Vertical transmission and concurrent infection of multiple bacterial pathogens in naturally infected red tilapia (Oreochromis spp.)

Horizontal transmission of various bacterial pathogens in tilapia is well described, but there is scant information regarding their vertical transmission. This study aimed to determine the possibility of vertical transmission of two commonly reported bacterial pathogens (Francisella noatunensis subsp. orientalis and Shewanella putrefaciens) in natural stocks of red tilapia (Oreochromis spp.). Vertical transmission of these pathogens via gametes was evaluated using in vitro fertilization from 10 different families and analysing for the presence of bacteria in milt, unfertilized eggs, fertilized eggs and offspring at various ages (1‐day‐old larvae, 10‐day‐old fry and 30‐day‐old fingerlings), as well as water samples using colorimetric LAMP assay. The study revealed that both F. n. orientalis (6/10) and S. putrefaciens (4/10) was transmitted vertically to the fertilized eggs. Analysis of the water samples from different water sources (brood stock tanks, hatching chamber and larval rearing tanks) showed that both the pathogens were present in water samples with highest prevalence for F. n. orientalis followed by S. putrefaciens. Analyses for the presence of two pathogens in various organs (gonads, gill, liver, spleen, kidney and brain) of the healthy tilapia broodstock without any clinical symptoms of disease demonstrated they were carriers of S. putrefaciens and F. n. orientalis. This is the first documented evidence that vertical transmission via the broodstock of tilapia may also play an important role in transmitting these problematic pathogens to their progeny and underlines the necessity to modify the current disease management strategies in tilapia aquaculture.     

Colorimetric Method of Loop-Mediated Isothermal Amplification with the Pre-Addition of Calcein for Detecting Flavobacterium columnare and its Assessment in Tilapia Farms

Flavobacterium columnare, the causative agent of columnaris disease in fish, affects many economically important freshwater fish species. A colorimetric method of loop-mediated isothermal amplification with the pre-addition of calcein (LAMP–calcein) was developed and used to detect the presence of F. columnare in farmed tilapia (Nile Tilapia Oreochromis niloticus and red tilapia [Nile Tilapia × Mozambique Tilapia O. mossambicus]) and rearing water. The detection method, based on a change in color from orange to green, could be performed within 45 min at 63°C. The method was highly specific, as it had no cross-detections with 14 other bacterial species, including other fish pathogens and two Flavobacterium species. The method has a minimum detection limit of 2.2 × 10² F. columnare CFU; thus, it is about 10 times more sensitive than conventional PCR. With this method, F. columnare was detected in gonad, gill, and blood samples from apparently healthy tilapia broodstock as well as in samples of fertilized eggs, newly hatched fry, and rearing water. The bacteria isolated from the blood were further characterized biochemically and found to be phenotypically identical to F. columnare. The amplified products from the LAMP–calcein method had 97% homology with the DNA sequence of F. columnare.

Received May 21, 2014; accepted August 10, 2014