First report on characterization and pathogenicity study of emerging Lactococcus garvieae infection in farmed rainbow trout, Oncorhynchus mykiss (Walbaum), from India

Warm water lactococcosis in farm-reared rainbow trout, Oncorhynchus mykiss (Walbaum) in the northern Himalayan region of India, caused by bacterium Lactococcus garvieae is described in this study. Nine bacterial isolates were recovered from the organs of haemorrhagic septicaemia rainbow trout and were subjected to biochemical and molecular identification. Cell surface characteristics and virulence of the bacterial isolates are also described. All the nine bacterial isolates had homogenous biochemical characteristics and were Gram-positive, short chains forming (two to eight cells long), -haemolytic, non-motile ovoid cocci. Partial 16S rDNA nucleotide sequence (similar to 1,400bp) of current isolates shared 99% identities with the 16S rDNA nucleotide sequence of L.garvieae R421, L.garvieae FMA395 and L.garvieae CAU:1730. The identity of the bacterial isolates was further confirmed by PCR amplification of L.garvieae-specific similar to 1,100bp fragment. Transmission electron microscopy (TEM) of one representative isolate, L.garvieae RTCLI04, indicates that the isolated strain lacks thick outer capsule and is of KG+ (non-capsulates) phenotype. An intraperitoneal and intramuscular injection (2.6x10(5) CFU ml(-1)) and also immersion in bacterial suspension @ of 2.6x10(5) CFU ml(-1) to healthy rainbow trout juveniles (body weight: 27.5 +/- 3.7g) with L.garvieae RTCLI04 caused 80%, 60% and 10% cumulative mortality in challenged fish, respectively, within 15days post-infection. The haemorrhagic septicaemic disease was reproduced experimentally. Histopathological examination of organs of experimentally infected fish revealed extensive degenerative and inflammatory changes in eye, kidney, gill and liver. PCR amplification of several putative virulence genes such as haemolysins, adhesins, LPxTG-containing surface proteins and adhesins cluster confirms the virulence of our Indian L.garvieae isolates. To the best of our knowledge, we are reporting for the first time that L.garvieae is associated with fatal haemorrhagic septicaemia in farmed rainbow trout in India.