They were linked with high mortality rates in young fish and were initially described as a picornavirus-like virus.

[4][5][6][7] The oldest evidence related to the virus, however, dates back to 1984 in Queensland (Australia) where a disease outbreak with the clinical signs and histopathological lesions corresponding to NNV was reported.

Signs include: abnormal behaviour like lethargy, anorexia, spiral swimming; and change in pigmentation.

[11] Microscopical lesions are mostly located in brain, retina and spinal cord where necrosis of the neurons and the presence of round empty spaces called vacoules are commonly associated with the disease.

[citation needed] The most recent common ancestor of the four extant genotypes has been dated to ~1300 CE.