Continuing the January report on the EAS workshop held in Verona last year
A WARNING of a new disease challenge, nodavirus, came from a paper by E.Sweetman and J.Sweetman, both of Ecomarine Ltd., Cephalonia, Greece, A. Le Breton, National Aquaculture Centre, Malta, and L. Grisez, Laboratory for Ecology and Aquaculture, Zoological Institue, Leuven, Belgium.
In July 1995, Ecomarine was contracted to investigate heavy mortalities in some age groups of sea-bass occurring at an ongrowing cage farm in Greece. The fish exhibited nervous symptoms and all previous diagnostic investigations had failed to identify the causative agent.
Classical histology revealed the presence of a vacuolating encephalopathy and retinopathy. Subsequently, a positive peroxydase reaction with the viral nervous necrosis (VNN) antiserum confirmed nodavirus as the causative pathogen.
Later in the summer, 11 other farms in Greece reported similar outbreaks and all were confirmed as being infected with nodavirus, using the same diagnostic technique. Sea-bream on the same sites showed no similar symptoms, but during the same period reports of outbreaks of nodavirus with heavy mortalities in sea-bass stocks were received from both Italy and Malta.
The appearance of viral encephalitis in sea-bass greatly concerns the industry and was the subject of a meeting of experts held by the DGXIV in the presence of DGVI at the European Commission in September 1995. At the Presidents' meeting of the Federation of European Aquaculture Producers (FEAP) on October 9 a resolution called for immediate action. A partnership of Ecomarine and FGM applied to the European Commission for a special action to study nodavirus infections and a six-month project was approved in December 1995.
In their report, the authors related the course of this study. Symptoms could appear, they explained, at day 10 post hatch, bu the usual onset of the disease was from day 25 to day 40. Deaths could reach 100 per cent of a batch of larvae, but were usually less for juveniles.
Infected larvae stopped feeding, failed to swim, and hyperinflation of the swimbladder was generally observed. Larvae gathered on the surface or floated suspended head down in the water column. Heavy daily mortality occurred for three to five days, resulting in more than 70 per cent of stock being lost.
Fish of 0.5-5g exhibited nervous symptoms and abnormal swimming, including uncoordinated darting, spiral or looping patterns, arched backs and listless behaviour.
A sub-acute form and an acute form of the disease were observed on all sizes of sea-bass from 10g to 600g. Mortalities ranged between 15 and 60 per cent of infected batches, depending on average weight and location. Deaths were also reported among broodstock fish with an average weight of 1.5kg.
The acute form developed at temperatures over 25°C, mainly with a display of nervous symptoms. Most victims were blind, some showing exophthalmia or a white cornea. Daily mortalities of up to 5 per cent were observed with total mortalities up to 60 per cent.
In the sub-acute form evolution of the disease was slower, with water temperatures of 20-25°C. Nervous symptoms were less severe, victims being mainly blind and with large, deep necrotic areas on the head and eyes. Daily mortalities rarely exceeded 0.5 per cent and total deaths of up to 15 per cent had been reported.
Some evidence supporting the possibility of vertical transmission of the disease was quoted in the report, other possible modes of transmission being influent water, juvenile fish held on the same site, carriage on utensils or vehicles. Horizontal transmission was suspected in many cases.
For the sea-bass Dicentrarchus labrax, effective methods of control and prevention will not be possible until the routes and modes of virus transmission are better understood, the reporters concluded. Strict hygiene measures involving routine disinfection of larval rearing areas and disinfection of inflow water using UV radiation or ozonization might help reduce the risk of infection by horizontal transmission. The development of vaccines and the use of antiviral agents remained speculative.
Exact assessment of losses in 1995 was still awaited, but if losses continued as reported, the economic impact on the sea-bass industry could be devastating. In 1996 there was a noticeable short supply of sea-bass juveniles, particularly in Greece.
F. Baudin Laurencin, of CNEVA, Brest, looking at future research work on nodavirus, also recommended sanitary measures as a first line of control, with measures favouring host resistance to the viral infection, vaccination and immunostimulation the subject of future work.
The 388-page A4 book of contributions and short communications to the conference entitled "Seabass and seabream culture: problems and prospects" is obtainable from the European Aquaculture Society (Slijkensesteenweg 4, B-8400 Oostende, Belgium. Tel: +32 59 32 38 59. Fax: +32 59 32 1005. e-mail: eas@unicall.be); the book costs 2,400 Belgian francs to EAS/WAS members, BF3,300 to others.
(from Fish Farmer International File, March/April 1997)
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