I suspect that the distension is caused by damage to the nerves regulating the function of the swim bladder. In advanced cases, lesions can be seen in ganglia in addition to brain, medulla and retina.  

Dear Hege,

Thanks for your comment.

As you know, swim bladder over-inflation/distention is a common finding in fish suffering VNN/VER caused by strains of piscine Nodavirus.

http://www.vetcare.gr/ARTPRES/pics_santiago_161_msw10.htm

Meanwhile, what do you think about Swim Bladder Inflammation (SBI) that can identifies by multiple etiological agents?

As you know, some Protozoan parasites have been associated with swim-bladder inflammation (SBI) in young carp. In other hand, Sphaerospora renicola (Myxosporea) infection could be considered as one of important causative agent of swim-bladder inflammation.

For more details please see site as follows:

http://www.parasitesandvectors.com/content/7/1/398

How can we use differential diagnosis methods in mentioned clinical sign (Swim bladder distention)?

Dr.Jalil,

If you have primer for detection of alpha or beta nodavirus, then all ambiguity could be solved, so try to have primers of VNN then have screening program, we can conclude it either VNN or Myxosporean infection

Dear Dr. Radkhah,

Thanks for your message, as dear Hege told before, distension is caused by damage to the nerves ( in the special location in the affected fish Brain) that regulating the function of the swim bladder could be main reason of the

swimbladder distension in VNN.

Am I right?

Thanks again