Elsevier

Aquaculture

Volume 484, 1 February 2018, Pages 168-174
Aquaculture

Emergence of Tilapia Lake Virus associated with mortalities of farmed Nile Tilapia Oreochromis niloticus (Linnaeus 1758) in India

https://doi.org/10.1016/j.aquaculture.2017.11.025Get rights and content

Highlights

  • Mortalities were observed in farmed tilapia from West Bengal and Kerala, India.

  • Samples were positive for tilapia lake virus(TiLV) in RT PCR amplification.

  • NCBI-BLAST of the PCR amplicons showed maximum similarity with TiLV from Israel.

  • Typical syncytial giant cells were observed in sections of liver.

  • Filtered tissue homogenate from liver and brain of affected tilapia produced CPE in CFF cell line.

Abstract

Tilapiines are one of the most widely farmed fish species and currently being cultured in > 100 countries in the World. Over the last few years, large-scale mortalities have been reported in tilapia due to infection with orthomyxo-like virus i.e. Tilapia Lake Virus (TiLV) which is considered as a threat to global tilapia industry. In the present study, we report outbreaks of TiLV disease in farmed tilapia in two states, West Bengal and Kerala from India. Diseased fish exhibited lethargy, inappetance and skin erosions with > 85% mortality. TiLV infection was confirmed on the basis of PCR amplification and sequencing of segment 3 of TiLV, histopathology, infection of fish cell line and bioassay. CLUSTALW alignment of the partial sequences of segment 3 of the TiLV revealed that North 24 Parganas (MF502419) and South 24 Parganas (MF582636) of West Bengal, India showed 97.2% and Ernakulam, Kerala, India (MF574205) showed 96.4% similarity with TiLV from Israel (KJ605629.1). In histopathology, typical syncytial giant cells in liver and congestion of the blood vessels along with haemorrhages in sections of brain tissue were observed. The filtered tissue homogenate prepared from liver and brain of affected tilapia produced cytopathic effects in CFF cell line derived from Pristolepis fasciatus. The disease was successfully reproduced in naive tilapia following injection of culture supernatant from infected cell line and TiLV was successfully reisolated from experimentally infected tilapia. This is the first report of TiLV from India and adds to the reports of TiLV outbreaks in five countries across three continents.

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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