بيماري نكروز عصبي ويروسي (VNN) در آبزيان: گذشته، حال، آينده

Viral Nervous Necrosis (VNN) Disease in Aquatic animals: Past, Present and Future

بيماري نكروز عصبي ويروسي يك بيماري مهلك به ويژه درماهيان با ارزش اقتصادي است. اين بيماري بيش از چهل گونه از ماهيان را تحت تاثير قرار مي دهد و حدود هفتاد گونه نيز به صورت حامل، فاقد علايم باليني ولي داراي ويروس عامل اين بيماري هستند. ماهيان مبتلا علايم باليني از قبيل شناي مارپيچي، شناي نيزه اي، كم اشتهايي، تغيير در رنگدانه ها، تورم كيسه شنا، برگشتن بر روي پشت وشناي خوابيده بر پشت و خونريزي هاي زيرجلدي را نشان مي دهند. در مطالعات ميكروسكوپيك علايم اصلي يعني واكويولاسيون مغز، چشم و نخاع مشاهده مي گردد در واقع سيستم عصبي مركزي هدف اصلي اين ويروس است. عامل ايجاد كننده اين بيماري ويروسي از نوع بتانوداويروس و از خانواده نوداويريده مي باشد. اين خانواده شامل دو جنس است، جنس آلفانوداويروس كه نوداويروس خاص حشرات مي باشد و در حشرات ايجاد بيماري مي كند. آلفا نودا ويروس همچنين مي تواند بچه موش و بچه هامسترهاي شيرخوار را مبتلاكند و سبب فلج و مرگ در اين جانوران شود. جنس بتانوداويروس نيزماهيان را تحت تاثير قرار مي دهد. عوامل متعلق به جنس بتانوداويروس ذراتي كوچك به قطر حدود 25 تا 30 نانومتر بدون پوشش و داراي شكل بيست وجهي بوده و ژنوم آن ها شامل دو RNA تك رشته اي است. اين بيماري از سراسر جهان به استثناي قاره آفريقا گزارش شده است. نكروز عصبي ويروسي عمدتاً از طريق انتقال افقي گسترش مي يابد اما انتقال عمودي آن نيز مطرح است. تاكنون هيچ واكسن تجاري موثري براي مقابله با اين بيماري تهيه نشده و عملاً راهي براي درمان آن وجود ندارد ولي آشنايي بيشتر با خصوصيات اين بيماري مي تواند در پيشگيري از انتفال آن موثر باشد.

Viral Nervous Necrosis (VNN) is a serious disease causing significant economic damages to marine aquaculture industry. The disease has a wide geographical distribution having been observed in tropical and temperate climates. Up to now, more than forty species mainly of marine origin have been so far affected and this number is likely to rise in future following the introduction of new species and the increase of aquaculture trade. According to OIE all fish mortalities characterized by abnormal swimming behaviours associated with vacuolar lesions in the nervous tissues containing viral particles of the Nodaviridae family should be ascribed to one single disease, officially identified as viral nervous necrosis (VNN) also known as viral encephalopathy and retinopathy (VER). Affected fish may show different clinical symptoms depending on species, age and temperature; furthermore acute and sub-acute forms characterized by slightly different symptoms and mortality rates are also known. The most characteristic and common clinical sign observed among the different species is an abnormal swimming behaviour characterized by fish showing difficulties to maintain the normal static and dynamic equilibrium, speed and swimming direction and to control the swim bladder inflation. The viral aetiology has been confirmed following the identification of small, non enveloped, RNA agents definitively assigned to the Nodaviridae family, genus Betanodavirus. The existence of four genotypes characterized by high homology has been proposed on the basis of the viral genome analysis. Although horizontal transmission undoubtedly represents the most common transmission route, vertical transmission has also been highly suspected, at least for some species. VER/VNN is commonly diagnosed after isolation of the causative agent in cell cultures followed by identification with immunological or molecular methods.The control of the disease is complicated by difficulties in both applying strict hygiene and preventive measures in open environments like the sea and in selecting broodfish free from infection. Although currently under study, commercial vaccines are unfortunately not yet available. VNN was identified in Golden grey mullet (Liza aurata) in the Caspian Sea as first time in 2005. A comprehensive research project was designed and carried out in IFRO from 2006 till 2010.Viral causative agent was isolated and identified by Cell culture, IHC, IFAT and RT-PCR and confirmed in Guppy fish as pathogenicity test.