Pathogen identification, risk factor and preventive measure of a columnaris disease outbreak in Tilapia (Oreochromis niloticus) eggs and larvae from a tilapia hatchery

Flavobacterium columnare is the causative agent of columnaris disease and brings about huge damages to the aquaculture industry worldwide. First classified as one specie with several genomovars/genetic groups, then reclassified into four distinct species, F. columnare was well-known for its high genetic diversities among different isolates. In this study, six bacterial strains, designated as HNU-01 to HNU-06, were isolated from diseased Nile tilapia eggs and larvae in Hainan, China. Based on colony characteristics, physiological/ biochemical test and 16S rRNA gene analysis results, all isolated strains were identified as Flavobacterium davisii according to a recently published research (which correspond to genetic group 3 of F. columnare). Virulence test showed that among the isolates, HNU-01 was most virulent to tilapia larvae. The lethal doses of 7 days (LD50) of F. davisii HNU-01 to tilapia eggs and larvae were 7.29 x 10(7) CFU/ml and 1.56 x 10(6) CFU/ml, respectively. The anti-microbial susceptibility test indicated that the bacterium showed reduced susceptibility towards ampicillin and oxytetracycline. In the disinfection-infection experiment, compared with tilapia larvae hatched from untreated eggs, tilapia larvae hatched from disinfected eggs (treated with formalin, potassium permanganate and chloramine T) showed significantly higher mortality rates/F. davisii loads after challenging with F. davisii HNU-01. Salinity test showed that elevated hatching system water salinity decreased the mortality of tilapia larvae infected with F. davisii. The best protective effect was seen in water salinity of 4.0-10.0 parts per thousand. This is the first report of F. davisii (or genetic group 3 of F. columnare) infecting tilapia eggs and larvae. This study revealed that the virulence of F. davisii in tilapia varied at different fish developmental stages. In addition, disinfectant treatment of tilapia eggs increased the susceptibility of tilapia larvae to F. davisii, and regulation of water salinity is a potential effective measure to prevent columnaris disease in tilapia hatcheries.