RSIVD is a significant cause of mortality in farmed red sea bream (Pagrus major) and more than 30 other species of
cultured marine fish (15, 21) belonging mainly to the orders Perciformes and Pleuronectiformes. The first outbreak of
RSIVD was recorded in cultured red sea bream in Shikoku Island, Japan in 1990 (8). Since then, the disease has
caused mass mortalities in cultured fish populations in the western part of Japan, mainly among juvenile red sea
bream. Affected fish become lethargic, exhibit severe anaemia, petechiae of the gills, and enlargement of the spleen
(8, 12, 24). The disease is characterised by the appearance of enlarged cells stained deeply with Giemsa solution in
the histopathological observations of the spleen, heart, kidney, intestine and gill of infected fish (8).