Emergence of Tilapia Lake Virus associated with mortalities of farmed Nile Tilapia Oreochromis niloticus (Linnaeus 1758) in India
Introduction
Tilapia, belonging to the family Cichlidae, includes over 100 species under three genera viz. Oreochromis, Sarotherodon and Tilapia (Ridha, 2006, Wang and Lu, 2016). These are native to Middle East and Africa (Trewavas, 1983) and described as the most important aquaculture species of the 21st century (Shelton, 2002). Presently, tilapias are being cultured in > 100 countries (FAO (Food and Agriculture Organization of the United Nations), 2013, Shelton and Popma, 2006) and interestingly, about 98% of total tilapia production comes from countries outside their native distribution (Shelton, 2002). These are the second most farmed fish after carps and are considered to be among the most important fish species to meet the growing demand for protein sources (Ng and Romano, 2013). The global production of tilapia was estimated to be 5.67 million mt in 2015 (FAO, 2017) and is expected to reach 7.3 million mt by 2030 (FAO, 2013). The top three producers are the People's Republic of China (1.78 MMT), Indonesia (1.12 MMT) and Egypt (0.88 MMT), and other leading producers include Bangladesh, Vietnam and Philippines (FAO, 2017). India ranks second in aquaculture production after China and tilapia culture is picking up during last few years. Rajiv Gandhi Centre for Aquaculture (RGCA) under Marine Products Export Development Authority, Government of India is promoting culture of genetically improved farmed tilapia (GIFT), Oreochromis niloticus (through all-male seed production) (RGCA, 2017). Though tilapia are considered to be relatively resistant to a number of diseases encountered in other farmed fishes (Del-Pozo et al., 2017), but, a number of bacterial, fungal, protozoan and parasitic diseases have been reported in these fish (FAO, 2005). In addition, some viral diseases, though not very common, have been reported in tilapia (Mcgrogan et al., 1998, Bigarre et al., 2009, Shlapobersky et al., 2010). But, the emergence of Tilapia Lake Virus (TiLV) disease, the first-ever major disease epidemic reported in tilapia aquaculture, has put the global tilapia industry at risk (Jansen and Mohan, 2017). This disease first reported from Israel (Eyngor et al., 2014, Bacharach et al., 2016), has subsequently been reported from Ecuador (Ferguson et al., 2014, Del–Pozo et al., 2017), Columbia (Kembou Tsofack et al., 2017), Egypt (Fathi et al., 2017, Nicholson et al., 2017) and Thailand (Surachetpong et al., 2017, Dong et al., 2017a, Dong et al., 2017b). Mortalities ranging from 80 to 90%, especially in fingerlings and juveniles, have been reported following infection with TiLV (Fathi et al., 2017, Surachetpong et al., 2017). Based on screening of archived and fresh tilapia samples from Thai hatcheries (collected during 2012–17), it has been speculated that over 40 countries including India may already have been imported TiLV-infected tilapia fry and fingerlings from the infected hatcheries (Dong et al., 2017b).
Tilapia Lake Virus (TiLV) is a novel orthomyxo-like virus and has 10 segmented negative sense RNA genome. The size of each segment ranges from 456 to 1641 nucleotides and the total genome size is 10.323 kb (Bacharach et al., 2016). The genome organization and ultrastructural morphology of TiLV resemble with other orthomyxoviruses (Del–Pozo et al., 2017, Eyngor et al., 2014).
In the present study, we report mortalities in tilapia from three geographical locations in two states, West Bengal and Kerala in India that were diagnosed to be caused by infection with TiLV on the basis of reverse transcription polymerase chain reaction, sequencing of segment 3 of TiLV, histopathology, infection of cell line with filtered tissue homogenate and bioassay.
Section snippets
Collection of fish samples
Diseased fish Nile Tilapia (Oreochromis niloticus) in the present study were collected during July 2016 from a polyculture farm located in North 24 Parganas district of West Bengal, India (Fig. 1). The farm had been stocked with tilapia along with Indian Major Carps (IMCs) in a ratio of 30:70, with a stocking density of 8000 fingerlings per hectare. In the farm, only tilapia (20–80 g) were affected with > 85% mortality, whereas cohabiting IMCs, Labeo rohita, Catla catla, Cirrhinus mrigala were
Pathological findings
Gross lesions in naturally infected fish were characterized by lethargy, skin discoloration and erosions and loss of scales (Fig. 2). The cumulative mortality of tilapia ranged from 80 to 90%. However, no clinical signs or gross lesions were observed in cohabiting fishes, IMCs, milk fish and pearl spot in the same farm. No bacteria could be isolated from kidney of sampled fish.
PCR and sequence analysis
PCR amplification for detection of herpes-like tilapia larvae encephalitis virus and Betanodavirus did not yield any
Discussion
Tilapia Lake Virus is a newly emerging virus that has been associated with significant mortalities in wild and farmed tilapia and represents a huge threat to the global tilapia industry (Eyngor et al., 2014, Jansen and Mohan, 2017). Till date, the virus has been reported from five countries across three continents i.e. Africa, Asia and South America. Recently, based on screening of archived tilapia samples (collected during 2012–17) from Thai hatcheries involved in export of tilapia fry and
Conflict of interest
The authors have declared no conflict of interest.
Acknowledgements
This work was carried out under “National Surveillance Program for Aquatic Animal Diseases”. The authors acknowledge National Fisheries Development Board, Department of Animal Husbandry and Dairying, Govt. of India (G/Nat. Surveillance/2013 dated 16.08.2013), for the funding support. The authors would like to express their sincere thanks to Mr. Asim Kumar Jana, Senior Technical Assistant for helping in sampling and laboratory assistance.
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Contributed equally.