Swelling of brain. Probable cause a viral infection. There are likely more than one such, that cause the condition. The nodovirus (DIEV) has been described as causing this condition in Dicenthrachus labrax (Seabass). However another virus (LcEV) has also been described as causing encephalitis in the same species. They are similar and both are closely related to the members of the Nodaviridae family.
The disease has also been reported in Tilapia larvae, and is described as a herpes-like virus. Partial analyses places this outbreak in the TLEV category. A primary morphological, biophysical and molecular characterisation of TLEV tends to confirm this assumption and places TLEV within the family of Herpesvirida.
Based on these reports above, it seems that Encephalitis in fish can be brought about by a number of viruses, and more detailed investigations will be needed.
DNA and RNA diagnostic references
Purification and characterization of two fish encephalitis viruses (FEV) infecting Lates calcarifer and Dicentrarchus labrax
From Aquaculture June 1994 M.Comps et al.