Aquatic animals have always been victims of many diseases of various forms, most of which were either non-infectious (tumors) or caused by common endemic pathogens such as virus, bacteria, fungi and parasites. In Pakistan, fish diseases are related to stocking density, aquaculture technology level, inputs to fish ponds, uncontrolled and unregulated fish transport, and poor management of fish exploitation. A total of 9 viruses lymphocystis, nodavirus, infectious pancreatic necrosis (IPN), infectious hematopoietic necrosis (IHN), viral hemorrhagic septicemia (VHS), spring viraemia of carp (SVC), catfish iridovirus, catfish herpervirus and eel herpervirus are reported to be present in the area for the years 1998, 1999 and 2000, although only 6 represent some threat.
Viral Haemorrhagic Septicaemia (VHS)
VHS is emerging disease caused by cold water rhinovirus. The virus belongs to the family Rhabdoviridae, genus Novirhabdovirus. The disease was first occurred in rainbow trout in Germany. It is responsible for severe economic losses in rainbow trout farms in Europe.
Symptoms
Fish may have pale gills, dark body color, and exophthalmos and in some cases intermittent periods of erratic spiraling swimming. Hemorrhage may be visible in the eyes and skin, within the muscle and internally in the viscera and intestine. In more chronic cases some of the above signs may be obvious with abdominal distension due to edema in visceral organs. VHSV was thought to have a predilection for salmonids, especially rainbow trout. Over the years, host range of the pathogen has expanded. For instance, Atlantic cod, pike, turbot, sea bass, Coho salmon, brown trout.
Transmissions
Horizontal and vertical transmissions of VHSV have been demonstrated. The virus is shed in the faeces, urine and sexual products. Once VHSV is established in a farm and, therefore, in the water catchment system, the disease becomes enzootic because of the virus carrier fish. Salmonid fries are the most susceptible fish species to the virus. Mortality rates can reach up to 80-90%. VHSV was first isolated in Trabzon, Turkey from cultured turbot fry and brood stock in 2004.
Spring Viraemia of Carp (SVC)
Spring viraemia of carp is an important disease affecting cyprinids and is reported to be present in several European countries. Although the common carp (Cyprinus carpio) is the main host of SVC virus, natural outbreaks of SVC were also found in other species of freshwater fishes. SVC is caused by a fish rhabdovirus (SVCV). The disease usually erupts in spring and causes mortality of the adults as well as the young stock. Viral outbreaks are often complicated with secondary bacterial infections.
Symptoms
The signs of disease are; lethargy, distended abdomen, petechiation on gills, skin and around eyes, oedematous vent and trailing mucoid casts, exophthalmia and internally ascites with focal hemorrhages in swim bladder and other visceral organs.
Transmissions
Transmission of SVCV is generally horizontal. Excretion of virus via faeces and urine from infected fish has been demonstrated. In addition, biological vectors such as leeches, carp louse (Argulus foliaceus) as well as fish-eating birds such as herons can play a role in the transmission of the disease.
Lymphocystis Disease (LD)
The LD of fishes is a well-known viral infection associated with hypertrophy of connective tissue cells presented as nodular skin lesions. LD has been described in many species of cultured and wild marine fish species such as seriola, flounder, gilthead seabream, red seabream, dab, sole. Taxonomically, lymphocystis virus (LV) belongs to the genus Lymphocystivirus within the Iridoviridae family. The LV is an icosahedral particle varying in size (from 215-240 nm across the apices) dependent on the fish host origin and contains double stranded DNA.
Symptoms
The LV induces chronic or slowly developing disease, often recurrent, with the highest incidence during the summer months. Although the disease causes very low mortalities, the external clinical signs of affected fish make them unmarketable. Infected fish have clusters of pear-like nodules up to 5 mm in diameter that develop on the skin, gills or fins resulting from an enlargement of tissue cells.
Transmissions
It can be transmitted by cohabitation and exposure to contaminated water. In addition, fish affected by LD are more susceptible to other bacterial, viral or parasitic infections. The best way to control the disease in the farms is to isolate the affected fish as early as possible to prevent cross-contamination and allow the lesions to heal.
Infectious Pancreatic Necrosis (IPN)
IPN is an acute contagious systemic disease. The causative agent is the IPN virus (IPNV), a bisegmented double-stranded, icosahedral, naked RNA virus which belongs to the genus Aquabirnavirus within the family Birnaviridae. Although this group was classically known for their high incidence in salmonids cultured in fresh water (where the viruses can cause 100% mortality in 1-4month old fish). Regarding the incidence of IPNV in the marine cultures, the virus has been associated with mortalities in Atlantic salmon in Norway and Scotland, especially during the first months after their transfer to seawater.
Symptoms
It is noteworthy that in these marine fishes, unlike in salmonids, the pancreatic tissue is not altered. In relation to the possible origin of the birnavirus in the marine cultures, it has been suggested that wild fish or invertebrates may be important vectors.
Transmissions
The virus can be vertically and horizontally transmitted. In addition, vertical transmission of the virus via the reproductive products has been demonstrated. Thus, the only way to get rid of the virus is the destruction of the stock
Nodavirosis (Viral Encephalopathy and Retinopathy; VER)
The VER disease, also called viral nervous necrosis (VNN), is caused by the group of piscine nodaviruses (genus Betanodavirus) and produces important economic losses in the larval culture of a great number of marine fish species (more than 20) such as seabass, striped jack, halibut and grouper.
Symptoms
The disease, as the name indicates, is characterised by a cell vacuolisation and neuronal degeneration in the central nervous system and the retina. Therefore, affected fish show a loss of equilibrium, failure of muscular control and visual dysfunction. The nodavirus is the smallest single-stranded RNA virus that affects fish. It is icosahedral, with a single coat protein and a bi-segmented genome. Although this disease was described for the first time in 1988. The disease causes percentages of mortality ranging from 50 to 100% in the larval stages. The fish susceptibility to the nodavirus is also dependent on temperature and strain.
Transmissions
Although the nodavirus can be transmitted horizontally by contact between diseased and healthy fish, the main transmission route is vertical, the broodstock being the reservoir of this virus, which is transmitted to the larvae through the fertilised eggs. The presumptive diagnosis of VER can be made on the basis of the light microscopic appearance of the brain, spinal cord and/or retina.
Infectious Hematopoietic Necrosis (IHN)
IHN occur in farms rearing rainbow trout in freshwater, both Pacific and Atlantic salmon cultured in freshwater or seawater may be severely affected. Large mortalities have also been recorded among some wild stocks of Pacific salmon. The etiological agent responsible for this disease is an enveloped single stranded RNA virus belonging.
Symptoms
The IHNV in susceptible fish causes viraemia with subsequent destruction of internal organs, impairment of osmotic balance, edema and haemorrhage. These clinical signs are a consequence of the high virus multiplication in endothelial cells of blood capillaries, haematopoietic tissues and nephron cells.
Transmissions
The transmission of IHNV between fish is primarily horizontal because the virus is shed via faeces, urine, sexual fluids and external mucus from both clinically infected fish and covert carriers. Vertical or egg-associated transmissions have also been demonstrated. This egg-associated transmission is significantly reduced by the common practice of surface disinfection of eggs with iodophors. The age of the fish appears to be extremely important to the susceptibility to the IHN: the older the fish, the more resistant to disease.