Economically, Infectious Haematopoietic Necrosis (IHN) and Viral Haemorrhagic Septicaemia (VHS) of salmon are the most important Rhabdovirus infections in aquaculture. In Europe, the causative agents of both fish diseases cause severe economic losses in both, fish farms and open bodies of water. IHN and VHS are listed as non-exotic notifiable diseases in European legislation.
VHS epidemics have been observed in trout farms in Germany and Central Europe since 1937. The pathogen has been detected in America, Asia and in almost all European countries. There are indications that VHS originally was a disease of marine fishes and was transmitted to fresh water fishes by feeding fish waste as a consequence of intensified aquaculture. Meanwhile the pathogen has been isolated from more than 100 fresh and salt water fish species. Transmission of VHS by other fish species is also possible.
The IHN virus (IHNV) was first detected in salmon along the Pacific coast of North America in 1957. Subsequently, the pathogen continued to spread in Canada and in the United States. Due to intensified aquaculture the agent meanwhile has caused considerable losses in fresh water fish populations. In 1976, the virus was first isolated in Japan. A few years later IHN was reported in other Asian countries (Taiwan, China, and Korea) and in the European countries France, Italy, Switzerland, and Belgium. In Germany, IHNV was first detected in 1992. The spectrum of IHNV susceptible species mainly incudes Pacific salmon, Atlantic salmon and rainbow trout. Other fish species can transmit IHN.
Differentiation of the two diseases based on clinical and pathological/anatomical symptoms is not possible. The clinical picture of IHN and VHS can include: edema, haemorrhages, bloated abdomen, exophthalmus (bulging eyes). The disease may lead to increased mortality. Under natural conditions the clinical symptoms manifest at water temperatures of up to 14 °C. In peracute (sudden) cases of death these symptoms may be missing. The incubation period ranges from one to two weeks and depends on the age of the fish, water temperature, infection dose and virulence of the pathogen. Possible routes of transmission are direct transmission between fish or indirect transmission through equipment or humans as well as by fish-eating animals (e.g. cormorant, grey heron, otter).
IHN and VHS outbreaks are reported repeatedly every year. IHN and VHS viruses can be classified into the respective European genogroup (M for IHN virus and Ia for VHS virus). In Germany, the viruses most commonly are spread by trade with infected fish.
The NRL for IHN and VHS has been accredited pursuant to DIN EN Iso/IEC 17025 and coordinates the diagnosis of these fish diseases based on EU and national legislation. The FLI clarifies questionable findings from regional diagnostic laboratories pursuant to legal regulations, particularly in case of an IHN and/or VHS outbreak in a previously officially disease-free compartment or zone.
To support epidemiological investigations on IHN and/or VHS, virus isolates may be sent to the FLI reference laboratory for genetic characterization. In the frame of these analyses the complete sequence of the pathogens‘ glycoprotein gene is determined and compared with data available in international and in the FLI databases. These analyses permit conclusions with regard to the genetic relationship of the pathogens. The VHS and IHN virus databases of the FLI include data from public databases and from isolates characterized at the FLI. The databases comprise information on pathogens isolated worldwide since 1962 (VHSV) and 1987 (IHNV).