Oncorhynchus masou virus
Oncorhynchus masou virus (OMV) disease is a herpes viral disease of salmonid fish which causes oncogenic and skin ulcerative conditions in Asia. Though diseased fish exhibits tumors (basal cell carcinoma) around the head and jaw, the disease is not fatal.
Oncorhynchus masou virus, the causative virus of OMVD, is caused by the Salmonid herpesvirus 2 (SalHV-2). On the basis of antigenic studies conducted with neutralising polyclonal rabbit antisera, OMV differs from Salmonid herpesvirus 1 (SalHV-1), which is present in the western United States of America and is only weakly pathogenic.
The reservoirs of OMV are clinically infected fish and covert carriers among groups of cultured, feral or wild fish. Infectious virus is shed via faeces, urine, sexual products and probably skin mucus, while the kidney, spleen, liver and tumours are the sites where virus is the most abundant during the course of overt infection. The transmission of OMV is horizontal and possibly 'egg-surface associated'. Horizontal transmission may be direct or vectorial, water being the major abiotic factor. Animate vectors and inanimate objects also act in OMV transmission. Disinfection of the eggs just after fertilisation and eyed stage is effective in preventing OMV infection. OMV disease was not reported in alevins originating from disinfected eggs that had been incubated and hatched in virus-free water.
Clinically, the initial infection by OMV appears as a systemic and frequently lethal infection that is associated with oedema and haemorrhages. Virus multiplication in endothelial cells of blood capillaries, haematopoietic tissue and hepatocytes underlies the clinical signs. Four months after this first clinical condition, a varying number of surviving fish exhibit epithelioma occurring mainly around the mouth (upper and lower jaw) and, to a lesser extent, on the caudal fin, operculum and body surface. These neoplasia may persist for up to 1 year post-infection. In the case of coho salmon, 1-year-old infected fish in particular show ulcers on the skin, white spots on the liver and neoplastic tissues around the mouth parts or body surface. In rainbow trout, the diseased fish exhibit almost no external signs, although some fish manifest ulcerative lesions on the skin. Internally, intestinal haemorrhage and white spots on the liver are observed.
Following the septicaemia phase of OMV infection, an immune response takes place that results in the synthesis of neutralising antibodies to OMV. A carrier state frequently occurs that leads to virus shedding via the sexual products at the time of spawning.
Salmonids are the only fish species susceptible to OMV infection; the order of the fish species from the most to the least susceptible is: kokanee salmon, chum salmon, masou salmon, coho salmon and rainbow trout. The age of the fish is critical and 1-month-old alevins are the most susceptible target for virus infection. The main environmental factor favouring OMV infection is low water temperature (below 14°C).
The screening procedures for OMV are based on direct isolation of the virus in cell culture and co-culture of neoplastic tissues with salmonid cell lines. Confirmatory testing is by immunological identification using neutralisation or immunofluorescence tests, and virus-specific gene detection using polymerase chain reaction.
Oncorhynchus masou virus disease should be suspected in salmon with epithelial tumors, and in young salmonids that develop a systemic disease with a high mortality rate. Infected rainbow trout may have few signs of disease other than skin ulcers. The differential diagnosis includes infectious hematopoietic necrosis, whirling disease, viral hemorrhagic septicemia, infection with atypical Aeromonas salmonicida and erythrocyte inclusion body syndrome.
Oncorhynchus masou virus disease can be diagnosed by virus isolation in cell cultures; appropriate cell lines include RTG-2 (Rainbow trout gonad) and CHSE-214 (Chinook salmon embryo) cells. Infections can also be diagnosed by co-culturing neoplastic tissues with salmonid cell lines. The identity of the virus is confirmed by virus neutralization, immunofluorescence, enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) tests. Viral antigens can be identified directly in tissues by immunofluorescence or ELISA techniques. PCR can be used to detect nucleic acids in tissues. Serologic tests including virus neutralization, indirect immunofluorescence, and ELISA may be available, but these methods remain to be validated for routine diagnosis.
Control methods currently rely on the implementation of avoidance and hygiene practices in the operating of salmonid husbandry. The thorough disinfection of fertilised eggs and the incubation of these eggs and rearing of fry and alevins in premises completely separated from those harbouring virus carriers and free from contact with inanimate objects are the key measures needed to decrease contamination of OMV in a defined fish production site.
- Yoshimizu, M (2004) Viral diseases Infectious and parasitic diseases of fish and shellfish. (ed. by Wakabayashi, H. and K. Muroga), Koseisha Koseikaku, pp:29-128
- Furihata M, Suzuki K, Hosoe A, Miyazaki T. (2005) Histopathological study on Oncorhynchus masou virus disease (OMVD) of cultured rainbow trout in natural outbreaks, and artificial infection. Fish Pathol 40:161-168
- World Organization for Animal Health [OIE]. Manual of diagnostic tests for aquatic animals [online]. Paris: OIE; 2003. Oncorhynchus masou virus disease. Available at:. http://www.oie.int/eng/normes/fmanual/A_00020.htm. Accessed 13 Jul 2007
- Furihata M, Suzuki K, Hosoe A, Miyazaki T. Histopathological study on Oncorhynchus masou virus disease (OMVD) of cultured rainbow trout in natural outbreaks, and artificial infection. Fish Pathol. 2005; 40:161-168.
- World Organization for Animal Health [OIE]. Disease card: Oncorhynchus masou virus disease. OIE; 2000 Sept. Available at: http://www.oie.int/aac/eng/Publicat/Cardsenglish/2.1.03.%20OMV%20SEPT%202000.DOC. Accessed 23 Jul 2007
- Yoshimizu M, Yoshinaka T, Hatori S, Kasai H. (2005) Survivability of fish pathogenic viruses in environmental water, and inactivation of fish viruses. Bull Fish Res Agen 2:47-54.