Infectious Hematopoietic Necrosis Virus

Infectious Hematopoietic Necrosis Disease (IHNV) is an acute fish infectious disease caused by the Infectious Hematopoietic Necrosis Virus (IHNV) of the Flavivirus genus. It mainly harms juvenile salmonids and has a high mortality rate. The virus is transmitted through water bodies, excreta, and the digestive tract. After infection, it presents symptoms such as necrosis of hematopoietic tissue, blackening of the body surface, internal bleeding, and spasms. The onset temperature is concentrated at 4-13 ℃. The virus was first discovered in Canada and the United States in the mid-20th century and is sensitive to high temperatures and acidic environments.The transmission is related to the migration of virus-infected fish and international trade activities, with fish eggs and adult fish as the main transmission vectors. The diagnostic methods include pathological observation, virus isolation and cultivation, and molecular testing. The prevention and control measures include quarantine management, disinfection of fish eggs with iodine preparations, temperature regulation to 17-20 ℃, and feeding of Chinese herbal medicine. Inactivated vaccines and attenuated vaccines have been applied to stage immunity.

Figure 1. Median-joining network based on the complete G gene sequence from 294 IHNV isolates.( Cieslak, Michael, et al.2017)

Pathological Changes

The main cause is severe necrosis of hematopoietic tissues in the kidneys and spleen. In severe cases, local necrosis also occurs in the renal tubules and liver, as well as degeneration and necrosis of granulosa cells in the gastric and intestinal lamina propria, and some of the pancreatic glandular and pancreatic islet cells. Inclusions are often seen in the cytoplasm. The white blood cells of diseased fish fry decrease, white blood cells and thrombocytes degenerate, and a considerable number of cells fragments appear. The levels of bicarbonate, bilirubin, calcium, chloride, osmotic pressure, and phosphorus in the blood all decrease. The activity of plasma LDH isoenzymes significantly increased. Abnormal regulation of plasma acidity and alkalinity, renal dysfunction leading to significant imbalance of electrolytes and water, may be the cause of fish death. Virus protein derived inclusion bodies are often observed in the cytoplasm and can be clearly observed and confirmed through specific immunostaining (such as immunohistochemistry) using anti infective hematopoietic necrosis virus (IHNV) polyclonal antibodies, enabling precise histological diagnosis of viral infection.

Classification

IHNV is the pathogen of fish infectious hematopoietic necrosis (IHN) disease, which is a negative sense single stranded RNA virus. IHNV is a member of the novel rhabdovirus genus, belonging to the rhabdovirus family. North American IHNV isolates were grouped based on partial glycoprotein (G) gene sequences. The three main IHNV genomes in North America are designated as U, M, and L, representing the upper, middle, and lower parts of the geographic range of IHNVs in North America. The Japanese and Korean isolates constitute a new genome of Japanese rainbow trout.

Vaccination

Significant progress has been made in vaccine development for IHNV, with several methods showing promise:

• DNA vaccine: A DNA vaccine targeting IHNV glycoprotein is available in Canada and the United States. This vaccine exhibits protective effects by inducing humoral and cellular immune responses.
• Inactivated vaccines: Traditional inactivated vaccines have been developed, but their protection period may be limited compared to new technologies.
• Attenuated live vaccine: The attenuated vaccine strain has been studied, but faces regulatory barriers due to safety issues.
• Recombinant vaccines: Virus vector vaccines expressing IHNV antigens (especially G proteins) are expected to become effective and safe alternatives.

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