Definite diagnosis of viral haemorrhagic septicaemia infected farmed rainbow trout and histopathologic study of acutely diseased fish in Iran

Viral haemorrhagic septicaemia (VHS) is one of the most important viral diseases in rainbow trout that has caused great losses to the Iranian rainbow trout aquaculture industry in few recent years. In the early autumn of 2015, in order to identify the possible causes of loss, farmed fish were sampled from the epicenter where massive mortalities and economic losses had occurred. Three samples were collected from relevant tissues (i.e. liver, kidney, spleen, heart, and brain) of the rainbow trout with clinical signs of VHS disease. The samples were stored in 70% ethanol, Eagle’s minimum essential medium (EMEM) and 10% formalin to perform RT-PCR, cell-culture and histopathology examinations, respectively. A virus was isolated using fish cell lines inoculated with a homogenate of the internal organs. Sequencing analysis of truncated N gene confirmed the existence of VHS virus. RT-PCR and histopathology tests indicated the suitability and simplicity of the test as a paraclinical diagnostic method for VHS virus in the region. Haemorrhage, necrosis and other changes in histological sections of the diseased fish demonstrated pathological changes identical with the typical VHS pathology in target organ of the virus (i.e. heart, spleen, liver and kidney). To our knowledge, this is one the first studies that fully focused on the identification of VHSV from farmed trout in Iran by using histopathology examinations.