Abstract
BackgroundLactococcus garvieae is one of the most important risk factors in the rainbow trout culture. Therefore, the purpose of this study was to identify and detect strains isolated from rainbow trout suspected of having Lactococcus garvieae using biochemical characteristics and PCR and determination of the degree of severity of isolated strains.MethodsIn this study, the cause of lactococcosis in selected rainbow trout farms in Kohkilooieh and Boyerahmad province was assayed. Gram-positive and catalase-negative bacterial isolates were first obtained from selected trout fish farms using conventional biochemical tests and PCR assay. The 10-day LD50 method (concentration causing 50% mortality in 10 days) was used to determine the severity of the isolated bacteria.ResultsOne bacterial isolate was detected from all sampled fish which confirmed as Lactococcus garvieae using a specific PCR assay based on the 16S rDNA gene by producing a single band of 1107 bp. Analysis of the rate of mortality showed that the 10-day LD50 was 4.6 × 105 CFU/fish. The results of this study showed that isolated bacteria had high severity for rainbow trout. The presence of bacteria in internal organs of suspected fish showed a severe systemic infection in challenged fish. Antibiogram assay also indicated that the isolated Lactococcus garvieae were resistant to some mostly used antibiotics in rainbow trout.ConclusionsAccording to current research, it can be concluded that the condition of lactococcosis in the studied area is not suitable, and despite the presence of disease, there is no proper action to control and prevent the disease. Unfortunately, isolated bacteria from the studied area have a very high severity compared to bacteria isolated from other regions of the country or other countries. Therefore, further investigation is needed to determine the cause of this difference and possibly in the design of the vaccine.
Highlights
Lactococcus garvieae is one of the most important risk factors in the rainbow trout culture
Lactococcosis is a systemic disease in marine fish including tilapia, sea bass, eel, and yellowfish and is recognized as one of the most important diseases of the rainbow trout in aquaculture (Austin and Austin 2007, 2016; Meyburgh et al 2017; Chapela et al 2018)
Several reports are presented of lactococcosis occurrence in marine fish, brackish water, and freshwater, especially when water temperature increases over 15 °C (Austin and Austin, 2007)
Summary
Lactococcus garvieae is one of the most important risk factors in the rainbow trout culture. The purpose of this study was to identify and detect strains isolated from rainbow trout suspected of having Lactococcus garvieae using biochemical characteristics and PCR and determination of the degree of severity of isolated strains. The first occurrence of lactococcosis in rainbow trout was reported in Spain in 1988 (Ghittino and Prearo 1992) This pathogen has caused serious economic losses in the cultivation of marine and freshwater fishes (Vendrell et al 2006; Wang et al 2007; López et al 2015; Meyburgh et al 2017). Considering that L. garvieae is the primary cause of mortality and risk factor in the rainbow trout culture industry during hot seasons; the purpose of this study was to identify and detect strains isolated from rainbow trout suspected of having L. garvieae using biochemical characteristics and PCR and determination of the degree of severity of isolated strains
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