The pathogen of the 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 occur in our latitudes, especially 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 100 %, especially in juvenile fish, and is thus associated with high economic losses in aquaculture. Infectious haematopoietic necrosis (IHN) is a fish disease caused by infectious haematopoietic necrosis virus (IHNV) that affects salmonids. The disease first appeared in North America, from where it spread to Europe and Asia through trade in fish and fish eggs. The disease affects salmonids of all ages, but can cause high losses of between 90 and 95%, particularly in juvenile fish, and is therefore associated with high economic losses in aquaculture. Epizootic Hematopoietic Necrosis (EHN) is caused by the Epizootic Hematopoietic Necrosis Virus (EHNV). It affects river perch and rainbow trout of all ages. EHN occurs mainly in Australia. To date, EHN has not been detected in Europe. Infectious salmon anemia, also known as infectious salmon anemia (ISA), is caused by the infectious salmon anemia virus (ISAV). The disease was first described in Norway in 1984 and has since been the most important fish disease in the salmon industry. ISA mainly affects Atlantic salmon and can cause major economic losses in 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; outbreaks have been documented in Pakistan, Peru, and Kuwait

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. Birds can act as mechanical vectors

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 herpes virus infection: Diseased fish and fish with evidence of the pathogen must be killed and disposed of harmlessly. For this reason, the main focus is on prevention.

Viral haemorrhagic septicaemia: Diseased fish and 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: diseased fish and 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. Attention must be paid to thorough cleaning and disinfection as this is a very resistant virus. The main focus is on the prevention of introduction

Infectious salmonid anaemia: Diseased fish must be killed and disposed of harmlessly. The main focus is on the prevention of introduction

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: In Austria, the first detection occurred in 2003. In 2019, an outbreak was documented.

Viral haemorrhagic septicaemia: VHS occurs in Austria. Most recently, several outbreaks occurred in 2019.

Infectious hematopoietic necrosis: IHN occurs in Austria. The last outbreak was documented in February 2021.

Epizootic haematopoietic ne crosis does not currently occur in Austria, and infectious salmonid anaemia has 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 of the family Alloherpesviridae. KHV infection can cause disease symptoms and high mortality in both 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 virus, but they do not become diseased.

CyHV-3 has 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 expression of disease symptoms is also dependent on water temperature in the case of KHV infections. 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 courses of the disease. So-called carrier fish - latently infected animals without disease symptoms, in which attempts to detect the pathogen are sometimes falsely negative - are responsible for maintaining the infection within a population and for its further spread. Stress or changes in water temperature can reactivate the virus, leading to virus excretion via the gills, skin and intestinal tract.

Viral haemorrhagic septicaemia is caused by rhabdovirus VHSV, which belongs to the genus Novirhabdovirus and the family Rhabdoviridae. The virus affects a variety of fish species, but in our latitudes rainbow trout and brown trout are most susceptible. The virus is labile to alkalis and acids. In water, the virus is stable at 4 °C for up to 35 days. Freezing VHSV-infected fish at commercial freezing temperatures and then 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 with high losses occurs. Below 4 °C and above 15 °C the disease occurs subclinically. In addition to the water temperature, other factors such as the fish species, condition and immune status of the fish as well as stress situations caused by husbandry 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 persons. The virus can also adhere to the egg surface and be spread further. For these reasons, it is important to follow thorough hygiene measures that include disinfection of equipment as well as fish eggs. Persistently infected carrier fish, which represent a reservoir of virus after infection, are an important factor in the further spread of the pathogen. Infectious haematopoietic necrosis (IHN) is caused by infectious haematopoietic necrosis virus (IHNV), which is classified in the genus Novirhabdovirus and the family Rhabdoviridae. There are several strains that differ in virulence. Susceptible are salmonids such as rainbow trout, brown trout as well as Atlantic salmon and various Pacific salmon species.

IHNV is labile to acids and heat and can be easily inactivated with common disinfectants and drying. The virus survives in cooler temperatures in freshwater for at least one month, especially if organic material is present. Water temperature has a significant influence on the course of the disease. Between 8 and 15 °C the acute form with high losses occurs. In addition to water temperature, stress situations caused by husbandry, such as housing density, transport or sorting, can also favour disease outbreaks. The virus is transmitted horizontally from fish to fish but also via contaminated water, urine and faeces as well as equipment and persons. The virus can also adhere to the egg surface and thus be spread further. For these reasons, it is important to observe thorough hygiene measures, which include disinfection of equipment as well as fish eggs. Persistently infected carrier fish, which represent a virus reservoir after infection, are an important factor for the further spread of the pathogen. Epizootic hematopoietic necrosis (EHN) is caused by the epizootic hematopoietic 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 endemic only in Australia. EHN is not found in Europe. However, there are closely related ranaviruses that cause the same disease pattern in catfish and turbot. These viruses include European catfish virus (ECV) and European sheatfish virus (ESV), which have caused disease outbreaks in several European countries (Germany, France, Denmark). Disease outbreaks occur at water temperatures between 11 and 20 °C. EHNV is highly resistant to desiccation 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 anticipated 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 route of transmission of EHN is not fully understood. Experimentally, fish can be infected by immersion in virus-containing water. Oral infection is also possible. The virus enters the environment when infected tissues and carcasses decay. Infectious salmonid anaemia (ISA) is caused by the infectious salmonid anaemia virus (ISAV), which belongs to the genus Isavirus and the family Orthomyxoviridae. 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, but also in non-salmonids such as herring and cod.

ISA viruses are capable of surviving outside their host for 48 hours. Therefore, the pathogen can be spread by equipment and personnel. The salmon louse is discussed as a mechanical vector. Fish that have survived the disease excrete virus for at least a month. This occurs through mucus, urine, feces, and gonadal fluid. Subclinically diseased carrier fish harbor the virus for up to 7 months. ISAV can be inactivated by UV radiation and ozone and at 56 °C for 30 minutes.

Symptomatology

Koi herpesvirus infection (KHV-I): The most striking feature is the sudden high mortality, 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 leading to respiratory symptoms. As the disease progresses, the mucus production of the skin decreases, giving it a sandpaper-like texture.

Viral haemorrhagic septicaemia (VHS): There are three different forms of VHS: 1) Acute form: Diseased fish are apathetic and show anorexia. Animals are standing at the edge ("marginals") and there is darkening of the skin. Furthermore, exophthalmos, bleeding in the eye, in the skin and especially at the base of the fins can be observed. 2) Chronic course: accompanied by pronounced anaemia. 3) Nervous form of progression: diseased fish begin to turn around the longitudinal axis.

Infectious Hematopoietic Necrosis (IHN): In peracute course, infected fish, mostly juvenile fish younger than 2 months, may die without symptoms. Symptoms include lethargy alternating with hyperactivity, anorexia, darkening of the skin, distended abdomen, exophthalmos, pale gills, haemorrhages in the skin and fin attachments, and in the yolk sacs of young fry. Furthermore, catarrhal enteritis results in the formation of stringy feces/mucus cylinders (pseudofaeces). Surviving fish often develop scoliosis.

Epizootic Hematopoietic Necrosis (EHN): EHN results in nonspecific disease symptoms such as increased mortality, darkening of the skin, inappetence, loss of equilibrium, and lethargy, as well as hemorrhage at the base of the fins and skin erosions. In river perch, sudden deaths occur without other signs of disease. In rainbow trout, outbreaks are related to poor husbandry, especially overcrowding, inadequate water flow, and pollution. Skin lesions caused by husbandry can be a portal of entry for EHNV. In this fish species, EHN can lead to enlargement of the abdomen as a result of ascites and enlargement of the kidney and spleen. Overall, disease-associated lesions and mortality are less in rainbow trout.

On the organ side, EHN infection in rainbow trout can result in enlargement of the kidney, spleen, and liver, as well as focal hemorrhages in the gills. In the liver, multiple white to yellowish discolored areas with focal necrosis may develop. Morbidity and mortality are high in this fish species, which can lead to population declines or losses during outbreaks in natural waters.

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

Infectious Salmonid Anemia (ISA):In the farmed form of Atlantic salmon, clinical signs include lethargy, anemia, leukopenia, ascites, exophthalmos, darkening of the skin, and increased mortality. The hematocrit may drop to as low as 2-3% but may also remain at a physiological level. As a result of anemia, there are pale gills. The following pathological changes may occur in the internal organs: yellowish to bloody fluid accumulation in the body cavities, petechiae in the organs and visceral fatty tissue and muscle tissue. Furthermore, blood stasis, enlargement and necrosis of the liver, and enlargement and darkening of the kidney may occur.

Morbidity and mortality rates can vary widely. At the beginning of an outbreak, often only a few fish are affected. Mortality at this time is often as low as 0.5 to 1%. As the duration of the outbreak progresses, accompanied by an uncontrolled increase in viral load, mortality may 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.