervous necrosis (VNN), or viral encephalopathy and retinopathy (VER) in a variety of cultured marine fishes. Approximately 40 species of marine finfish species are known to be affected with this disease with recent introduction in freshwater and aquarium fish as well. In the present study, we examined two species of freshwater aquarium fishes, viz. gold fish (Carassius auratus auratus), rainbow shark (Epalzeorhynchos frenatum) and its colour varieties (albino rainbow shark) reported to be affected with nervous system associated symptoms in a fresh water aquarium fish rearing centre near Chennai (India) by histopathology and nested RT-PCR. The only visible clinical symptom was the appearance of a congested brain which appeared as a translucent patch clearer to the naked eye through the transparent skin and cartilage of the young fry of albino forms and the low-grade morbidity among adult fish. Amplification of 430 and 280 bp by 2-step RT-PCR was achieved using primer sets which are known to detect all the genotypic variants of fish nodavirus in the Asian region. Histopathology of these fishes also revealed typical lesions of VNN in eye and brain tissues. No gill lesions or other abnormalities in the internal organs were detected at necropsy examination. To our knowledge, this is the first description of a natural infection of VNN in freshwater aquarium fishes in India causing morbidity and mortality.