Photographs by Dr. Panos Varvarigos

Nodavirosis (VNN, VER) of farmed sea bass and corb
(Piscine Neuropathy Nodavirus -PNNV)

Lesions and symptoms

Common demoralising scene of a sea bass fry cage during a VNN outbreak. Most dead and moribund fish are on the surface due to the inflated swim bladders. Arched fish, ataxia and violent bursts of activity, especially when the infected fish are disturbed, leave no doubt for the diagnosis of VNN. Within days the disease spreads to other cages.


Young caged bass fry with distended/hyper-inflated swim bladders due to VNN. All other organs were normal.

Close-up on a sea bass fry dying from VNN. Apart from the swim bladder hyperinflation, inflammatory lesions start to appear on the skin of the lower jaw and lower part of the operculum due to collisions with the nets.


Umbrina cirrosa suffering from VNN. The internal organs are normal, the alimentary tract empty, but the swim bladder is hyper-inflated. No external lesions are usually found since the clinical symptoms of this species are characterised more with lethargy rather than violent ataxic swimming.


60 days old sea bass juveniles suffering from VNN. 65% of the juveniles were lost prior to destroying the rest of the batch. The source of infection was the introduction of uncertified fertilised ova. Some fish were placed in a small glass aquarium for observation:

The fish seemed suspended head-down or were resting on their sides on the bottom of the aquarium. Some floated in the water column with the characteristic arching of the body (opisthotonos -muscle cramps). When the aquarium walls were tapped, some of these fish darted, spiralled or looped before resuming their lethargic posture.

Most of the juveniles were floating on the surface with visible hyperinflation of the swim bladder and brain congestion. Exophthalmia was not a common sign. The most frequent postures were arched body and head-down suspension in the water.

Seen from above, the swim bladder hyperinflation and the congested brain were clear to the naked eye through the transparent skin and cartilage of the young fry.


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Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.


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