What is it?
Red sea bream iridoviral disease (RSIVD) is a significant cause of mortality among cultured marine fish caused by the causative agent, red sea bream iridovirus (RSIV). Overt infections have been recognised not only in red sea bream, but also among other cultured marine fish including yellowtail, sea bass and Japanese parrotfish.
The first outbreak of an RSIVD was recorded in cultured red sea bream in Shikoku Island, Japan in 1990. Since 1991, the disease has produced mass mortalities in cultured fish populations in the western part of Japan, mainly among juvenile red sea bream. However, mortality of market-sized fish has also been reported.
Where and when might it occur?
Currently, RSIVD has only been reported from cultured marine fish in Japan, though a serologically and genetically related iridovirus has been isolated from brown-spotted grouper from Thailand. This latter isolate did not induce disease in experimentally exposed red sea bream.
The principal mode of transmission of RSIVD is by horizontal means via the water.
Diagnosis
Affected fish are lethargic, exhibit severe anaemia, petechiae of the gills and enlargement of the spleen. The disease is characterised by the appearance of enlarged cells stained deeply with Giemsa solution on microscopic observation of tissue sections of the spleen, heart, kidney, liver and gills of infected fish.
Control and treatment
Control methods currently rely on the implementation of hygiene practices at the farm. A commercial vaccine may soon be available for RSIVD in red sea bream. Vaccination for other marine fish species is at the experimental stage.
Source: OIE