Investigating biosecurity strategies to promote improved disease management practices in farmed African catfish (Clarias gariepinus)

Abstract

African catfish is the most cultured fish species in Nigeria, but aquaculture production has remained stagnant for over a decade. Several factors including infectious disease outbreaks has been implicated as one of the drivers affecting production in Nigeria due to poor sanitation practices common among fish farm operators. Available data has identified research gaps including limited information about factors influencing disinfection efficacy within the Nigerian catfish sector. Furthermore, poor knowledge about host-pathogen interaction and immune mechanisms in African catfish are exacerbating these issues. This thesis investigated two factors contributing towards biosecurity strategies to support improved fish health and welfare applicable to African catfish aquaculture in Nigeria. To support hygiene and sanitation practices, disinfection efficacy studies using Virkon™ aquatic was explored in vitro against common bacterial pathogens Aeromonas hydrophila, Streptococcus agalactiae and Edwardsiella tarda. The result revealed that temperature fluctuations, disinfectant concentration and bacterial innate properties were the principal factors that influenced efficacy of Virkon™ aquatic. The least disinfectant concentration 0.125 % was MIC and MBC for the S. agalactiae strains at all experimental conditions in both tests. E. tarda strain was more resistant to the disinfectant with 1 % as MBC in carrier test at 22 and 28 °C. In addition, 0.25 % emerged as MIC value for strains of A. hydrophila and E. tarda at 28 and 34 °C in the suspension test, and both species had the same MBC value of 0.25 % at 28 °C in suspension test and 0.5 % at 34 °C in carrier test. Data from the study highlighted that carrier test using nets was more suitable to evaluate efficacy of chemical disinfectants applied in aquaculture systems compared with microdilution suspension test. To provide information regarding immune functions in the African catfish, relevant immune targets including NOD1, NOD2, TLR3, MHC II, IL1β, COX2 and MPO genes were identified, cloned and characterized using PCR assays. Then Absolute qPCR assays were performed to screen the expression pattern of NOD1, NOD2, TLR3 and MHC II genes in the spleen, gill, head kidney and liver of two fish groups. The PCR assays detected the transcript expression of all selected immune genes and results from qPCR assays showed that mRNA of evaluated genes was expressed in the selected tissues of fish. Gene expression levels were statistically significant among the tissues; NOD1 (P = 0.0006), NOD2 (P = 0.0067), TLR3 (P < 0.0001) and MHC II (P = 0.0426). And between the two fish groups, expression levels differed statistically only in the spleen tissue for NOD1 (P = 0.0042), NOD2 and (P = 0.0100), and in the gill and liver for TLR3 (P < 0.0001) respectively but not in MHC II gene (P = 0.2895). Also, higher copy numbers of NOD1 mRNA with over 3 to 5-fold differences more than NOD2 was recorded in the tissues; spleen (5.59), gill (5.01), head kidney (4.49), and liver (3.97). The study concluded that African catfish possess immune relevant genes as previously found in other fish species. A further study was conducted to quantify innate immune responses with 72 African catfish exposed by intraperitoneal injection to a heat-killed A. hydrophila experimental vaccine which acted as a proxy immunostimulant. In the spleen, the result revealed that expression of both receptors was elevated in vaccine group after 24 h and statistically different among treatment groups at 24 hours PT (NOD1, P = 0.035 and NOD2, P = 0.0113) and at 7 days PT (NOD1, P = 0.008 and NOD2, P = 0.0025) with highest and least expression in the vaccine group at 24 h PT and bacterin group at 7 days PT respectively. All treatments triggered continuous increased expression of both receptors in gill with highest level of expression observed at 34 days PT in bacterin group and downregulated expression after 7 days PT in vaccine group. At 34 days PT, NOD1 expression in gill of vaccine group was statistically low in comparison to other treatment groups (P = 0.025) whereas NOD2 expression in bacterin group was statistically higher compared to the expression in vaccine group (P = 0.0297). The study concluded that A. hydrophila derived immunostimulant and mineral oil-based adjuvant induced innate immune responses in African catfish. Overall, results from this study will inform the biosecurity practices, from basic hygiene and sanitation to novel vaccination programmes, adopted by the Nigerian Clarias sector to promote improved health and welfare of their farmed catfish sustainably.