Fish Diseases and Management

5.3.7 RNA viral diseases

 Infectious pancreatic necrosis (IPN) : The disease is caused by a highly contagious virus, infectious pancreatic necrosis virus (IPNV) belonging to the Birnaviridae. It affects both warmwater and coldwater fishes of freshwater and marine environment. IPNV enjoys a wide geographical distribution, reported from America, Europe and Asia. IPN is characterized by sudden mortality with a progressive increase in severity. Cumulative mortalities may vary from less than 10% to more than 90%. The virus is also isolated from ornamental fishes in which it causes severe mortality. Affected fish die very quickly. In outbreaks fish lose appetite, become lethargic and eventually disorientated. Some become ascites and haemorrhages at the base of the fins are common. Cork screwing/spiral swimming motion, darkening pigmentation, a pronounced distended abdomen, pop-eye and/or pale faecal casts are also seen in IPN infected fishes. In this infection, survivors of the disease become carriers of the virus for the rest of the life. The IPN is transmitted both horizontally by viral uptake across the gills and by ingestion and vertically via the egg. Horizontal transmission is  The virus shows strong survival in open water conditions and can survive a wide range of environmental parameters. Prevention methods include avoidance of fertilised eggs from IPNV carrier broodstock, surface disinfection of eggs to reduce transmission rate, vaccination etc. Control of losses during outbreaks involves reducing stocking densities and dropping water temperatures (if possible).

Viral nervous necrosis : The viral nervous necrosis (VNN) has caused havoc internationally in both European and Asian Seabass populations. To date, the disease has been reported in at least 30 fish species, including freshwater ornamental fishes. The disease has typical characteristics of neurological abnormalities with clinical symptoms like abnormal swimming and whirling leading to mass mortalities of the young ones. The lesions of the disease develop in brain and retina. The common clinical signs noticed in the disease include abnormal swimming behaviour, pale colour, anorexia and swim bladder hyperinflation. Histologically, the larvae are characterised by vacuolation of cells of the nervous system, retinal layers and brain. Intracytoplasmic inclusions in brain cells are also noticed.

Spring viraemia of carp : This disease is one of the most important disease of cultured and ornamental fishes.  It is caused by spring viraemia of carp virus (SVCV), coming under the rhabdovirus group. This group of virus share many similarities with the swim bladder inflammation virus (SBIV) and both the viruses are mostly indistinguishable. Clinically the infection is characterized by lethargy, darkening of the skin, exophthalmia, petechial haemorrhages of the skin and gills, inflamed vent, trailing mucoid faecal casts and loss of balance. Internal symptoms include viral septicaemia, oedema and necrosis in liver, pancreas, kidney, heart, brain, intestine and swimbladder, visceral haemorrhages and ascitic fluid in abdominal cavity. Swimbladder inflammation leads to the loss of balance and the fish often vertically hang in the water. The disease outbreaks are often associated with increasing water temperatures, and survivors become carriers of latent infection. During periods of stress, virus is shed with mucus and faeces and enters the uninfected fish through the gills that are considered the site of entry. As the SVCV is also transmitted by vectors, good husbandry practices are essential to keep the infection under check.

Viral haemorrhagic septicaemia: Viral haemorrhagic septicaemia(VHS) virus is an RNA virus belonging to the rhabdovirus genus and is related to the viruses causing infectious haematopoietic necrosis and spring viraemia of carp (SVC). Natural outbreaks of VHS occur in marine reared rainbow trout and turbot. Other fishes like sea bass and sea bream are susceptible to the virus and VHSV has been isolated from Atlantic herring, sprat, dab and Atlantic and Pacific cod in Alaska. In freshwater, rainbow and brown trout also get infected by VHSV. In acute infection, clinical signs of the disease include darkening of body color, pop eye (exophthalmia), bleeding around the eyes, pectoral and pelvic fins, pale gills with pin point hemorrhage.  Internally, petechial hemorrhages on the surfaces of fatty tissue, intestine, liver, swim bladder and in the muscle, presence of fluid in the abdominal cavity are the signs noticed. In subacute or chronic stage, some of the above signs are noticed with severe anaemia. Mortality is variable depending on the serotype ranging from 10 to 80%. The virus is transmitted through the water, from infected carrier fish, birds, equipment, transport, water or by some blood sucking parasites.

Infectious hematopoietic necrosis: IHN is caused by a RNA virus, which causes large scale mortality. During outbreaks, fish are typically lethargic with bouts of frenzied, abnormal activity such as spiral swimming and flashing. Affected fish exhibit darkening of the skin, pale gills, ascites, distended abdomen, exophthalmia and petechial haemorrhages internally and externally. A trailing faecal cast is observed in some species. Spinal deformities are present among some of the surviving fish. Internally, fish appear anaemic and lack food in the gut. Degenerative necrosis in haematopoietic tissues, kidney, spleen, liver, pancreas and digestive tract. The cellular debris, termed necrobiotic bodies, can be seen in stained tissue imprints from the anterior kidney and has diagnostic value.

Infectious salmon anaemia: ISA is caused by an enveloped RNA virus, which buds from plasma membranes after replication. The disease is observed in Atlantic salmon. Experimentally, ISA has been transferred to sea trout and rainbow trout. Clinical signs of ISA are lethargy, haemorrhagic eyes, pale gills and a distended abdomen filled with fluid. The liver shows focal necrosis, petechial haemorrhage. Haemorrhage may occur in the stomach wall and the organ becomes filled with a serous or viscous fluid. The liver may be yellowish or pale with haemorrhage. Histological examination shows haemorrhagic, focal and diffuse liver necrosis. Anaemia is accompanied by leucopenia. Mortality up to 90% has been recorded. Only horizontal transmission has been recorded. Infected live salmon, and infected biological waste from slaughter,  processing plants and water are the main source of infection. Sea lice may act as vectors. Towards control, as certain management practices that reduce exposure to biological material. Avoid locations close to infected farms or close to slaughter and processing plants. Frequent removal of dead fish from net pens also reduces the risk of ISA outbreaks.

Salmon pancreas disease (Pancreas disease): The virus responsible for the disease is identified as a togavirus and named salmon pancreas disease virus (SPDV). Pancreas disease (PD) is a subacute to chronic disease of farmed Atlantic salmon post molts and growers. PD has been found in Europe and rainbow and brown trout are also susceptible. Clinical signs include reduced feeding response and slow movement near the water surface. As the disease progresses the fish stop feeding, become emaciated and frequently show a loss of balance. Absence of fat body, localised pinpoint haemorrhage and an empty gut are also noticed. Mortality of about 50% is noticed. Pancreas disease is diagnosed by microscopical examination of stained tissue sections which shows areas of necrosis and fibrosis with total loss of the acinar cells of the exocrine pancreas. Pancreas disease has also been linked with the disease known as ‘sleeping disease’ (SD) in freshwater rainbow trout. PD has been experimentally transmitted by cohabitation and infective injection of kidney homogenate.