Enhanced susceptibility of hybrid tilapia to Flavobacterium columnare after parasitism by Ichthyophthirius multifiliis

• Evaluated susceptibility of tilapia to Flavobacterium columnare after parasitism.
• The loads of F. columnare in parasitized fish were higher than non-parasitized fish.
• Tilapia showed high mortality when co-infected with Ichthyophthirius and F. columnare.
• Prevention of parasite infection will reduce fish loss due to parasites and bacteria.

Bacterium Flavobacterium columnare and protozoan Ichthyophthirius multifiliis are two common pathogens of cultured fish. The objective of this study was to evaluate the susceptibility of hybrid tilapia (Oreochromis spp.) to F. columnare, including fish mortality and bacterial loads in fish gill and kidney after parasitism by I. multifiliis. Fish received the following treatments: 1) non-infected control; 2) infected by I. multifiliis at 30,000 theronts fish− 1 alone; 3) infected by F. columnare ALM-05-53 at 4.59 × 107 CFU mL− 1 alone; 4) infected by I. multifiliis at 30,000 theronts fish− 1 and exposed to F. columnare ALM-05-53 at 4.59 × 107 CFU mL− 1; 5) infected by F. columnare TN-3-2012 at 4.27 × 107 CFU mL− 1 alone; and 6) infected by I. multifiliis at 30,000 theronts fish− 1 and exposed to F. columnare TN-3-2012 at 4.27 × 107 CFU mL− 1. F. columnare in fish tissues were quantified by quantitative real-time polymerase chain reaction and reported as genome equivalents per mg of tissue (GEs mg− 1). The results demonstrated that the I. multifiliis-parasitized tilapia showed significantly (P < 0.05) higher mortality (60.4%) when exposed to F. columnare ALM-05-53 than non-parasitized fish (29.1%). The bacterial loads of F. columnare ALM-05-53 in fish infected by 30,000 theronts fish− 1 were ≥ 5703 GEs mg− 1 which was between 13 and 17 fold higher than those of non-parasitized fish (≤ 472 GEs mg− 1). Similarly, parasitized tilapia showed significantly higher mortality (25%) and bacterial loads (≥ 1586 GEs mg− 1) at day 3 post-exposure to F. columnare TN-3-2012 than non-parasitized fish (0% and ≤ 197 GEs mg− 1). I. multifiliis parasitism of tilapia enhanced F. columnare invasion and resulted in higher fish mortality.