Nota Técnica Tilapias

Viral Diseases in Tilapias Dr. Marco Rozas-Serri DVM, MSc, PhD 2020 VIRAL DISEASES IN TILAPIAS The viral infections have the potential to cause relatively high mortalities of up to 90% in some affected populations. The actual impact and geographical distribution of the viruses are not known so there is a potential danger of the viruses being introduced to new countries or regions unintentionally through movement of sub-clinically infected fish that are destined for aquaculture farms lacking appropriate control measures (Table 1). The priority focus of the Brazilian tilapia industry As outlined by the OIE Guide for aquatic animal, surveillance may be should be on the active relatively simple in the form of passive surveillance or highly sophisticated in surveillance of two important the form of active surveillance that implements specific sampling strategies exotic viruses: Infectious and that may target specific disease agents. In all the viral diseases that spleen kidney necrosis virus affect tilapia, the correlation between virulence, genetic type, survival outside host as well as environmental factors, is an area of research requiring infection and Tilapia lake virus attention. Table 1. Summary of viral diseases affecting tilapines. Infectious spleen kidney necrosis virus - ISKNV The first DNA viruses discovered in tilapia were iridoviruses. Although the family Iridoviridae is composed of 5 genera, only members of the genera Megalocytivirus, Lymphocystivirus, and Ranavirus infect fish. The ISKNV is the only formally accepted into the Megalocytivirus genus. The ISKNV virus has been isolated from both marine and freshwater fish: rock bream iridovirus (RBIV), red seabream iridovirus (RSIV), orange spotted grouper iridovirus (OSGIV), turbot reddish body iridovirus (TRBIV), large yellow croaker iridovirus (LYCIV), giant seaperch The disease was described in tilapia after a US Midwestern iridovirus (GSIV-K1), scale drop disease virus aquaculture tilapia facility experienced heavy mortalities of 50– (SDDV). 75% from November to December 2012. The moribund fish The ISKNV contains naked double-stranded DNA genome displayed lethargy, branchial pallor, and distension of the encapsulated by an outer nucleocapsid layer, forming an coelomic cavity due to a clear ascitic coelomic fluid (Table 2) icosahedral shape with a diameter of 140–200 nm (Table 1). (Figure 2). Table 2. Diagnosis and characterization of etiological agent. Tilapia lake virus - TiLV Tilapia lake virus (TiLV) affected wild and farmed tilapines (Table 1). It has been reported in Asia, Africa and South America, and the number of countries where the agent has been detected is likely to increase rapidly as a result of increased awareness, surveillance and availability of diagnostic methods (Table 2). There are currently severe knowledge gaps relating to TiLV and no effective, affordable vaccines are yet available. The virus is an enveloped, negative-sense, single-stranded RNA virus with 10 segments and a diameter between 55 and 100 nm. A taxonomic proposal is a new genus Tilapinevirus, specie Tilapia tilapinevirus. The reported clinical signs and gross pathological lesions associated with TiLV infections are somewhat variable, depending on geographical origin (Figure 2). Based on the available information, it seems that a list of pathognomonic signs, to allow a reliable diagnosis based on clinical signs alone, is not currently feasible. Figure 2. Tilapia lake virus (TiLV). (A) Naturally diseased red tilapia showing skin haemorrhages. (B) Experimentally diseased Nile tilapia showing exophthalmia, abdominal swelling and scale protrusion. (C) The infected liver tissue showed syncytial hepatocytes and foamy cytoplasm, intracytoplasmic inclusion bodies. Jansen et al. 2018 Figure 1. Infectious spleen kidney necrosis virus (ISKNV). (A) Broodstock with microphthalmia (left arrow head), skin haemorrhages (middle arrow head) and skin ulcers (right arrow head). (B) Juvenile with exophthalmia (left arrow head), ascites Pathovet Chile is carrying out PCR and RT-PCR tests (arrow) and loss of scales, excess of mucus and haemorrhages to detect all exotic and enzootic viruses and bacteria (right arrow head). (C) Ventral view of juvenile fish showing for the tilapia industry in Brazil. An active severe ascites black arrow and skin haemorrhages (arrow head). Bar= 1cm. (D) Spleen. Affected cells show pronounced epidemiological surveillance program based on eosinophilia and are distributed throughout the section (*). regular sampling to detect specific disease agents is Numerous vacuolated macrophage aggregates are also present the technical basis from which it must to build the (M). Bar = 50μm. (E) Pronephros showing extensive distribution comprehensive health management strategy for of megalocytes without associated tissue necrosis (*). disease prevention and control in aquaculture. Bar=100μm. Inset shows pale staining of DNA in the cytoplasm of Histological diagnosis and histoscores are also affected cells (arrows). Feulgen stain. Bar=25μm. Ramirez- available to complement the health surveillance plan Paredes et al. 2019. Palena 280, Puerto Montt, CHILE / www.pathovet.cl / +56 65 2 77 31 75.

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