Catfish farming is of great economic importance for Southern U.S. agriculture and is the largest production sector within U.S. aquaculture. Flavobacterium covae and virulent Aeromonas hydrophila are prevalent bacterial pathogens within this industry that can cause high mortality in production ponds. In addition, bacterial co-infections in catfish production may go unreported or misdiagnosed, resulting in a lack of proper mitigation or inadequate treatment. These polymicrobial infections are predicted to increase disease severity and farmed fish mortality.
A preliminary assessment of in vivo bacterial co-infection with virulent A. hydrophila (ML09-119) and F. covae (ALG-00-530) was conducted in juvenile channel catfish (Ictalurus punctatus). For the pathogen challenge, catfish were divided into seven treatment regimes: 1) mock control; 2 and 3) full and half doses of virulent A. hydrophila; 4 and 5) full and half doses of F. covae; 6 and 7) co-infective full and half doses of virulent A. hydrophila and F. covae. In addition to the mortality assessment, head kidney and spleen tissues were collected to evaluate immune gene expression and quantify bacterial load using qPCR. At 96 h post-challenge (hpc), the full-dose, single virulent A. hydrophila infection (immersed in 2.3 x 107 CFU mL-1) resulted in final cumulative percent mortality (CPM) of 28.3 ± 9.5 %. The full-dose F. covae group (immersed in 5.2 x 106 CFU mL-1) was 23.3 ± 12.9 %. When the single pathogens were compared to the polymicrobial infections, the co-infective full-dose combination (98.3 ± 1.4 %) and half-dose combination (76.7 ±17.1 %) significantly increased mortality (P< 0.001).
Sera lysozyme content between treatment groups was not significantly different; however, differences were observed across time. At 12 hpc, a significant increase (P<0.002) in lysozyme content was observed compared to 6, 48, and 96 hpc. Conversely, lysozyme content at 96 hpc significantly decreased (P<0.001) compared to 6, 12, 24, and 48 hpc. An increase in expression of three pro-inflammatory cytokines (tnfa, il8, il-1b) occurred at 48 hpc. For instance, tnfa demonstrated a significant increase in expression at 48 hpc (P< 0.001) followed by a significant decrease at 96 hpc (P< 0.001).
Trial results emphasize the importance of evaluating co-infections and demonstrate dramatic increases in mortality when two pathogens are combined, even at half-doses. The synthesis of these mortality and health metrics will aid fish health diagnosticians and channel catfish producers in developing therapeutants and prevention methods to control bacterial co-infections better.