WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2023 67 et al. 2017). Medium- to largesized, adult stages of tilapia are also reported to be affected, but with lower mortality rates of just over 9 percent (Fathi et al. 2017). Mode of Transmission TiLV can be transmitted through direct horizontal transmission by cohabitation or transfer of live aquatic animals. Until now, there is no evidence of vertical transmission. The biophysical properties of TiLV are not well characterized. Hence, the risk of indirect transmission of TiLV associated with animal products is difficult to determine. Still, research suggests that the eye, brain and liver are likely to contain the highest concentrations of TiLV and therefore solid and liquid waste were likely to be contaminated (OIE 2018). Clinical Signs and Symptoms The clinical signs and symptoms associated with TiLV infection vary with the geographical location of the disease outbreak. Gross clinical signs include ocular alterations, including opacity of the lens and, in advanced cases, ruptured lens, lethargy, skin erosion, discoloration (darkening), haemorrhages in the leptomeninges and congestion of the spleen (Eyngor et al. 2014). Other signs and symptoms include exophthalmia, abdominal distension, scale protrusion, pale gills, ulcers, loss of appetite, abnormal behavior, such as reduced schooling behavior and swimming at the surface (Fig. 3) (Ferguson et al. 2014, OIE 2018, Dong et al. 2017, Surachetpong et al. 2017). Additionally, brain congestion and paleness of the liver have been observed (Surachetpong et al. 2017). The main organs that are primarily affected are eyes, brain and liver (Eyngor et al. 2014). Although affected eyes can easily be observed, as the lesions are macroscopic, lesions in the brain and liver are microscopic and thus require histological examination for observation and confirmation. Socio-economic Impact The emergence and spread of TiLV has had a global impact as tilapia is currently the second most important group of farmed fish worldwide after carps. Global production is estimated at 6.4 million t in 2015, with an estimated market value of US$ 9.8 billion annually (FAO 2017). This industry is a major component of aquaculture sectors in China, Egypt, Thailand, Indonesia, Philippines, Laos, Colombia, Costa Rica, Honduras and Ecuador, with the United States as the lead importer (Bacharach et al. 2016). Tilapia are also ecologically important in some regions as they are beneficial in mosquito and algae control and habitat maintenance for shrimp farming. They are also an important wild capture species. Thus, the global emergence of TiLV has caused significant commercial, economic and ecological losses in the tilapia industry. Diagnostic Methods The preliminary diagnostic methods to examine affected tilapias includes observation of gross clinical signs and symptoms. High levels of mortality associated with ocular alterations such as opacity of the lens, or more severe pathology, should be considered suspicious of TiLV infection (OIE 2018). Other signs, such as skin erosions, haemorrhages in the leptomeninges and moderate congestion of the spleen and kidney may be observed post-mortem; however, these signs are only useful as a presumptive diagnosis and a histological examination or molecular methods are required for further confirmation. Cell culture could be used as a presumptive test method where TiLV could be cultured in various cell lines and then observed for cytopathic effects. Multiple cell lines are suitable for TiLV cell culture (Eyngor et al. 2014). Tilapia lake virus can be cultured in the E-11 cell line or primary tilapia brain cells, inducing a cytopathic effect 3-10 days after inoculation (Eyngoret al. 2014, Liamnimitr et al. 2018). Tsofack et al. (2017) reported the E-11 cell line at 25 C provided optimal conditions for TiLV replication. Several molecular methods based on polymerase chain reaction (PCR), such as reverse transcriptase (RT) PCR have been developed and PCR primers, specifically targeting TiLV genes have been designed (Eyngor et al.2014). A more sensitive, nested RT-PCR has been developed, which is 10,000 times more sensitive than the single RT-PCR (Tsofack et al .2017). Subsequently, a semi-nested RT-PCR method with improved analytical sensitivity (i.e., detection limit of 7.5 viral copies per reaction) has been developed and can detect TiLV in clinically healthy fish (Dong et al. 2017, Senapin et al. 2018). A SYBR green-based reverse transcription-quantitative PCR (RT-qPCR) method with a reported sensitivity of two copies/ μL has been developed (Tattiyapong et al. 2018). All currently available PCR methods have not been fully validated and thus should be combined with sequencing of representative PCR products for agent confirmation. Control and PreventionMethods Tilapia lake virus can be prevented by following good sanitary and biosecurity practices. Movement of live tilapias from farms where the virus is known to occur would limit the spread of the disease. Standard measures, such as cleaning and disinfection should be followed to minimize spread via equipment, vehicles or staff. At present, there is no effective protocol or medication to cure TiLV diseases in aquaculture and, therefore, the importance of biosecurity measures needs to be promoted by the competent authorities. Biosecurity measures, in combination with the breeding of fish for improved resistance, or the development of a vaccine may offer the long term prospects for managing the disease (Ferguson et al. 2014). FIGURE 3. Gross clinical signs and symptoms associated with tilapia lake virus infection. (CONTINUED ON PAGE 68)
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