Cause not known. Thought to be caused by a viral infection with a bacterial secondary infection, or subjection of the fish to extreme and sudden changes of temperature. Some fungal infections exhibit similar symptoms.
If viral, the outlook is not good. If bacterial, treatment is possible.
DNA and RNA diagnostic references
Transcriptome Analysis of the Gill and Swimbladder of Takifugu rubripes by RNA-Seq.
From 2014 Jan 16. doi:10.1371/journal.pone.0085505
Jun Cui et al.
Key Laboratory of Mariculture, N.China Sea, Min. of Ag. Dalian Ocean Univ, Dalian China
Note:- This dissertation is on a sp. of Puffer fish, a delicacy in Japan and Asia. Can find bo reference to DNA in swimbladder disease per se, however this article may serve as a useful basis for comparison if a researcher wishes to make a comparison.