EPIZOOTIC HAEMATOPOIETIC NECROSIS EHN is an infection caused by EHNV, a member of the genus Ranavirus in the family Iridoviridae with the type species Frog virus. It causes systemic clinical or subclinical infection in rainbow trout and redfin perch. The virus is currently restricted to Australia. INFECTION WITH BONAMIA EXITIOSA Bonamia exitiosa is a protistan parasite of the phylum Haplosporidia. It causes a lethal infection of the haemocytes of flat oysters sometimes accompanied by yellow discoloration and extensive lesions on the gills and mantle. However, most of the infected oysters appear normal. INFECTION WITH PERKINSUS MARINUS Perkinsus marinus is a pathogenic dinoflagellate of oysters. Perkinsus marinus causes disease of economic importance in Crassostrea virginica. Dead of gaping oysters are the main clinical signs; thin, watery tissue and pale digestive gland are the gross signs. However these signs are not specific to infection with Perkinsus marinus. Crassostrea gigas can be infected but do not develop the disease. The parasite has been detected in the East Coast of USA and Mexico. INFECTION WITH MICROCYTOS MACKINI Mikrocytos mackini is an eukaryotic protistan of uncertain taxonomy. M. mackini is a microcell responsible for the disease called Denman Island Disease, named after the location in British Columbia (Pacific coast of Canada) were it was first described in 1960. It has been experimentally demonstrated in laboratory and in the field that M. mackini can be highly infective for several other species of oysters: Crassostreae virginica, Ostrea edulis and Ostrea conchaphila. The parasite has been detected in Canadian west coast and adjacent areas of the State of Washington, USA. TAURA SYNDROME Taura Syndrome is due to a small icosahedral virus not enveloped. The agent is a RNAvirus classified in the Dicistroviridae family. Strain variation may cause difficulties in performing an accurate diagnosis. At least three genotypic variants were identified based on the sequence of VP1 structural protein. Taura syndrome is known as a nursery or post larval disease, occurring 14-40 days after stocking PLs. Infected shrimp are typically small juveniles from 0.05 to 5 g but larger juveniles and adults can be affected. Taura Syndrome is widely distributed in the shrimp-farming regions of the Americas and South-East Asia. YELLOWHEAD DISEASE Taxonomy of Yellowhead Virus (YHV) has been the subject of some controversy. Recent work has shown YHV to be a positive-sense single stranded RNA virus in the genus Okavirus in a new family (Roniviridae) of the order Nidovirales. It exists in at least three different genotypes. Yellowhead disease caused by YHV has been reported in the People’s Republic of China, India, Indonesia, Malaysia, the Philippines, Sri Lanka, Thailand and Vietnam. .