The tilapia farming industry has consistently been challenged by multiple bacterial pathogens, with streptococcosis and columnaris being some of the most threatening. Antibiotic intervention has become critical in disease management, necessitating sustainable and cost-effective alternatives, such as functional additives, to improve fish health. A Nile tilapia Oreochromis niloticus feeding study investigated the potential of a humic substance and butyrate blend (HSB) alone or combined with yeast cell wall (YCW) components as feed additives for improving growth performance and resistance to two bacterial tilapia pathogens and their coinfection.
A 50-day feeding trial was conducted in twelve outdoor raceway tanks (1,200 L) that received flow-through water from a traditional earthen pond. Each raceway was stocked with 155 Nile tilapia (21.8 ± 0.1g) and was randomly assigned to one of four practical diet treatments in triplicate: a basal diet (Control; 36% protein), a HSB-supplemented diet (HSB; 2 g kg-1), a YCW-supplemented diet (YCW; 0.5 g kg-1), or a HSB (1 g kg-1) and YCW (0.25 g kg-1) diet (HSB+YCW). After the feeding trial, growth performance was evaluated, followed by the collection of blood, spleen, and kidney from five fish per raceway to assess immune responses. Subsequently, all treatment groups were challenged (in triplicate tanks) with either an immersion challenge with Flavobacterium oreochromis (R18-27; 1.4 × 106 CFU mL-1), an intramuscular injection with Streptococcus agalactiae capsular type Ib (LADL-05-108A; 1.3 × 104 fish-1), or co-infection with simultaneous half-doses of F. oreochromis and S. agalactiae.
At trial endpoint, differences were observed in final body weight (BW; p=0.026), weight gain (WG; p=0.032), and feed conversion ratio (FCR; p=0.042) across groups, with the HSB-fed tilapia demonstrating an increased BW, WG, and reduced FCR compared to the control group (p<0.05). In the bacterial challenge, all treatment groups reached 100% mortality after 72 h of F. oreochromis exposure. However, there was a significant difference in Kaplan-Meier survival curves (p<0.0001), with HSB shown to prolong tilapia survival to F. oreochromis than the control (p<0.008). Meanwhile, survival curves also differed in the 21-day S. agalactiae challenge (p=0.016), with the highest survival observed in the YCW groups (45%) compared to the control (15%). Within coinfected fish, survival curves were significantly different (p<0.0001), with an increased survival in HSB+YCW (42%) and HSB (37%) compared to the control (0%). Based on these findings, feeding these additives to tilapia improved their ability to combat streptococcosis and coinfections of F. oreochromis and S. agalactiae. The use of these additives in the industry may provide farmers with an alternative to antibiotics to enhance growth performance and mitigate bacterial diseases.