Pathogenicity of Anguillid herpesvirus to Anguilla anguilla

Anguillid herpesvirus (AngHV) is an important viral pathogen of cultured Anguilla anguilla, and also an important factor in the decline of wild A. anguilla populations. Since the 1990s, A. anguilla culture industry has developed rapidly in China, accompanied by frequent outbreaks of an eel epidemic disease with symptoms of “skin mucus sloughing, red head, and septicemia”, which was named eel “mucus sloughing and hemorrhagic septicemia disease”. The disease is the most common and harmful infectious disease of the cultured juvenile A. anguilla, and occurs every year in the eel farms in Fujian Province, causing huge economic losses to the breeders. We detected and isolated AngHV from the diseased A. anguilla samples. In order to determine the pathogenicity of AngHV, an AngHV strain (NA16108) isolated from “mucus sloughing and hemorrhagic septicemia disease” samples was used to study its pathogenesis to juvenile Anguilla anguilla. 15-20 g body weight A. anguilla were intraperitoneally injected with 10⁶ PFU of NA16108, and clinical symptoms, histopathological changes, viral replications in the major organs and tissues, etc, were used to evaluate its pathogenicity. The results showed that the body surface of the AngHV-injected A. anguilla had the symptoms of mucus exfoliation, fin hemorrhages, red head, etc. The diseased A. anguilla had pathological changes of mucus increasing and hemorrhages of gill, discoloration of liver and swelling of spleen and kidney, etc; further histopathological analysis showed that the skin had exfoliation of mucus and mucosa flat epithelial cells; gill had secondary lamellar hyperplasia, blood vessel hemorrhages, swelling and necrosis of respiratory cells; spleen had necrosis and aggregation of melanocytes, and kidney had necrosis wall cells and narrowing lumen of renal tubular, which are consistent with the clinical characteristics of A. anguilla "mucus sloughing and hemorrhagic septicemia disease". The mortality analysis showed that AngHV infected A. anguilla began to die from the 4th day post injection, and the cumulative mortality on the 7th and 14th day reached up to 26.7% and 56.7%, respectively. However, the eels in the control group had no obvious pathological symptoms during the experimental period. Quantitative real-time PCR analysis showed that AngHV could be detected in the liver, spleen, kidney, intestine, gill and skin mucus of AngHV infected A. anguilla with higher copy number in the gill and skin mucus, and AngHV was not detected in the corresponding tissues of the control group. Viral isolation experiment showed that AngHV could be re-isolated by EO cell line from the main internal organs of the injected A. anguilla, indicating that the injected A. anguilla had a systemic infection of AngHV. In addition, we found that the quantity of Pseudodactylogyrus in the challenged group increased rapidly, reaching to about 200 and 800 per A. anguilla on the 7th and 14th day post viral injection, respectively, however, the quantity of Pseudodactylogyrus in the control group did not change significantly, and remained within 100 per A. anguilla. The results indicated that AngHV is the pathogenic agent of "mucus sloughing and hemorrhagic septicemia disease", which laid foundation for further study on the pathogenesis and prevention and control of the disease.