The causative agent of koi herpesvirus infection (KHV-I) - the koi herpesvirus (KHV) - occurs worldwide in areas where carp or koi are kept. Depending on the water temperature, clinically manifest forms of the disease occur in our latitudes, particularly in spring and summer.

The causative agent of viral haemorrhagic septicaemia is the VHS virus (VHSV). The disease affects salmonids of all age classes, but can lead to high losses of up to 90 %, particularly in juvenile fish, and is therefore associated with high economic losses in aquaculture.

Infectious haematopoietic necrosis (IHN) is a fish disease caused by the infectious haematopoietic necrosis virus (IHNV) that affects salmonids. The disease affects salmonids of all age classes, but can lead to high losses of up to 100 %, particularly in juvenile fish, and is therefore associated with high economic losses in aquaculture.

Epizootic haematopoietic necrosis (EHN ) is caused by the epizootic haematopoietic necrosis virus (EHNV). Perch and rainbow trout of all ages are affected. EHN has not yet been detected in Europe.

Infectious salmon anaemia, also known as infectious salmon anaemia (ISA), is caused by the infectious salmon anaemia virus (ISAV). ISA mainly affects Atlantic salmon and can lead to major economic losses on salmon farms.

Koi herpesvirus infection: worldwide

Viral haemorrhagic septicaemia: Europe, North America, Asia

Infectious hematopoietic necrosis: Europe, North America, Asia

Epizootic hematopoietic necrosis: endemic in Australia

Infectious salmonid anemia: Norway, Faroe Islands, Scotland, Canada, USA, and Chile.

Koi herpesvirus infection: Susceptible fish species are carp as well as the eponymous variegated carp (koi).

Viral hemorrhagic septicemia: Primarily fish of the salmonid family. However, the pathogen has been isolated in a variety of fish species

Infectious haematopoietic necrosis: Susceptible fish species are those of the salmonid family e.g. rainbow trout and brown trout.

Epizootic haematopoietic necrosis: Susceptible fish species are perch and rainbow trout. Experimentally, other fish species, such as silvereye, goblin carp, and Atlantic salmon, could be infected.

Infectious salmonid anemia: Atlantic salmon are especially susceptible. Rainbow trout and brown trout are other hosts. The virus has also been detected in brook trout, herring, chinook salmon and silver salmon

Koi herpesvirus infection: Transmission occurs horizontally from fish to fish as well as through water containing the virus. The virus enters the fish through the gills, intestinal tract and skin.

Viral haemorrhagic septicaemia: transmission is horizontal from fish to fish through virus-containing water as well as through contaminated equipment. The gills and gastrointestinal tract are the ports of entry through which the virus enters the fish.

Infectious haematopoietic necrosis: transmission is horizontal from fish to fish, through water containing the virus and through contaminated equipment. The virus enters the host through the gills, gastrointestinal tract, and through the skin at the bases of the fins

Epizootic hematopoietic necrosis: transmission occurs horizontally through virus-containing water.

Infectious salmonid anemia: Transmission occurs horizontally from fish to fish but also through contaminated equipment. The salmon louse is discussed as a vector. The virus enters the fish through the gills. Excretion occurs via urine, faeces and other body fluids.

Koi herpesvirus infection: 3 to 9 days at 20-24°C Viral haemorrhagic septicaemia: 1 to 2 weeks depending on fish species and water temperature Infectious haematopoietic necrosis: 5 to 45 days Epizootic haematopoietic necrosis: 3 to 32 days depending on fish species and water temperature Infectious salmonid anaemia: 2 to 4 weeks

Koi herpes virus infection: The severity of the disease symptoms depends on the water temperature. From 16 °C onwards, disease-related mortality occurs, which can be between 80 and 100 %. Symptoms include behavioural changes, excessive mucus production, enophthalmos and dyspnoea caused by gill necrosis.

Viral haemorrhagic septicaemia: symptoms depend on the course. Acutely affected fish show behavioural changes, dark colouration, exophthalmos, haemorrhages in the eyes and skin and pale gills.

Infectious haematopoietic necrosis: The clinical signs include lethargy alternating with hyperactivity, darkening of the skin, exophthalmos, haemorrhages in the skin, pale gills and distension of the abdominal cavity. Often, diseased animals excrete transparent filamentous feces

Epizootic hematopoietic necrosis: nonspecific disease symptoms include increased mortality, darkening of the skin, inappetence, loss of equilibrium, and lethargy, as well as hemorrhages at the base of the fins and skin erosions. In river perch, sudden deaths occur without further signs of disease

Infectious salmonid anemia: In Atlantic salmon, infection leads to lethargy, anemia, darkening of the skin, ascites, and exophthalmos. Often there is pale gills due to the decrease in red blood cells.

Koi herpesvirus infection: Infected fish with positive pathogen detection must be killed and destroyed. Ready-to-eat fish without clinical symptoms may be placed on the market under certain conditions. The main focus is on prevention.

Viral haemorrhagic septicaemia: Fish with evidence of the pathogen must be killed and destroyed. Edible fish without clinical symptoms may be placed on the market under certain conditions. The main focus is on prevention.

Infectious haematopoietic necrosis: Fish with evidence of the pathogen must be killed and destroyed. Edible fish without clinical symptoms may be placed on the market under certain conditions. The main focus is on prevention.

Epizootic haematopoietic necrosis: Diseased fish and fish with evidence of the pathogen must be killed and destroyed. Care must be taken to ensure thorough cleaning and disinfection, as this is a very resistant virus. The main focus is on preventing the introduction of the virus.

Infectious anaemia of salmonids: Infected fish must be killed and disposed of safely. The main focus is on preventing the introduction of the virus

Koi herpes virus infection: The greatest risk of introduction is posed by the import of ornamental carp. A stress quarantine with subsequent PCR test should be carried out for purchased fish. In order to prevent the spread into the pond industry, fish from private garden ponds should not be introduced into farm ponds or natural waters.

Preventive measures for all the fish diseases mentioned above include, above all, the prevention of introduction. Special caution is advised when trading fish as well as fish eggs. In particular, asymptomatic carrier fish and contaminated fish eggs play a major role in the spread of disease. In addition, attention should be paid to hygiene, including thorough cleaning and disinfection of equipment immediately after use.

Koi herpesvirus infection, viral haemorrhagic septicaemia, infectious haematopoietic necrosis: there are isolated outbreaks, see brief reports on animal health.

Epizootic haematopoietic necrosis and infectious anaemia of salmonids have not yet been detected in Austria.

Koi herpesvirus infection (KHV-I): KHV-I, also known as koi herpesvirus disease (KHVD), is caused by the koi herpesvirus (Cyprines herpesvirus-3 (CyHV-3)), an aquatic herpesvirus from the Alloherpesviridae family. A KHV infection can lead to symptoms of disease and high mortality in carp(Cyprinus carpio), koi and their hybrids. Other carp-like fish such as grass carp, goldfish, crucian carp and tench can become infected and excrete the virus, but do not become ill.

CyHV-3 has a low tenacity to common disinfectants and is inactivated by temperatures above 50 °C for one minute and UV radiation. As with most fish diseases, the severity of the symptoms of KHV infections also depends on the water temperature. Between 16 °C and 25 °C, disease symptoms develop and mortality is high (80-100 %). Water temperatures below 13 °C and above 30 °C cause clinically inapparent disease progression. So-called carrier fish - latently infected animals without symptoms of disease, in which the attempt to detect the pathogen is sometimes falsely negative - are responsible for maintaining the infection within a population and spreading it further. The virus can be reactivated by stress or changes in water temperature, which leads to virus excretion via the gills, skin and intestinal tract.

Viral haemorrhagic septicaemia is caused by the rhabdovirus VHSV, which belongs to the genus Novirhabdovirus and the family Rhabdoviridae. The virus affects a variety of fish species, but in our latitudes it is mainly rainbow trout and brown trout that are susceptible. The virus is unstable to alkalis and acids. The virus is stable in water at 4 °C for up to 35 days. Freezing VHSV-infected fish at standard freezing temperatures and subsequent thawing does not completely kill the virus.

The water temperature has a significant influence on the course of the disease. Between 10 and 15 °C, the acute form occurs with high losses. Below 4 °C and above 15 °C the disease is subclinical. In addition to the water temperature, other factors such as the fish species, condition and immune status of the fish as well as husbandry-related stress situations are also decisive for the course of the disease. The VHS virus is transmitted horizontally from fish to fish, but also via contaminated water, urine and faeces as well as equipment and people. The virus can also adhere to the egg surface and thus be spread further. For these reasons, it is important to ensure thorough hygiene measures, including the disinfection of equipment and fish eggs. An important factor in the spread of the pathogen is persistently infected carrier fish, which represent a virus reservoir once the infection has been overcome.

Infectious haematopoietic necrosis (IHN) is caused by the infectious haematopoietic necrosis virus (IHNV), which belongs to the genus Novirhabdovirus and the family Rhabdoviridae. There are various strains that differ in their virulence. Salmonids such as rainbow trout, brown trout, Atlantic salmon and various Pacific salmon species are susceptible.

IHNV is unstable to acids and heat and can be easily inactivated with common disinfectants and drying. The virus survives at cooler temperatures in freshwater for at least a month, especially if organic material is present. The water temperature has a significant influence on the course of the disease. Between 8 and 15 °C, the acute form occurs with high losses. In addition to the water temperature, husbandry-related stress situations such as housing density, transport or sorting can also favour outbreaks of the disease. The virus is transmitted horizontally from fish to fish, but also via contaminated water, urine and faeces as well as equipment and people. The virus can also adhere to the egg surface and thus be spread further. For these reasons, it is important to ensure thorough hygiene measures, including the disinfection of equipment and fish eggs. An important factor in the spread of the pathogen is persistently infected carrier fish, which represent a virus reservoir once the infection has been overcome.

Epizootic haematopoietic necrosis (EHN) is caused by the epizootic haematopoietic necrosis virus, which belongs to the genus Ranavirus and the family Iridoviridae. The susceptible fish species are rainbow trout and river perch. The virus is only endemic in Australia. EHN does not occur in Europe. However, there are closely related ranaviruses that cause the same clinical picture in catfish and turbot. These viruses include the European catfish virus (ECV) and the European sheatfish virus (ESV), which have caused outbreaks of disease in several European countries (Germany, France, Denmark). Disease outbreaks occur at water temperatures between 11 and 20 °C. EHNV is very resistant to drying and can survive in water for months. The virus can remain infectious in frozen fish tissue for more than 2 years and in frozen fish carcasses for at least 1 year. For these reasons, it is assumed that EHNV would persist on a fish farm for months to years in water and sediment as well as on plants and equipment. EHNV can be inactivated by 70% ethanol (2 hours exposure time), sodium hypochlorite and heating to 60 °C for 15 minutes. The transmission route of EHN is not fully understood. Experimentally, fish can be infected by immersion in water containing the virus. Oral infection is also possible. The virus is released into the environment when infected tissue and carcasses decompose.

Infectious salmonid anaemia (ISA) is caused by the infectious salmonid anaemia virus (ISAV), which belongs to the genus Isavirus and the Orthomyxoviridae family. A distinction is made between HPR-deleted and non-pathogenic (HPR0) variants of the virus. Only infections with HPR-deleted ISAV are covered by EU animal health legislation.

Disease outbreaks have been observed in both the farmed and wild forms of Atlantic salmon. Other salmonid species rarely become ill. Subclinical infections have been documented in brown trout, rainbow trout, brook trout, silver salmon, chinook salmon and also in non-salmonids such as herring and cod.

ISA viruses can survive outside their host for 48 hours. The pathogen can therefore be spread by equipment and personnel. The salmon louse is being discussed as a mechanical vector. Fish that have survived the disease excrete virus for at least a month. This occurs via mucus, urine, faeces and gonadal fluid. Subclinically infected carrier fish harbour the virus for up to 7 months. ISAV can be inactivated by UV radiation and ozone as well as at 56 °C for 30 minutes.

Symptoms

Koi herpesvirus infection (KHV-I): The most striking feature is the sudden high mortality rate, which can rise to 100% within a few days. The most important clinical symptoms include behavioural changes such as apathy and anorexia, excessive mucus production, bleeding at the base of the fins, enophthalmos and gill necrosis, which lead to respiratory symptoms. As the disease progresses, the skin's mucus production decreases, giving it a sandpaper-like texture.

Viral haemorrhagic septicaemia (VHS ): There are three different forms of VHS:
1) Acute form: Affected fish are apathetic and show anorexia. Animals stand on the edge ("marginal fish") and darkening of the skin occurs. In addition, exophthalmos, haemorrhages in the eye, in the skin and especially at the base of the fins can be observed.
2) Chronic form: is accompanied by pronounced anaemia.
3) Nervous form: affected fish begin to turn around their longitudinal axis.

Infectious haematopoietic necrosis (IHN): In a peracute course, infected fish, usually juvenile fish younger than 2 months, can die without symptoms. Symptoms include lethargy alternating with hyperactivity, anorexia, darkening of the skin, distended abdomen, exophthalmos, pale gills, haemorrhages in the skin and at the base of the fins, as well as in the yolk sacs of young fry. In addition, catarrhal enteritis results in the formation of stringy faeces/mucus cylinders (pseudofaeces). Surviving fish often develop scoliosis.

Epizootic haematopoietic necrosis (EHN): EHN leads to non-specific disease symptoms such as increased mortality, darkening of the skin, inappetence, loss of balance and lethargy as well as bleeding at the base of the fins and skin erosions. In perch, sudden deaths occur without any other signs of disease. In rainbow trout, outbreaks are linked to poor husbandry, particularly overcrowding, insufficient water flow and pollution. Husbandry-related skin lesions can be a portal of entry for EHNV. In this species, EHN can lead to enlargement of the abdomen as a result of ascites and enlargement of the kidneys and spleen. Overall, the disease-associated lesions and mortality are less pronounced in rainbow trout.

In the organs, an EHN infection in perch can lead to enlargement of the kidneys, spleen and liver as well as focal haemorrhages in the gills. In the liver, multiple white to yellowish discoloured areas with focal necrosis may develop. Morbidity and mortality are high in this fish species, which can lead to stock declines or losses in the event of outbreaks in natural waters.

ESV/ ECV can cause high morbidity and mortality rates in susceptible species. ESV outbreaks have been associated with mortality rates of up to 100 % in catfish.

Infectious salmonid anaemia (ISA): In the farmed form of Atlantic salmon, clinical signs include lethargy, anaemia, leucopenia, ascites, exophthalmos, darkening of the skin and increased mortality. The haematocrit can drop to 2-3 % but can also remain at a physiological level. The anaemia results in pale gills. The following pathological changes may occur in the internal organs: yellowish to bloody fluid accumulations in the body cavities, petechiae in the organs and in the visceral fatty tissue as well as in the muscle tissue. Blood stasis, enlargement and necrosis of the liver and enlargement and darkening of the kidneys may also occur.

Morbidity and mortality rates can vary greatly. At the beginning of an outbreak, often only a few fish are affected. Mortality at this stage is often only 0.5 to 1 %. As the outbreak progresses and is accompanied by an uncontrolled increase in viral load, mortality can increase either gradually or suddenly. Cumulative mortality varies from insignificant to moderate or severe. Infections with highly virulent viral strains can lead to mortality rates of more than 90 % within a few months.

Koi herpesvirus infection (KHV-I): The most commonly used method for detecting KHV from fish tissue is PCR. Additionally, cultivation by cell culture can be attempted. However, this method is more laborious and less sensitive than PCR.

Viral haemorrhagic septicaemia (VHS): Depending on the length of the fish, the whole fish (< 4 cm), the internal organs including kidney (4-6 cm) or heart, spleen, kidney and brain (> 6 cm) as well as ovarian fluid in farmed fish are used for virus isolation on cell culture. Genome detection by PCR is also commonly used. Sequencing of PCR products can provide important epidemiological data given the presence of different strains.

Infectious haematopoietic necrosis (IHN): Tissue from rainbow trout, or alternatively tissue from species listed as susceptible, should preferably be used for diagnosis. Depending on the length of the fish, the whole fish up to the anus (< 4 cm), the internal organs including the kidney (4-6 cm) or the spleen, anterior kidney and brain or heart (> 6 cm) are used for virus isolation on cell culture. Genome detection by PCR is also common.

Epizootic haematopoietic necrosis (EHN) and infectious anaemia of salmonids: Virus isolation in cell culture and genome detection by PCR are currently recommended as diagnostic procedures.