IHN is notifiable in both Norway and the EU (Categories C+D+E), and any detection or suspicion of IHN must be reported immediately to the Norwegian Food Safety Authority.
Pathogen and Transmission
The disease is caused by Infectious Hematopoietic Necrosis Virus (IHNV), of the genus Novirhabdovirus in the Rhabdoviridae family. These are enveloped, single-stranded RNA viruses. Based on a specific region of the genome, the virus is classified into five genotypes: U, M, L, J, and E. The letters U, M, and L refer to the Upper, Middle, and Lower regions of the North American west coast where these genotypes were originally found. Genotype E in Europe and Genotype J in Japan both originated from virus strains introduced from North America.
Susceptible species include rainbow trout, Arctic char, and Atlantic salmon. The disease can also be experimentally induced in other species such as pike. Juvenile fish are generally more susceptible than older individuals.
IHN has historically been most problematic in intensive rainbow trout farming. Fry are most severely affected, but larger fish can also develop the disease. Most outbreaks occur during spring and autumn at relatively low water temperatures (8–15°C).
Clinical Signs
The most prominent clinical signs relate to impaired osmoregulation and blood circulation. This manifests as exophthalmia (bulging eyes) and accumulation of fluid in the abdominal cavity.
Diagnosis
Post-mortem examination often reveals small hemorrhages in the organs and swollen kidneys. Microscopic examination of tissue sections shows widespread necrosis in hematopoietic (blood-forming) tissues, changes that give the disease its name.
IHNV can be detected in tissues using specific antibody staining, as well as by cultivation in cell culture and molecular methods such as PCR.
If IHN is detected in Norway, samples will be forwarded to the EU Reference Laboratory for Fish and Crustacean Diseases (EURL Fish & Crustacean Diseases) in Denmark for confirmation.
Differential Diagnoses
Viral Hemorrhagic Septicemia (VHS), caused by a different rhabdovirus, and bacterial infections leading to septicemia are relevant differential diagnoses. Hemorrhagic Smolt Syndrome (HSS) may also resemble IHN in gross pathology.
Occurrence
IHN has never been detected in Norway.
Originally, IHN was a problem in the United States and Canada, particularly on the west coast. The disease has since been reported in countries that imported broodstock from these regions, including Japan, Chile, Italy, France, Germany, and Belgium. In November 2017, IHNV was detected for the first time in Finland.
In May 2021, Denmark reported IHNV in rainbow trout at a freshwater facility in Jutland. The fish showed no clinical signs. The virus was Genotype E, believed to have been introduced from Germany. Over the summer and early autumn, the virus was confirmed at a total of eight aquaculture sites and three recreational fishing lakes (“put & take” lakes). On December 10, 2021, Denmark informed the EU Commission that it was relinquishing its IHN-free status. During spring and summer 2022, the disease was detected at an additional ten aquaculture facilities in Denmark.
Surveillance
Norway has maintained IHN-free status since 1994. A surveillance program was established in 2004.
The program is currently risk-based and includes active sites with Atlantic salmon and rainbow trout. Samples submitted to the Norwegian Veterinary Institute for suspicion of or monitoring of other listed diseases, or for general disease diagnostics, are tested for IHNV using PCR. Pink salmon samples have been included in some years. Since 2021, samples from inland rainbow trout farms have been tested for IHNV. In 2022, brown trout from hatcheries and food production facilities were also included. That same year, samples from wild fish were inoculated into cell lines susceptible to IHNV, and some were tested by PCR based on high mortality or histological findings.
Surveillance Program for VHSV and IHNV
Control Measures
IHN is notifiable in both Norway and the EU (Categories C+D+E). If detected, all fish at the affected site must be culled (“stamping out”). In addition, a restriction zone covering nearby sites will be established.
There is currently no approved vaccine for IHN in Norway.
