Temporal trends of tilapia lake virus disease in Israel, 2017-2018

Tilapia lake virus (TiLV) is an emerging viral disease that affects several tilapia species in different countries since 2014. In 2017-2018, 129 samples were collected from 14 tilapia farms in Israel. Ninety samples represented mortality events (ME), and 39 were used as control samples (CS). RT-qPCR was performed on 89 and 39 duplicate brain and liver tissue samples from ME samples and CS, respectively. TiLV was diagnosed in 37 (41.6%) ME, while only two of the CS samples (5%) were positive for TiLV (OR = 13.2, 95% CI = 3.0-58.1). Additional RT-PCR was performed on positive samples, and amplified products were sequenced. Maximum likelihood phylogenetic analysis of segment-3 revealed three distinct clades: the first clade (A) includes 25 sequences of TiLV, detected previously in Israel (2011), Ecuador (2012), Egypt (2015), Thailand (2015-2019), India (2017), Peru (2018) and USA (2018-2019) and 11 sequences of TiLV from the current study (2017-2018); the second clade (B) includes only four sequences from Thailand (2018) and Bangladesh (2017 and 2019); and a third clade (C) which includes a single sequence from Bangladesh (2019). Out of the 39 sequences included in clade A, 14 closely related sequences of TiLV from the current study (2018) formed a distinctive sub-clade (IL-2018). Mann-Whitney U test showed differences in the distribution of survival rates between Israeli sequences (from 2011, 2017 and 2018) of clade A (p = 0.004) and Israeli sequences (from 2018 solely) of sub-clade IL-2018. The average survival rates of clade A and sub-clade IL-2018 were 58.1% (SD = 21.5) and 31.2% (SD = 25.6), respectively. This is one of only few field studies which show direct association of TiLV with mortality events in tilapia farms. The decrease in survival rate in the newly evolved clade might raise concern regarding virus evolution towards increased virulence, which should be further explored.