PATHOLOGICAL CHANGES AND ETIOLOGICAL DIAGNOSIS IN FARMED ADULT RAINBOW TROUT (ONCORHYNCHUS MYKISS) ASSOCIATED WITH A LOW MORTALITY

Infectious haematopoietic necrosis (IHN) is a severe infectious disease in a variety of salmonids such as sockeye salmon (Oncorhynchus nerka) and rainbow trout (Oncorhynchus mykiss), and leads to necrosis in the haematopoietic organs and tissues including kidney and spleen and causes up to 100% mortality in larvae salmons. However, it can also infect adult trout and cause lower mortality. In September 2014, an outbreak emerged in a trout farm in Chongzhou, Sichuan Province, with about 30% cumulative mortality in 2-years rainbow trout (Oncorhynchus mykiss). Gross findings included dark skin color, hemorrhagic spots in muscle and abdominal wall. Histopathology showed severe necrosis in the hematopoietic tissues including spleen and interstitial kidney. A multiplex RT-PCR method was conducted to detect IHNV, VHSV and IPNV of fish tissues and cell culture. Only a 371 bp segment was amplified which was confirmed as an IHNV gene through sequencing and phylogenetic tree. Furthermore, the obvious cytopathic effect (CPE) was observed in epithelioma papulosum cyprini (EPC) cells inoculated with tissue filtrates. And 20 rainbow trouts (1.5 kg mean weight) received a 0.5 mL i.p. injection of the tissue filtrate of 104 TCID50 per mL, and the cumulative mortality reached 35% within 10 days. No bacteria and parasites were detected in the case. All the results demonstrated that it was due to IHNV infection in spite of the low mortality.