Study On Histopathology In Cruciancarp Infected With CyHV-2

Crucian carp(Carassius auratus) is the most important economic freshwater species which has many variants including edible types and others with ornamental value. In the past few years, there has been continuous threat of diseases in carp aquaculture industry of which the disease caused by Cy HV-2 is the most potent. Cy HV-2 is a pathogen causing herpesviral hematopoietic necrosis which only infects goldfish, carp and its variants. When water temperature holds on the range of 15 to 25 ℃, the mortality of carp infected with Cy HV-2 is often near 100%. At present, the literature regarding Cy HV-2 concerns its morphology and taxonomic status, epidemic characteristics, molecular diagnostic techniques and the complete genome sequences of several Cy HV-2 strains. However, research on the histopathology of Cy HV-2 infected fish is limited and less detailed. To fully understand the histopathology of lesions caused by Cy HV-2 in different tissues of goldfish, carp and its variants, an experimental model of infection was developed which combines naturally infected wild fish to carry out a detailed analysis of the Cy HV-2 propagation and the histopathology of lesions.(1) In this research, the TK-gene of strain SY-C1 strain of Cy HV-2 was chosen to represent the equivalent of the complete genome sequence of the pathogen in order to develop a fluorogenic quantitative PCR method. The accurate quantitative range of the standard curve was 102 to 1011.(2) Combined with the naturally infected crucian carp,experimental infection with Cy HV-2 in crucian carp was performed to analyze the propagation of Cy HV-2. In the acute infection trial, fish death occurred at 3 d post-infection. The results of quantification showed that the viral load rapidly reached above 108 copies/μg DNA in spleen and kidney tissue. After this point, the viral load continued to increase until mortality declined. In the chronic infection trial, the results of quantification showed that the viral load of Cy HV-2 with chronic infection was maily under 106 copies/μg DNA and no mortality occurred during the experimental period. We concluded that when the viral load of Cy HV-2 exceedes 108 copies/μg DNA in spleen and kidney tissue, fish death occured easily. Data from the chronic infection trial and naturally infected Cy HV-2 specimens share the same distribution range indicating that these two different scenarios had similar conditions during infection. Comparing changing characteristics of the viral load in gill, spleen, kidney and brain tissue of these two different scenarios implied that Cy HV-2 was transmitted in fish via the blood circulatory system with gill and kidney tissue both highly sensitive towards Cy HV-2.(3) In this research, we systematically analyzed the histopathology of the Cy HV-2 infection of gill, spleen, kidney, liver, intestinal and brain tissue from different infection pathways. In the acute infection trial, pathological damage strength was serious in the gills, spleen, kidney and brain with necrosis as the main histopathological injury. The most typical characteristic was extensive necrotic plaque buildup which showed in the spleen tissue. In chronically infected and naturally infected fish, the histopathology changes were hyperplasia of the primary lamellae, curling of secondary lamellae and shedding of secondary lamellae epithelial cells in gill tissue. We also observed coagulative necrosis of parenchyma tissue and cell tropism changes of reticular hematopoietic cells ranging from weak basophilia to acidophilia made reticular structures with strong acidophilia in the spleen. The ratio of renal tubular epithelial cells with new nuclei rose along with inflammation, internal hemorrhaging, granulation, coagulation necrosis and granular degeneration of renal tubular epithelial cell in the kidney. In liver tissue, hepatic cytochrome metabolic disorder with lipofuscin accumulation occurred. In fish intestines, increased numbers of goblet cells along with secretion intensity were observed. Additionally, necrosis of neurocytes, neurogliocytes and Purkinje cells occurred in brain tissue along with breakage of nerve fibers. Combining the load of Cy HV-2 and histopathology of lesions caused by the virus, the chronic infection pattern was a reiteration of the natural infection as they showed high similarity. Histopathologcial lesions and propagation of Cy HV-2 implied the location of extensive necrosis plaque build-up in the spleen of acutely infected fish to be the same with naturally and chronically infected fish.In addition, using the kidney as the target tissue, cell sensitivity of Cy HV-2 had explored by in situ hybridization. The primary results showed there were a large amount of positive signals that appeared in kidney tissue from the Cy HV-2 infection with most signals distributed on renal tubular epithelial cells. Our results indicated Cy HV-2 is highly sensitive to renal tubular epithelial cells.